diff options
Diffstat (limited to 'src/runtime/mgc0.c')
| -rw-r--r-- | src/runtime/mgc0.c | 1426 |
1 files changed, 1048 insertions, 378 deletions
diff --git a/src/runtime/mgc0.c b/src/runtime/mgc0.c index 897dc1415..f37c01af0 100644 --- a/src/runtime/mgc0.c +++ b/src/runtime/mgc0.c @@ -4,22 +4,72 @@ // Garbage collector (GC). // -// GC is: -// - mark&sweep -// - mostly precise (with the exception of some C-allocated objects, assembly frames/arguments, etc) -// - parallel (up to MaxGcproc threads) -// - partially concurrent (mark is stop-the-world, while sweep is concurrent) -// - non-moving/non-compacting -// - full (non-partial) +// The GC runs concurrently with mutator threads, is type accurate (aka precise), allows multiple GC +// thread to run in parallel. It is a concurrent mark and sweep that uses a write barrier. It is +// non-generational and non-compacting. Allocation is done using size segregated per P allocation +// areas to minimize fragmentation while eliminating locks in the common case. // -// GC rate. -// Next GC is after we've allocated an extra amount of memory proportional to -// the amount already in use. The proportion is controlled by GOGC environment variable -// (100 by default). If GOGC=100 and we're using 4M, we'll GC again when we get to 8M -// (this mark is tracked in next_gc variable). This keeps the GC cost in linear -// proportion to the allocation cost. Adjusting GOGC just changes the linear constant -// (and also the amount of extra memory used). +// The algorithm decomposes into several steps. +// This is a high level description of the algorithm being used. For an overview of GC a good +// place to start is Richard Jones' gchandbook.org. +// +// The algorithm's intellectual heritage includes Dijkstra's on-the-fly algorithm, see +// Edsger W. Dijkstra, Leslie Lamport, A. J. Martin, C. S. Scholten, and E. F. M. Steffens. 1978. +// On-the-fly garbage collection: an exercise in cooperation. Commun. ACM 21, 11 (November 1978), 966-975. +// For journal quality proofs that these steps are complete, correct, and terminate see +// Hudson, R., and Moss, J.E.B. Copying Garbage Collection without stopping the world. +// Concurrency and Computation: Practice and Experience 15(3-5), 2003. // +// 0. Set phase = GCscan from GCoff. +// 1. Wait for all P's to acknowledge phase change. +// At this point all goroutines have passed through a GC safepoint and +// know we are in the GCscan phase. +// 2. GC scans all goroutine stacks, mark and enqueues all encountered pointers +// (marking avoids most duplicate enqueuing but races may produce duplication which is benign). +// Preempted goroutines are scanned before P schedules next goroutine. +// 3. Set phase = GCmark. +// 4. Wait for all P's to acknowledge phase change. +// 5. Now write barrier marks and enqueues black, grey, or white to white pointers. +// Malloc still allocates white (non-marked) objects. +// 6. Meanwhile GC transitively walks the heap marking reachable objects. +// 7. When GC finishes marking heap, it preempts P's one-by-one and +// retakes partial wbufs (filled by write barrier or during a stack scan of the goroutine +// currently scheduled on the P). +// 8. Once the GC has exhausted all available marking work it sets phase = marktermination. +// 9. Wait for all P's to acknowledge phase change. +// 10. Malloc now allocates black objects, so number of unmarked reachable objects +// monotonically decreases. +// 11. GC preempts P's one-by-one taking partial wbufs and marks all unmarked yet reachable objects. +// 12. When GC completes a full cycle over P's and discovers no new grey +// objects, (which means all reachable objects are marked) set phase = GCsweep. +// 13. Wait for all P's to acknowledge phase change. +// 14. Now malloc allocates white (but sweeps spans before use). +// Write barrier becomes nop. +// 15. GC does background sweeping, see description below. +// 16. When sweeping is complete set phase to GCoff. +// 17. When sufficient allocation has taken place replay the sequence starting at 0 above, +// see discussion of GC rate below. + +// Changing phases. +// Phases are changed by setting the gcphase to the next phase and possibly calling ackgcphase. +// All phase action must be benign in the presence of a change. +// Starting with GCoff +// GCoff to GCscan +// GSscan scans stacks and globals greying them and never marks an object black. +// Once all the P's are aware of the new phase they will scan gs on preemption. +// This means that the scanning of preempted gs can't start until all the Ps +// have acknowledged. +// GCscan to GCmark +// GCMark turns on the write barrier which also only greys objects. No scanning +// of objects (making them black) can happen until all the Ps have acknowledged +// the phase change. +// GCmark to GCmarktermination +// The only change here is that we start allocating black so the Ps must acknowledge +// the change before we begin the termination algorithm +// GCmarktermination to GSsweep +// Object currently on the freelist must be marked black for this to work. +// Are things on the free lists black or white? How does the sweep phase work? + // Concurrent sweep. // The sweep phase proceeds concurrently with normal program execution. // The heap is swept span-by-span both lazily (when a goroutine needs another span) @@ -50,6 +100,14 @@ // The finalizer goroutine is kicked off only when all spans are swept. // When the next GC starts, it sweeps all not-yet-swept spans (if any). +// GC rate. +// Next GC is after we've allocated an extra amount of memory proportional to +// the amount already in use. The proportion is controlled by GOGC environment variable +// (100 by default). If GOGC=100 and we're using 4M, we'll GC again when we get to 8M +// (this mark is tracked in next_gc variable). This keeps the GC cost in linear +// proportion to the allocation cost. Adjusting GOGC just changes the linear constant +// (and also the amount of extra memory used). + #include "runtime.h" #include "arch_GOARCH.h" #include "malloc.h" @@ -65,9 +123,8 @@ enum { Debug = 0, DebugPtrs = 0, // if 1, print trace of every pointer load during GC - ConcurrentSweep = 0, + ConcurrentSweep = 1, - WorkbufSize = 4*1024, FinBlockSize = 4*1024, RootData = 0, RootBss = 1, @@ -80,7 +137,7 @@ enum { // ptrmask for an allocation containing a single pointer. static byte oneptr[] = {BitsPointer}; -// Initialized from $GOGC. GOGC=off means no gc. +// Initialized from $GOGC. GOGC=off means no GC. extern int32 runtime·gcpercent; // Holding worldsema grants an M the right to try to stop the world. @@ -98,12 +155,16 @@ extern int32 runtime·gcpercent; // uint32 runtime·worldsema = 1; -typedef struct Workbuf Workbuf; -struct Workbuf -{ - LFNode node; // must be first - uintptr nobj; - byte* obj[(WorkbufSize-sizeof(LFNode)-sizeof(uintptr))/PtrSize]; +// It is a bug if bits does not have bitBoundary set but +// there are still some cases where this happens related +// to stack spans. +typedef struct Markbits Markbits; +struct Markbits { + byte *bitp; // pointer to the byte holding xbits + byte shift; // bits xbits needs to be shifted to get bits + byte xbits; // byte holding all the bits from *bitp + byte bits; // mark and boundary bits relevant to corresponding slot. + byte tbits; // pointer||scalar bits relevant to corresponding slot. }; extern byte runtime·data[]; @@ -128,26 +189,40 @@ BitVector runtime·gcbssmask; Mutex runtime·gclock; -static uintptr badblock[1024]; -static int32 nbadblock; - +static Workbuf* getpartialorempty(void); +static void putpartial(Workbuf*); static Workbuf* getempty(Workbuf*); static Workbuf* getfull(Workbuf*); static void putempty(Workbuf*); +static void putfull(Workbuf*); static Workbuf* handoff(Workbuf*); static void gchelperstart(void); static void flushallmcaches(void); -static bool scanframe(Stkframe *frame, void *unused); -static void scanstack(G *gp); -static BitVector unrollglobgcprog(byte *prog, uintptr size); +static bool scanframe(Stkframe*, void*); +static void scanstack(G*); +static BitVector unrollglobgcprog(byte*, uintptr); +static void scanblock(byte*, uintptr, byte*); +static byte* objectstart(byte*, Markbits*); +static Workbuf* greyobject(byte*, Markbits*, Workbuf*); +static bool inheap(byte*); +static bool shaded(byte*); +static void shade(byte*); +static void slottombits(byte*, Markbits*); +static void atomicxor8(byte*, byte); +static bool ischeckmarked(Markbits*); +static bool ismarked(Markbits*); +static void clearcheckmarkbits(void); +static void clearcheckmarkbitsspan(MSpan*); void runtime·bgsweep(void); +void runtime·finishsweep_m(void); static FuncVal bgsweepv = {runtime·bgsweep}; typedef struct WorkData WorkData; struct WorkData { - uint64 full; // lock-free list of full blocks - uint64 empty; // lock-free list of empty blocks + uint64 full; // lock-free list of full blocks + uint64 empty; // lock-free list of empty blocks + uint64 partial; // lock-free list of partially filled blocks byte pad0[CacheLineSize]; // prevents false-sharing between full/empty and nproc/nwait uint32 nproc; int64 tstart; @@ -162,315 +237,422 @@ struct WorkData { }; WorkData runtime·work; -// Is _cgo_allocate linked into the binary? +// To help debug the concurrent GC we remark with the world +// stopped ensuring that any object encountered has their normal +// mark bit set. To do this we use an orthogonal bit +// pattern to indicate the object is marked. The following pattern +// uses the upper two bits in the object's bounday nibble. +// 01: scalar not marked +// 10: pointer not marked +// 11: pointer marked +// 00: scalar marked +// Xoring with 01 will flip the pattern from marked to unmarked and vica versa. +// The higher bit is 1 for pointers and 0 for scalars, whether the object +// is marked or not. +// The first nibble no longer holds the bitsDead pattern indicating that the +// there are no more pointers in the object. This information is held +// in the second nibble. + +// When marking an object if the bool checkmark is true one uses the above +// encoding, otherwise one uses the bitMarked bit in the lower two bits +// of the nibble. +static bool checkmark = false; +static bool gccheckmarkenable = true; + +// Is address b in the known heap. If it doesn't have a valid gcmap +// returns false. For example pointers into stacks will return false. static bool -have_cgo_allocate(void) +inheap(byte *b) { - extern byte go·weak·runtime·_cgo_allocate_internal[1]; - return go·weak·runtime·_cgo_allocate_internal != nil; + MSpan *s; + pageID k; + uintptr x; + + if(b == nil || b < runtime·mheap.arena_start || b >= runtime·mheap.arena_used) + return false; + // Not a beginning of a block, consult span table to find the block beginning. + k = (uintptr)b>>PageShift; + x = k; + x -= (uintptr)runtime·mheap.arena_start>>PageShift; + s = runtime·mheap.spans[x]; + if(s == nil || k < s->start || b >= s->limit || s->state != MSpanInUse) + return false; + return true; } -// scanblock scans a block of n bytes starting at pointer b for references -// to other objects, scanning any it finds recursively until there are no -// unscanned objects left. Instead of using an explicit recursion, it keeps -// a work list in the Workbuf* structures and loops in the main function -// body. Keeping an explicit work list is easier on the stack allocator and -// more efficient. +// Given an address in the heap return the relevant byte from the gcmap. This routine +// can be used on addresses to the start of an object or to the interior of the an object. static void -scanblock(byte *b, uintptr n, byte *ptrmask) +slottombits(byte *obj, Markbits *mbits) { - byte *obj, *obj0, *p, *arena_start, *arena_used, **wp, *scanbuf[8], *ptrbitp, *bitp; - uintptr i, j, nobj, size, idx, x, off, scanbufpos, bits, xbits, shift; - Workbuf *wbuf; - Iface *iface; - Eface *eface; - Type *typ; + uintptr off; + + off = (uintptr*)((uintptr)obj&~(PtrSize-1)) - (uintptr*)runtime·mheap.arena_start; + mbits->bitp = runtime·mheap.arena_start - off/wordsPerBitmapByte - 1; + mbits->shift = (off % wordsPerBitmapByte) * gcBits; + mbits->xbits = *mbits->bitp; + mbits->bits = (mbits->xbits >> mbits->shift) & bitMask; + mbits->tbits = ((mbits->xbits >> mbits->shift) & bitPtrMask) >> 2; +} + +// b is a pointer into the heap. +// Find the start of the object refered to by b. +// Set mbits to the associated bits from the bit map. +// If b is not a valid heap object return nil and +// undefined values in mbits. +static byte* +objectstart(byte *b, Markbits *mbits) +{ + byte *obj, *p; MSpan *s; pageID k; - bool keepworking; + uintptr x, size, idx; - // Cache memory arena parameters in local vars. - arena_start = runtime·mheap.arena_start; - arena_used = runtime·mheap.arena_used; + obj = (byte*)((uintptr)b&~(PtrSize-1)); + for(;;) { + slottombits(obj, mbits); + if((mbits->bits&bitBoundary) == bitBoundary) + break; - wbuf = getempty(nil); - nobj = wbuf->nobj; - wp = &wbuf->obj[nobj]; - keepworking = b == nil; - scanbufpos = 0; - for(i = 0; i < nelem(scanbuf); i++) - scanbuf[i] = nil; + // Not a beginning of a block, consult span table to find the block beginning. + k = (uintptr)obj>>PageShift; + x = k; + x -= (uintptr)runtime·mheap.arena_start>>PageShift; + s = runtime·mheap.spans[x]; + if(s == nil || k < s->start || obj >= s->limit || s->state != MSpanInUse){ + if(s != nil && s->state == MSpanStack) { + return nil; // This is legit. + } + // The following ensures that we are rigorous about what data + // structures hold valid pointers + if(0) { + // Still happens sometimes. We don't know why. + runtime·printf("runtime:objectstart Span weird: obj=%p, k=%p", obj, k); + if (s == nil) + runtime·printf(" s=nil\n"); + else + runtime·printf(" s->start=%p s->limit=%p, s->state=%d\n", s->start*PageSize, s->limit, s->state); + runtime·throw("objectstart: bad pointer in unexpected span"); + } + return nil; + } + p = (byte*)((uintptr)s->start<<PageShift); + if(s->sizeclass != 0) { + size = s->elemsize; + idx = ((byte*)obj - p)/size; + p = p+idx*size; + } + if(p == obj) { + runtime·printf("runtime: failed to find block beginning for %p s=%p s->limit=%p\n", + p, s->start*PageSize, s->limit); + runtime·throw("failed to find block beginning"); + } + obj = p; + } + // if size(obj.firstfield) < PtrSize, the &obj.secondfield could map to the boundary bit + // Clear any low bits to get to the start of the object. + // greyobject depends on this. + return obj; +} + +// Slow for now as we serialize this, since this is on a debug path +// speed is not critical at this point. +static Mutex andlock; +static void +atomicand8(byte *src, byte val) +{ + runtime·lock(&andlock); + *src = *src&val; + runtime·unlock(&andlock); +} + +// Mark using the checkmark scheme. +void +docheckmark(Markbits *mbits) +{ + // xor 01 moves 01(scalar unmarked) to 00(scalar marked) + // and 10(pointer unmarked) to 11(pointer marked) + if(mbits->tbits == BitsScalar) + atomicand8(mbits->bitp, ~(byte)(BitsCheckMarkXor<<mbits->shift<<2)); + else if(mbits->tbits == BitsPointer) + runtime·atomicor8(mbits->bitp, BitsCheckMarkXor<<mbits->shift<<2); + + // reload bits for ischeckmarked + mbits->xbits = *mbits->bitp; + mbits->bits = (mbits->xbits >> mbits->shift) & bitMask; + mbits->tbits = ((mbits->xbits >> mbits->shift) & bitPtrMask) >> 2; + + return; +} + +// In the default scheme does mbits refer to a marked object. +static bool +ismarked(Markbits *mbits) +{ + if((mbits->bits&bitBoundary) != bitBoundary) + runtime·throw("ismarked: bits should have boundary bit set"); + return (mbits->bits&bitMarked) == bitMarked; +} + +// In the checkmark scheme does mbits refer to a marked object. +static bool +ischeckmarked(Markbits *mbits) +{ + if((mbits->bits&bitBoundary) != bitBoundary) + runtime·printf("runtime:ischeckmarked: bits should have boundary bit set\n"); + return mbits->tbits==BitsScalarMarked || mbits->tbits==BitsPointerMarked; +} + +// When in GCmarkterminate phase we allocate black. +void +runtime·gcmarknewobject_m(void) +{ + Markbits mbits; + byte *obj; + + if(runtime·gcphase != GCmarktermination) + runtime·throw("marking new object while not in mark termination phase"); + if(checkmark) // The world should be stopped so this should not happen. + runtime·throw("gcmarknewobject called while doing checkmark"); + + obj = g->m->ptrarg[0]; + slottombits((byte*)((uintptr)obj & (PtrSize-1)), &mbits); + + if((mbits.bits&bitMarked) != 0) + return; + + // Each byte of GC bitmap holds info for two words. + // If the current object is larger than two words, or if the object is one word + // but the object it shares the byte with is already marked, + // then all the possible concurrent updates are trying to set the same bit, + // so we can use a non-atomic update. + if((mbits.xbits&(bitMask|(bitMask<<gcBits))) != (bitBoundary|(bitBoundary<<gcBits)) || runtime·work.nproc == 1) + *mbits.bitp = mbits.xbits | (bitMarked<<mbits.shift); + else + runtime·atomicor8(mbits.bitp, bitMarked<<mbits.shift); + return; +} + +// obj is the start of an object with mark mbits. +// If it isn't already marked, mark it and enqueue into workbuf. +// Return possibly new workbuf to use. +static Workbuf* +greyobject(byte *obj, Markbits *mbits, Workbuf *wbuf) +{ + // obj should be start of allocation, and so must be at least pointer-aligned. + if(((uintptr)obj & (PtrSize-1)) != 0) + runtime·throw("greyobject: obj not pointer-aligned"); + + if(checkmark) { + if(!ismarked(mbits)) { + MSpan *s; + pageID k; + uintptr x, i; + + runtime·printf("runtime:greyobject: checkmarks finds unexpected unmarked object obj=%p, mbits->bits=%x, *mbits->bitp=%x\n", obj, mbits->bits, *mbits->bitp); + + k = (uintptr)obj>>PageShift; + x = k; + x -= (uintptr)runtime·mheap.arena_start>>PageShift; + s = runtime·mheap.spans[x]; + runtime·printf("runtime:greyobject Span: obj=%p, k=%p", obj, k); + if (s == nil) { + runtime·printf(" s=nil\n"); + } else { + runtime·printf(" s->start=%p s->limit=%p, s->state=%d, s->sizeclass=%d, s->elemsize=%D \n", s->start*PageSize, s->limit, s->state, s->sizeclass, s->elemsize); + for(i=0; i<s->sizeclass; i++) { + runtime·printf(" ((uintptr*)obj)[%D]=%p\n", i, ((uintptr*)obj)[i]); + } + } + runtime·throw("checkmark found unmarked object"); + } + if(ischeckmarked(mbits)) + return wbuf; + docheckmark(mbits); + if(!ischeckmarked(mbits)) { + runtime·printf("mbits xbits=%x bits=%x tbits=%x shift=%d\n", mbits->xbits, mbits->bits, mbits->tbits, mbits->shift); + runtime·throw("docheckmark and ischeckmarked disagree"); + } + } else { + // If marked we have nothing to do. + if((mbits->bits&bitMarked) != 0) + return wbuf; + + // Each byte of GC bitmap holds info for two words. + // If the current object is larger than two words, or if the object is one word + // but the object it shares the byte with is already marked, + // then all the possible concurrent updates are trying to set the same bit, + // so we can use a non-atomic update. + if((mbits->xbits&(bitMask|(bitMask<<gcBits))) != (bitBoundary|(bitBoundary<<gcBits)) || runtime·work.nproc == 1) + *mbits->bitp = mbits->xbits | (bitMarked<<mbits->shift); + else + runtime·atomicor8(mbits->bitp, bitMarked<<mbits->shift); + } + + if (!checkmark && (((mbits->xbits>>(mbits->shift+2))&BitsMask) == BitsDead)) + return wbuf; // noscan object + + // Queue the obj for scanning. The PREFETCH(obj) logic has been removed but + // seems like a nice optimization that can be added back in. + // There needs to be time between the PREFETCH and the use. + // Previously we put the obj in an 8 element buffer that is drained at a rate + // to give the PREFETCH time to do its work. + // Use of PREFETCHNTA might be more appropriate than PREFETCH + + // If workbuf is full, obtain an empty one. + if(wbuf->nobj >= nelem(wbuf->obj)) { + wbuf = getempty(wbuf); + } + + wbuf->obj[wbuf->nobj] = obj; + wbuf->nobj++; + return wbuf; +} + +// Scan the object b of size n, adding pointers to wbuf. +// Return possibly new wbuf to use. +// If ptrmask != nil, it specifies where pointers are in b. +// If ptrmask == nil, the GC bitmap should be consulted. +// In this case, n may be an overestimate of the size; the GC bitmap +// must also be used to make sure the scan stops at the end of b. +static Workbuf* +scanobject(byte *b, uintptr n, byte *ptrmask, Workbuf *wbuf) +{ + byte *obj, *arena_start, *arena_used, *ptrbitp; + uintptr i, j; + int32 bits; + Markbits mbits; + + arena_start = (byte*)runtime·mheap.arena_start; + arena_used = runtime·mheap.arena_used; ptrbitp = nil; + // Find bits of the beginning of the object. + if(ptrmask == nil) { + b = objectstart(b, &mbits); + if(b == nil) + return wbuf; + ptrbitp = mbits.bitp; //arena_start - off/wordsPerBitmapByte - 1; + } + for(i = 0; i < n; i += PtrSize) { + // Find bits for this word. + if(ptrmask != nil) { + // dense mask (stack or data) + bits = (ptrmask[(i/PtrSize)/4]>>(((i/PtrSize)%4)*BitsPerPointer))&BitsMask; + } else { + // Check if we have reached end of span. + // n is an overestimate of the size of the object. + if((((uintptr)b+i)%PageSize) == 0 && + runtime·mheap.spans[(b-arena_start)>>PageShift] != runtime·mheap.spans[(b+i-arena_start)>>PageShift]) + break; + // Consult GC bitmap. + bits = *ptrbitp; + if(wordsPerBitmapByte != 2) + runtime·throw("alg doesn't work for wordsPerBitmapByte != 2"); + j = ((uintptr)b+i)/PtrSize & 1; // j indicates upper nibble or lower nibble + bits >>= gcBits*j; + if(i == 0) + bits &= ~bitBoundary; + ptrbitp -= j; + + if((bits&bitBoundary) != 0 && i != 0) + break; // reached beginning of the next object + bits = (bits&bitPtrMask)>>2; // bits refer to the type bits. + + if(i != 0 && bits == BitsDead) // BitsDead in first nibble not valid during checkmark + break; // reached no-scan part of the object + } + + if(bits <= BitsScalar) // Bits Scalar || + // BitsDead || // default encoding + // BitsScalarMarked // checkmark encoding + continue; + + if((bits&BitsPointer) != BitsPointer) { + runtime·printf("gc checkmark=%d, b=%p ptrmask=%p, mbits.bitp=%p, mbits.xbits=%x, bits=%x\n", checkmark, b, ptrmask, mbits.bitp, mbits.xbits, bits); + runtime·throw("unexpected garbage collection bits"); + } + + obj = *(byte**)(b+i); + // At this point we have extracted the next potential pointer. + // Check if it points into heap. + if(obj == nil || obj < arena_start || obj >= arena_used) + continue; + // Mark the object. return some important bits. + // We we combine the following two rotines we don't have to pass mbits or obj around. + obj = objectstart(obj, &mbits); + // In the case of the span being MSpan_Stack mbits is useless and will not have + // the boundary bit set. It does not need to be greyed since it will be + // scanned using the scan stack mechanism. + if(obj == nil) + continue; + wbuf = greyobject(obj, &mbits, wbuf); + } + return wbuf; +} + +// scanblock starts by scanning b as scanobject would. +// If the gcphase is GCscan, that's all scanblock does. +// Otherwise it traverses some fraction of the pointers it found in b, recursively. +// As a special case, scanblock(nil, 0, nil) means to scan previously queued work, +// stopping only when no work is left in the system. +static void +scanblock(byte *b, uintptr n, byte *ptrmask) +{ + Workbuf *wbuf; + bool keepworking; + + wbuf = getpartialorempty(); + if(b != nil) { + wbuf = scanobject(b, n, ptrmask, wbuf); + if(runtime·gcphase == GCscan) { + if(inheap(b) && !ptrmask) + // b is in heap, we are in GCscan so there should be a ptrmask. + runtime·throw("scanblock: In GCscan phase and inheap is true."); + // GCscan only goes one level deep since mark wb not turned on. + putpartial(wbuf); + return; + } + } + if(runtime·gcphase == GCscan) { + runtime·throw("scanblock: In GCscan phase but no b passed in."); + } + + keepworking = b == nil; + // ptrmask can have 2 possible values: // 1. nil - obtain pointer mask from GC bitmap. // 2. pointer to a compact mask (for stacks and data). - if(b != nil) - goto scanobj; for(;;) { - if(nobj == 0) { - // Out of work in workbuf. - // First, see is there is any work in scanbuf. - for(i = 0; i < nelem(scanbuf); i++) { - b = scanbuf[scanbufpos]; - scanbuf[scanbufpos++] = nil; - scanbufpos %= nelem(scanbuf); - if(b != nil) { - n = arena_used - b; // scan until bitBoundary or BitsDead - ptrmask = nil; // use GC bitmap for pointer info - goto scanobj; - } - } + if(wbuf->nobj == 0) { if(!keepworking) { putempty(wbuf); return; } // Refill workbuf from global queue. wbuf = getfull(wbuf); - if(wbuf == nil) + if(wbuf == nil) // nil means out of work barrier reached return; - nobj = wbuf->nobj; - wp = &wbuf->obj[nobj]; + + if(wbuf->nobj<=0) { + runtime·throw("runtime:scanblock getfull returns empty buffer"); + } + } // If another proc wants a pointer, give it some. - if(runtime·work.nwait > 0 && nobj > 4 && runtime·work.full == 0) { - wbuf->nobj = nobj; + if(runtime·work.nwait > 0 && wbuf->nobj > 4 && runtime·work.full == 0) { wbuf = handoff(wbuf); - nobj = wbuf->nobj; - wp = &wbuf->obj[nobj]; - } - - wp--; - nobj--; - b = *wp; - n = arena_used - b; // scan until next bitBoundary or BitsDead - ptrmask = nil; // use GC bitmap for pointer info - - scanobj: - if(DebugPtrs) - runtime·printf("scanblock %p +%p %p\n", b, n, ptrmask); - // Find bits of the beginning of the object. - if(ptrmask == nil) { - off = (uintptr*)b - (uintptr*)arena_start; - ptrbitp = arena_start - off/wordsPerBitmapByte - 1; } - for(i = 0; i < n; i += PtrSize) { - obj = nil; - // Find bits for this word. - if(ptrmask == nil) { - // Check is we have reached end of span. - if((((uintptr)b+i)%PageSize) == 0 && - runtime·mheap.spans[(b-arena_start)>>PageShift] != runtime·mheap.spans[(b+i-arena_start)>>PageShift]) - break; - // Consult GC bitmap. - bits = *ptrbitp; - - if(wordsPerBitmapByte != 2) - runtime·throw("alg doesn't work for wordsPerBitmapByte != 2"); - j = ((uintptr)b+i)/PtrSize & 1; - ptrbitp -= j; - bits >>= gcBits*j; - - if((bits&bitBoundary) != 0 && i != 0) - break; // reached beginning of the next object - bits = (bits>>2)&BitsMask; - if(bits == BitsDead) - break; // reached no-scan part of the object - } else // dense mask (stack or data) - bits = (ptrmask[(i/PtrSize)/4]>>(((i/PtrSize)%4)*BitsPerPointer))&BitsMask; - - if(bits <= BitsScalar) // BitsScalar || BitsDead - continue; - if(bits == BitsPointer) { - obj = *(byte**)(b+i); - obj0 = obj; - goto markobj; - } - - // With those three out of the way, must be multi-word. - if(Debug && bits != BitsMultiWord) - runtime·throw("unexpected garbage collection bits"); - // Find the next pair of bits. - if(ptrmask == nil) { - bits = *ptrbitp; - j = ((uintptr)b+i+PtrSize)/PtrSize & 1; - ptrbitp -= j; - bits >>= gcBits*j; - bits = (bits>>2)&BitsMask; - } else - bits = (ptrmask[((i+PtrSize)/PtrSize)/4]>>((((i+PtrSize)/PtrSize)%4)*BitsPerPointer))&BitsMask; - - if(Debug && bits != BitsIface && bits != BitsEface) - runtime·throw("unexpected garbage collection bits"); - - if(bits == BitsIface) { - iface = (Iface*)(b+i); - if(iface->tab != nil) { - typ = iface->tab->type; - if(!(typ->kind&KindDirectIface) || !(typ->kind&KindNoPointers)) - obj = iface->data; - } - } else { - eface = (Eface*)(b+i); - typ = eface->type; - if(typ != nil) { - if(!(typ->kind&KindDirectIface) || !(typ->kind&KindNoPointers)) - obj = eface->data; - } - } - i += PtrSize; - - obj0 = obj; - markobj: - // At this point we have extracted the next potential pointer. - // Check if it points into heap. - if(obj == nil) - continue; - if(obj < arena_start || obj >= arena_used) { - if((uintptr)obj < PhysPageSize && runtime·invalidptr) { - s = nil; - goto badobj; - } - continue; - } - // Mark the object. - obj = (byte*)((uintptr)obj & ~(PtrSize-1)); - off = (uintptr*)obj - (uintptr*)arena_start; - bitp = arena_start - off/wordsPerBitmapByte - 1; - shift = (off % wordsPerBitmapByte) * gcBits; - xbits = *bitp; - bits = (xbits >> shift) & bitMask; - if((bits&bitBoundary) == 0) { - // Not a beginning of a block, consult span table to find the block beginning. - k = (uintptr)obj>>PageShift; - x = k; - x -= (uintptr)arena_start>>PageShift; - s = runtime·mheap.spans[x]; - if(s == nil || k < s->start || obj >= s->limit || s->state != MSpanInUse) { - // Stack pointers lie within the arena bounds but are not part of the GC heap. - // Ignore them. - if(s != nil && s->state == MSpanStack) - continue; - - badobj: - // If cgo_allocate is linked into the binary, it can allocate - // memory as []unsafe.Pointer that may not contain actual - // pointers and must be scanned conservatively. - // In this case alone, allow the bad pointer. - if(have_cgo_allocate() && ptrmask == nil) - continue; - - // Anything else indicates a bug somewhere. - // If we're in the middle of chasing down a different bad pointer, - // don't confuse the trace by printing about this one. - if(nbadblock > 0) - continue; - - runtime·printf("runtime: garbage collector found invalid heap pointer *(%p+%p)=%p", b, i, obj); - if(s == nil) - runtime·printf(" s=nil\n"); - else - runtime·printf(" span=%p-%p-%p state=%d\n", (uintptr)s->start<<PageShift, s->limit, (uintptr)(s->start+s->npages)<<PageShift, s->state); - if(ptrmask != nil) - runtime·throw("invalid heap pointer"); - // Add to badblock list, which will cause the garbage collection - // to keep repeating until it has traced the chain of pointers - // leading to obj all the way back to a root. - if(nbadblock == 0) - badblock[nbadblock++] = (uintptr)b; - continue; - } - p = (byte*)((uintptr)s->start<<PageShift); - if(s->sizeclass != 0) { - size = s->elemsize; - idx = ((byte*)obj - p)/size; - p = p+idx*size; - } - if(p == obj) { - runtime·printf("runtime: failed to find block beginning for %p s=%p s->limit=%p\n", - p, s->start*PageSize, s->limit); - runtime·throw("failed to find block beginning"); - } - obj = p; - goto markobj; - } - if(DebugPtrs) - runtime·printf("scan *%p = %p => base %p\n", b+i, obj0, obj); - - if(nbadblock > 0 && (uintptr)obj == badblock[nbadblock-1]) { - // Running garbage collection again because - // we want to find the path from a root to a bad pointer. - // Found possible next step; extend or finish path. - for(j=0; j<nbadblock; j++) - if(badblock[j] == (uintptr)b) - goto AlreadyBad; - runtime·printf("runtime: found *(%p+%p) = %p+%p\n", b, i, obj0, (uintptr)(obj-obj0)); - if(ptrmask != nil) - runtime·throw("bad pointer"); - if(nbadblock >= nelem(badblock)) - runtime·throw("badblock trace too long"); - badblock[nbadblock++] = (uintptr)b; - AlreadyBad:; - } - - // Now we have bits, bitp, and shift correct for - // obj pointing at the base of the object. - // Only care about not marked objects. - if((bits&bitMarked) != 0) - continue; - // If obj size is greater than 8, then each byte of GC bitmap - // contains info for at most one object. In such case we use - // non-atomic byte store to mark the object. This can lead - // to double enqueue of the object for scanning, but scanning - // is an idempotent operation, so it is OK. This cannot lead - // to bitmap corruption because the single marked bit is the - // only thing that can change in the byte. - // For 8-byte objects we use non-atomic store, if the other - // quadruple is already marked. Otherwise we resort to CAS - // loop for marking. - if((xbits&(bitMask|(bitMask<<gcBits))) != (bitBoundary|(bitBoundary<<gcBits)) || - runtime·work.nproc == 1) - *bitp = xbits | (bitMarked<<shift); - else - runtime·atomicor8(bitp, bitMarked<<shift); - - if(((xbits>>(shift+2))&BitsMask) == BitsDead) - continue; // noscan object - - // Queue the obj for scanning. - PREFETCH(obj); - p = scanbuf[scanbufpos]; - scanbuf[scanbufpos++] = obj; - scanbufpos %= nelem(scanbuf); - if(p == nil) - continue; - - // If workbuf is full, obtain an empty one. - if(nobj >= nelem(wbuf->obj)) { - wbuf->nobj = nobj; - wbuf = getempty(wbuf); - nobj = wbuf->nobj; - wp = &wbuf->obj[nobj]; - } - *wp = p; - wp++; - nobj++; - } - if(DebugPtrs) - runtime·printf("end scanblock %p +%p %p\n", b, n, ptrmask); - - if(Debug && ptrmask == nil) { - // For heap objects ensure that we did not overscan. - n = 0; - p = nil; - if(!runtime·mlookup(b, &p, &n, nil) || b != p || i > n) { - runtime·printf("runtime: scanned (%p,%p), heap object (%p,%p)\n", b, i, p, n); - runtime·throw("scanblock: scanned invalid object"); - } - } + // This might be a good place to add prefetch code... + // if(wbuf->nobj > 4) { + // PREFETCH(wbuf->obj[wbuf->nobj - 3]; + // } + --wbuf->nobj; + b = wbuf->obj[wbuf->nobj]; + wbuf = scanobject(b, runtime·mheap.arena_used - b, nil, wbuf); } } @@ -484,7 +666,7 @@ markroot(ParFor *desc, uint32 i) void *p; uint32 status; bool restart; - + USED(&desc); // Note: if you add a case here, please also update heapdump.c:dumproots. switch(i) { @@ -511,7 +693,8 @@ markroot(ParFor *desc, uint32 i) s = runtime·work.spans[spanidx]; if(s->state != MSpanInUse) continue; - if(s->sweepgen != sg) { + if(!checkmark && s->sweepgen != sg) { + // sweepgen was updated (+2) during non-checkmark GC pass runtime·printf("sweep %d %d\n", s->sweepgen, sg); runtime·throw("gc: unswept span"); } @@ -523,14 +706,16 @@ markroot(ParFor *desc, uint32 i) spf = (SpecialFinalizer*)sp; // A finalizer can be set for an inner byte of an object, find object beginning. p = (void*)((s->start << PageShift) + spf->special.offset/s->elemsize*s->elemsize); - scanblock(p, s->elemsize, nil); + if(runtime·gcphase != GCscan) + scanblock(p, s->elemsize, nil); // Scanned during mark phase scanblock((void*)&spf->fn, PtrSize, oneptr); } } break; case RootFlushCaches: - flushallmcaches(); + if (runtime·gcphase != GCscan) // Do not flush mcaches during GCscan phase. + flushallmcaches(); break; default: @@ -540,17 +725,37 @@ markroot(ParFor *desc, uint32 i) gp = runtime·allg[i - RootCount]; // remember when we've first observed the G blocked // needed only to output in traceback - status = runtime·readgstatus(gp); + status = runtime·readgstatus(gp); // We are not in a scan state if((status == Gwaiting || status == Gsyscall) && gp->waitsince == 0) gp->waitsince = runtime·work.tstart; - // Shrink a stack if not much of it is being used. - runtime·shrinkstack(gp); - if(runtime·readgstatus(gp) == Gdead) + // Shrink a stack if not much of it is being used but not in the scan phase. + if (runtime·gcphase != GCscan) // Do not shrink during GCscan phase. + runtime·shrinkstack(gp); + if(runtime·readgstatus(gp) == Gdead) gp->gcworkdone = true; else gp->gcworkdone = false; restart = runtime·stopg(gp); - scanstack(gp); + + // goroutine will scan its own stack when it stops running. + // Wait until it has. + while(runtime·readgstatus(gp) == Grunning && !gp->gcworkdone) { + } + + // scanstack(gp) is done as part of gcphasework + // But to make sure we finished we need to make sure that + // the stack traps have all responded so drop into + // this while loop until they respond. + while(!gp->gcworkdone){ + status = runtime·readgstatus(gp); + if(status == Gdead) { + gp->gcworkdone = true; // scan is a noop + break; + //do nothing, scan not needed. + } + if(status == Gwaiting || status == Grunnable) + restart = runtime·stopg(gp); + } if(restart) runtime·restartg(gp); break; @@ -562,53 +767,95 @@ markroot(ParFor *desc, uint32 i) static Workbuf* getempty(Workbuf *b) { - MCache *c; - - if(b != nil) - runtime·lfstackpush(&runtime·work.full, &b->node); - b = nil; - c = g->m->mcache; - if(c->gcworkbuf != nil) { - b = c->gcworkbuf; - c->gcworkbuf = nil; + if(b != nil) { + putfull(b); + b = nil; } - if(b == nil) + if(runtime·work.empty) b = (Workbuf*)runtime·lfstackpop(&runtime·work.empty); - if(b == nil) + + if(b && b->nobj != 0) { + runtime·printf("m%d: getempty: popped b=%p with non-zero b->nobj=%d\n", g->m->id, b, (uint32)b->nobj); + runtime·throw("getempty: workbuffer not empty, b->nobj not 0"); + } + if(b == nil) { b = runtime·persistentalloc(sizeof(*b), CacheLineSize, &mstats.gc_sys); - b->nobj = 0; + b->nobj = 0; + } return b; } static void putempty(Workbuf *b) { - MCache *c; - - c = g->m->mcache; - if(c->gcworkbuf == nil) { - c->gcworkbuf = b; - return; + if(b->nobj != 0) { + runtime·throw("putempty: b->nobj not 0\n"); } runtime·lfstackpush(&runtime·work.empty, &b->node); } -void -runtime·gcworkbuffree(void *b) +// Put a full or partially full workbuf on the full list. +static void +putfull(Workbuf *b) { - if(b != nil) - putempty(b); + if(b->nobj <= 0) { + runtime·throw("putfull: b->nobj <= 0\n"); + } + runtime·lfstackpush(&runtime·work.full, &b->node); } -// Get a full work buffer off the work.full list, or return nil. +// Get an partially empty work buffer +// if none are available get an empty one. +static Workbuf* +getpartialorempty(void) +{ + Workbuf *b; + + b = (Workbuf*)runtime·lfstackpop(&runtime·work.partial); + if(b == nil) + b = getempty(nil); + return b; +} + +static void +putpartial(Workbuf *b) +{ + + if(b->nobj == 0) + runtime·lfstackpush(&runtime·work.empty, &b->node); + else if (b->nobj < nelem(b->obj)) + runtime·lfstackpush(&runtime·work.partial, &b->node); + else if (b->nobj == nelem(b->obj)) + runtime·lfstackpush(&runtime·work.full, &b->node); + else { + runtime·printf("b=%p, b->nobj=%d, nelem(b->obj)=%d\n", b, (uint32)b->nobj, (uint32)nelem(b->obj)); + runtime·throw("putpartial: bad Workbuf b->nobj"); + } +} + +// Get a full work buffer off the work.full or a partially +// filled one off the work.partial list. If nothing is available +// wait until all the other gc helpers have finished and then +// return nil. +// getfull acts as a barrier for work.nproc helpers. As long as one +// gchelper is actively marking objects it +// may create a workbuffer that the other helpers can work on. +// The for loop either exits when a work buffer is found +// or when _all_ of the work.nproc GC helpers are in the loop +// looking for work and thus not capable of creating new work. +// This is in fact the termination condition for the STW mark +// phase. static Workbuf* getfull(Workbuf *b) { int32 i; if(b != nil) - runtime·lfstackpush(&runtime·work.empty, &b->node); + putempty(b); + b = (Workbuf*)runtime·lfstackpop(&runtime·work.full); + if(b==nil) + b = (Workbuf*)runtime·lfstackpop(&runtime·work.partial); if(b != nil || runtime·work.nproc == 1) return b; @@ -617,7 +864,9 @@ getfull(Workbuf *b) if(runtime·work.full != 0) { runtime·xadd(&runtime·work.nwait, -1); b = (Workbuf*)runtime·lfstackpop(&runtime·work.full); - if(b != nil) + if(b==nil) + b = (Workbuf*)runtime·lfstackpop(&runtime·work.partial); + if(b != nil) return b; runtime·xadd(&runtime·work.nwait, +1); } @@ -737,7 +986,7 @@ scanframe(Stkframe *frame, void *unused) } bv = runtime·stackmapdata(stackmap, pcdata); } - scanblock((byte*)frame->argp, bv.n/BitsPerPointer*PtrSize, bv.bytedata); + scanblock((byte*)frame->argp, bv.n/BitsPerPointer*PtrSize, bv.bytedata); } return true; } @@ -760,8 +1009,7 @@ scanstack(G *gp) case Gdead: return; case Grunning: - runtime·printf("runtime: gp=%p, goid=%D, gp->atomicstatus=%d\n", gp, gp->goid, runtime·readgstatus(gp)); - runtime·throw("mark - world not stopped"); + runtime·throw("scanstack: - goroutine not stopped"); case Grunnable: case Gsyscall: case Gwaiting: @@ -778,8 +1026,117 @@ scanstack(G *gp) runtime·tracebackdefers(gp, &fn, nil); } -// The gp has been moved to a gc safepoint. If there is gcphase specific -// work it is done here. +// If the slot is grey or black return true, if white return false. +// If the slot is not in the known heap and thus does not have a valid GC bitmap then +// it is considered grey. Globals and stacks can hold such slots. +// The slot is grey if its mark bit is set and it is enqueued to be scanned. +// The slot is black if it has already been scanned. +// It is white if it has a valid mark bit and the bit is not set. +static bool +shaded(byte *slot) +{ + Markbits mbits; + byte *valid; + + if(!inheap(slot)) // non-heap slots considered grey + return true; + + valid = objectstart(slot, &mbits); + if(valid == nil) + return true; + + if(checkmark) + return ischeckmarked(&mbits); + + return (mbits.bits&bitMarked) != 0; +} + +// Shade the object if it isn't already. +// The object is not nil and known to be in the heap. +static void +shade(byte *b) +{ + byte *obj; + Workbuf *wbuf; + Markbits mbits; + + if(!inheap(b)) + runtime·throw("shade: passed an address not in the heap"); + + wbuf = getpartialorempty(); + // Mark the object, return some important bits. + // If we combine the following two rotines we don't have to pass mbits or obj around. + obj = objectstart(b, &mbits); + if(obj != nil) + wbuf = greyobject(obj, &mbits, wbuf); // augments the wbuf + + putpartial(wbuf); + return; +} + +// This is the Dijkstra barrier coarsened to always shade the ptr (dst) object. +// The original Dijkstra barrier only shaded ptrs being placed in black slots. +// +// Shade indicates that it has seen a white pointer by adding the referent +// to wbuf as well as marking it. +// +// slot is the destination (dst) in go code +// ptr is the value that goes into the slot (src) in the go code +// +// Dijkstra pointed out that maintaining the no black to white +// pointers means that white to white pointers not need +// to be noted by the write barrier. Furthermore if either +// white object dies before it is reached by the +// GC then the object can be collected during this GC cycle +// instead of waiting for the next cycle. Unfortunately the cost of +// ensure that the object holding the slot doesn't concurrently +// change to black without the mutator noticing seems prohibitive. +// +// Consider the following example where the mutator writes into +// a slot and then loads the slot's mark bit while the GC thread +// writes to the slot's mark bit and then as part of scanning reads +// the slot. +// +// Initially both [slot] and [slotmark] are 0 (nil) +// Mutator thread GC thread +// st [slot], ptr st [slotmark], 1 +// +// ld r1, [slotmark] ld r2, [slot] +// +// This is a classic example of independent reads of independent writes, +// aka IRIW. The question is if r1==r2==0 is allowed and for most HW the +// answer is yes without inserting a memory barriers between the st and the ld. +// These barriers are expensive so we have decided that we will +// always grey the ptr object regardless of the slot's color. +// +void +runtime·gcmarkwb_m() +{ + byte *ptr; + ptr = (byte*)g->m->scalararg[1]; + + switch(runtime·gcphase) { + default: + runtime·throw("gcphasework in bad gcphase"); + case GCoff: + case GCquiesce: + case GCstw: + case GCsweep: + case GCscan: + break; + case GCmark: + if(ptr != nil && inheap(ptr)) + shade(ptr); + break; + case GCmarktermination: + if(ptr != nil && inheap(ptr)) + shade(ptr); + break; + } +} + +// The gp has been moved to a GC safepoint. GC phase specific +// work is done here. void runtime·gcphasework(G *gp) { @@ -790,12 +1147,18 @@ runtime·gcphasework(G *gp) case GCquiesce: case GCstw: case GCsweep: - // No work for now. + // No work. + break; + case GCscan: + // scan the stack, mark the objects, put pointers in work buffers + // hanging off the P where this is being run. + scanstack(gp); break; case GCmark: - // Disabled until concurrent GC is implemented - // but indicate the scan has been done. - // scanstack(gp); + break; + case GCmarktermination: + scanstack(gp); + // All available mark work will be emptied before returning. break; } gp->gcworkdone = true; @@ -885,6 +1248,7 @@ runtime·iterate_finq(void (*callback)(FuncVal*, byte*, uintptr, Type*, PtrType* } } +// Returns only when span s has been swept. void runtime·MSpan_EnsureSwept(MSpan *s) { @@ -899,6 +1263,7 @@ runtime·MSpan_EnsureSwept(MSpan *s) sg = runtime·mheap.sweepgen; if(runtime·atomicload(&s->sweepgen) == sg) return; + // The caller must be sure that the span is a MSpanInUse span. if(runtime·cas(&s->sweepgen, sg-2, sg-1)) { runtime·MSpan_Sweep(s, false); return; @@ -926,6 +1291,9 @@ runtime·MSpan_Sweep(MSpan *s, bool preserve) Special *special, **specialp, *y; bool res, sweepgenset; + if(checkmark) + runtime·throw("MSpan_Sweep: checkmark only runs in STW and after the sweep."); + // It's critical that we enter this function with preemption disabled, // GC must not start while we are in the middle of this function. if(g->m->locks == 0 && g->m->mallocing == 0 && g != g->m->g0) @@ -1173,6 +1541,7 @@ runtime·gosweepdone(void) return runtime·mheap.sweepdone; } + void runtime·gchelper(void) { @@ -1181,13 +1550,11 @@ runtime·gchelper(void) g->m->traceback = 2; gchelperstart(); - // parallel mark for over gc roots + // parallel mark for over GC roots runtime·parfordo(runtime·work.markfor); - - // help other threads scan secondary blocks - scanblock(nil, 0, nil); - - nproc = runtime·work.nproc; // runtime·work.nproc can change right after we increment runtime·work.ndone + if(runtime·gcphase != GCscan) + scanblock(nil, 0, nil); // blocks in getfull + nproc = runtime·work.nproc; // work.nproc can change right after we increment work.ndone if(runtime·xadd(&runtime·work.ndone, +1) == nproc-1) runtime·notewakeup(&runtime·work.alldone); g->m->traceback = 0; @@ -1353,6 +1720,7 @@ runtime·gcinit(void) runtime·gcbssmask = unrollglobgcprog(runtime·gcbss, runtime·ebss - runtime·bss); } +// Called from malloc.go using onM, stopping and starting the world handled in caller. void runtime·gc_m(void) { @@ -1366,17 +1734,296 @@ runtime·gc_m(void) a.start_time = (uint64)(g->m->scalararg[0]) | ((uint64)(g->m->scalararg[1]) << 32); a.eagersweep = g->m->scalararg[2]; gc(&a); + runtime·casgstatus(gp, Gwaiting, Grunning); +} + +// Similar to clearcheckmarkbits but works on a single span. +// It preforms two tasks. +// 1. When used before the checkmark phase it converts BitsDead (00) to bitsScalar (01) +// for nibbles with the BoundaryBit set. +// 2. When used after the checkmark phase it converts BitsPointerMark (11) to BitsPointer 10 and +// BitsScalarMark (00) to BitsScalar (01), thus clearing the checkmark mark encoding. +// For the second case it is possible to restore the BitsDead pattern but since +// clearmark is a debug tool performance has a lower priority than simplicity. +// The span is MSpanInUse and the world is stopped. +static void +clearcheckmarkbitsspan(MSpan *s) +{ + int32 cl, n, npages, i; + uintptr size, off, step; + byte *p, *bitp, *arena_start, b; + + if(s->state != MSpanInUse) { + runtime·printf("runtime:clearcheckmarkbitsspan: state=%d\n", + s->state); + runtime·throw("clearcheckmarkbitsspan: bad span state"); + } + arena_start = runtime·mheap.arena_start; + cl = s->sizeclass; + size = s->elemsize; + if(cl == 0) { + n = 1; + } else { + // Chunk full of small blocks. + npages = runtime·class_to_allocnpages[cl]; + n = (npages << PageShift) / size; + } + + // MSpan_Sweep has similar code but instead of overloading and + // complicating that routine we do a simpler walk here. + // Sweep through n objects of given size starting at p. + // This thread owns the span now, so it can manipulate + // the block bitmap without atomic operations. + p = (byte*)(s->start << PageShift); + // Find bits for the beginning of the span. + off = (uintptr*)p - (uintptr*)arena_start; + bitp = arena_start - off/wordsPerBitmapByte - 1; + step = size/(PtrSize*wordsPerBitmapByte); + + // The type bit values are: + // 00 - BitsDead, for us BitsScalarMarked + // 01 - BitsScalar + // 10 - BitsPointer + // 11 - unused, for us BitsPointerMarked + // + // When called to prepare for the checkmark phase (checkmark==1), + // we change BitsDead to BitsScalar, so that there are no BitsScalarMarked + // type bits anywhere. + // + // The checkmark phase marks by changing BitsScalar to BitsScalarMarked + // and BitsPointer to BitsPointerMarked. + // + // When called to clean up after the checkmark phase (checkmark==0), + // we unmark by changing BitsScalarMarked back to BitsScalar and + // BitsPointerMarked back to BitsPointer. + // + // There are two problems with the scheme as just described. + // First, the setup rewrites BitsDead to BitsScalar, but the type bits + // following a BitsDead are uninitialized and must not be used. + // Second, objects that are free are expected to have their type + // bits zeroed (BitsDead), so in the cleanup we need to restore + // any BitsDeads that were there originally. + // + // In a one-word object (8-byte allocation on 64-bit system), + // there is no difference between BitsScalar and BitsDead, because + // neither is a pointer and there are no more words in the object, + // so using BitsScalar during the checkmark is safe and mapping + // both back to BitsDead during cleanup is also safe. + // + // In a larger object, we need to be more careful. During setup, + // if the type of the first word is BitsDead, we change it to BitsScalar + // (as we must) but also initialize the type of the second + // word to BitsDead, so that a scan during the checkmark phase + // will still stop before seeing the uninitialized type bits in the + // rest of the object. The sequence 'BitsScalar BitsDead' never + // happens in real type bitmaps - BitsDead is always as early + // as possible, so immediately after the last BitsPointer. + // During cleanup, if we see a BitsScalar, we can check to see if it + // is followed by BitsDead. If so, it was originally BitsDead and + // we can change it back. - if(nbadblock > 0) { - // Work out path from root to bad block. - for(;;) { - gc(&a); - if(nbadblock >= nelem(badblock)) - runtime·throw("cannot find path to bad pointer"); + if(step == 0) { + // updating top and bottom nibbles, all boundaries + for(i=0; i<n/2; i++, bitp--) { + if((*bitp & bitBoundary) != bitBoundary) + runtime·throw("missing bitBoundary"); + b = (*bitp & bitPtrMask)>>2; + if(!checkmark && (b == BitsScalar || b == BitsScalarMarked)) + *bitp &= ~0x0c; // convert to BitsDead + else if(b == BitsScalarMarked || b == BitsPointerMarked) + *bitp ^= BitsCheckMarkXor<<2; + + if(((*bitp>>gcBits) & bitBoundary) != bitBoundary) + runtime·throw("missing bitBoundary"); + b = ((*bitp>>gcBits) & bitPtrMask)>>2; + if(!checkmark && (b == BitsScalar || b == BitsScalarMarked)) + *bitp &= ~0xc0; // convert to BitsDead + else if(b == BitsScalarMarked || b == BitsPointerMarked) + *bitp ^= BitsCheckMarkXor<<(2+gcBits); + } + } else { + // updating bottom nibble for first word of each object + for(i=0; i<n; i++, bitp -= step) { + if((*bitp & bitBoundary) != bitBoundary) + runtime·throw("missing bitBoundary"); + b = (*bitp & bitPtrMask)>>2; + + if(checkmark && b == BitsDead) { + // move BitsDead into second word. + // set bits to BitsScalar in preparation for checkmark phase. + *bitp &= ~0xc0; + *bitp |= BitsScalar<<2; + } else if(!checkmark && (b == BitsScalar || b == BitsScalarMarked) && (*bitp & 0xc0) == 0) { + // Cleaning up after checkmark phase. + // First word is scalar or dead (we forgot) + // and second word is dead. + // First word might as well be dead too. + *bitp &= ~0x0c; + } else if(b == BitsScalarMarked || b == BitsPointerMarked) + *bitp ^= BitsCheckMarkXor<<2; } } +} - runtime·casgstatus(gp, Gwaiting, Grunning); +// clearcheckmarkbits preforms two tasks. +// 1. When used before the checkmark phase it converts BitsDead (00) to bitsScalar (01) +// for nibbles with the BoundaryBit set. +// 2. When used after the checkmark phase it converts BitsPointerMark (11) to BitsPointer 10 and +// BitsScalarMark (00) to BitsScalar (01), thus clearing the checkmark mark encoding. +// This is a bit expensive but preserves the BitsDead encoding during the normal marking. +// BitsDead remains valid for every nibble except the ones with BitsBoundary set. +static void +clearcheckmarkbits(void) +{ + uint32 idx; + MSpan *s; + for(idx=0; idx<runtime·work.nspan; idx++) { + s = runtime·work.spans[idx]; + if(s->state == MSpanInUse) { + clearcheckmarkbitsspan(s); + } + } +} + +// Called from malloc.go using onM. +// The world is stopped. Rerun the scan and mark phases +// using the bitMarkedCheck bit instead of the +// bitMarked bit. If the marking encounters an +// bitMarked bit that is not set then we throw. +void +runtime·gccheckmark_m(void) +{ + if(!gccheckmarkenable) + return; + + if(checkmark) + runtime·throw("gccheckmark_m, entered with checkmark already true."); + + checkmark = true; + clearcheckmarkbits(); // Converts BitsDead to BitsScalar. + runtime·gc_m(); // turns off checkmark + // Work done, fixed up the GC bitmap to remove the checkmark bits. + clearcheckmarkbits(); +} + +// checkmarkenable is initially false +void +runtime·gccheckmarkenable_m(void) +{ + gccheckmarkenable = true; +} + +void +runtime·gccheckmarkdisable_m(void) +{ + gccheckmarkenable = false; +} + +void +runtime·finishsweep_m(void) +{ + uint32 i, sg; + MSpan *s; + + // The world is stopped so we should be able to complete the sweeps + // quickly. + while(runtime·sweepone() != -1) + runtime·sweep.npausesweep++; + + // There may be some other spans being swept concurrently that + // we need to wait for. If finishsweep_m is done with the world stopped + // this code is not required. + sg = runtime·mheap.sweepgen; + for(i=0; i<runtime·work.nspan; i++) { + s = runtime·work.spans[i]; + if(s->sweepgen == sg) { + continue; + } + if(s->state != MSpanInUse) // Span is not part of the GCed heap so no need to ensure it is swept. + continue; + runtime·MSpan_EnsureSwept(s); + } +} + +// Scan all of the stacks, greying (or graying if in America) the referents +// but not blackening them since the mark write barrier isn't installed. +void +runtime·gcscan_m(void) +{ + uint32 i, allglen, oldphase; + G *gp, *mastergp, **allg; + + // Grab the g that called us and potentially allow rescheduling. + // This allows it to be scanned like other goroutines. + mastergp = g->m->curg; + + runtime·casgstatus(mastergp, Grunning, Gwaiting); + mastergp->waitreason = runtime·gostringnocopy((byte*)"garbage collection scan"); + + // Span sweeping has been done by finishsweep_m. + // Long term we will want to make this goroutine runnable + // by placing it onto a scanenqueue state and then calling + // runtime·restartg(mastergp) to make it Grunnable. + // At the bottom we will want to return this p back to the scheduler. + + oldphase = runtime·gcphase; + + runtime·lock(&runtime·allglock); + allglen = runtime·allglen; + allg = runtime·allg; + // Prepare flag indicating that the scan has not been completed. + for(i = 0; i < allglen; i++) { + gp = allg[i]; + gp->gcworkdone = false; // set to true in gcphasework + } + runtime·unlock(&runtime·allglock); + + runtime·work.nwait = 0; + runtime·work.ndone = 0; + runtime·work.nproc = 1; // For now do not do this in parallel. + runtime·gcphase = GCscan; + // ackgcphase is not needed since we are not scanning running goroutines. + runtime·parforsetup(runtime·work.markfor, runtime·work.nproc, RootCount + allglen, nil, false, markroot); + runtime·parfordo(runtime·work.markfor); + + runtime·lock(&runtime·allglock); + + allg = runtime·allg; + // Check that gc work is done. + for(i = 0; i < allglen; i++) { + gp = allg[i]; + if(!gp->gcworkdone) { + runtime·throw("scan missed a g"); + } + } + runtime·unlock(&runtime·allglock); + + runtime·gcphase = oldphase; + runtime·casgstatus(mastergp, Gwaiting, Grunning); + // Let the g that called us continue to run. +} + +// Mark all objects that are known about. +void +runtime·gcmark_m(void) +{ + scanblock(nil, 0, nil); +} + +// For now this must be bracketed with a stoptheworld and a starttheworld to ensure +// all go routines see the new barrier. +void +runtime·gcinstallmarkwb_m(void) +{ + runtime·gcphase = GCmark; +} + +// For now this must be bracketed with a stoptheworld and a starttheworld to ensure +// all go routines see the new barrier. +void +runtime·gcinstalloffwb_m(void) +{ + runtime·gcphase = GCoff; } static void @@ -1385,9 +2032,9 @@ gc(struct gc_args *args) int64 t0, t1, t2, t3, t4; uint64 heap0, heap1, obj; GCStats stats; - - if(DebugPtrs) - runtime·printf("GC start\n"); + uint32 oldphase; + uint32 i; + G *gp; if(runtime·debug.allocfreetrace) runtime·tracegc(); @@ -1400,11 +2047,10 @@ gc(struct gc_args *args) if(runtime·debug.gctrace) t1 = runtime·nanotime(); - // Sweep what is not sweeped by bgsweep. - while(runtime·sweepone() != -1) - runtime·sweep.npausesweep++; + if(!checkmark) + runtime·finishsweep_m(); // skip during checkmark debug phase. - // Cache runtime.mheap.allspans in work.spans to avoid conflicts with + // Cache runtime·mheap.allspans in work.spans to avoid conflicts with // resizing/freeing allspans. // New spans can be created while GC progresses, but they are not garbage for // this round: @@ -1421,10 +2067,19 @@ gc(struct gc_args *args) runtime·work.spans = runtime·mheap.allspans; runtime·work.nspan = runtime·mheap.nspan; runtime·unlock(&runtime·mheap.lock); + oldphase = runtime·gcphase; runtime·work.nwait = 0; runtime·work.ndone = 0; - runtime·work.nproc = runtime·gcprocs(); + runtime·work.nproc = runtime·gcprocs(); + runtime·gcphase = GCmarktermination; + + // World is stopped so allglen will not change. + for(i = 0; i < runtime·allglen; i++) { + gp = runtime·allg[i]; + gp->gcworkdone = false; // set to true in gcphasework + } + runtime·parforsetup(runtime·work.markfor, runtime·work.nproc, RootCount + runtime·allglen, nil, false, markroot); if(runtime·work.nproc > 1) { runtime·noteclear(&runtime·work.alldone); @@ -1437,8 +2092,15 @@ gc(struct gc_args *args) gchelperstart(); runtime·parfordo(runtime·work.markfor); + scanblock(nil, 0, nil); + if(runtime·work.full) + runtime·throw("runtime·work.full != nil"); + if(runtime·work.partial) + runtime·throw("runtime·work.partial != nil"); + + runtime·gcphase = oldphase; t3 = 0; if(runtime·debug.gctrace) t3 = runtime·nanotime(); @@ -1499,6 +2161,16 @@ gc(struct gc_args *args) // Free the old cached mark array if necessary. if(runtime·work.spans != nil && runtime·work.spans != runtime·mheap.allspans) runtime·SysFree(runtime·work.spans, runtime·work.nspan*sizeof(runtime·work.spans[0]), &mstats.other_sys); + + if(gccheckmarkenable) { + if(!checkmark) { + // first half of two-pass; don't set up sweep + runtime·unlock(&runtime·mheap.lock); + return; + } + checkmark = false; // done checking marks + } + // Cache the current array for sweeping. runtime·mheap.gcspans = runtime·mheap.allspans; runtime·mheap.sweepgen += 2; @@ -1508,6 +2180,7 @@ gc(struct gc_args *args) runtime·sweep.spanidx = 0; runtime·unlock(&runtime·mheap.lock); + if(ConcurrentSweep && !args->eagersweep) { runtime·lock(&runtime·gclock); if(runtime·sweep.g == nil) @@ -1527,9 +2200,6 @@ gc(struct gc_args *args) runtime·mProf_GC(); g->m->traceback = 0; - - if(DebugPtrs) - runtime·printf("GC end\n"); } extern uintptr runtime·sizeof_C_MStats; @@ -1802,7 +2472,7 @@ runtime·unrollgcprog_m(void) prog = (byte*)typ->gc[1]; unrollgcprog1(mask, prog, &pos, false, true); } - + // atomic way to say mask[0] = 1 x = *(uintptr*)mask; ((byte*)&x)[0] = 1; |