rts: gc stats: account properly for copied bytes in sequential collectionswip/dougwilson-21745

We were not updating the [copied,any_work,scav_find_work, max_n_todo_overflow]
counters during sequential collections. As well, we were double counting for
parallel collections.

To fix this we add an `else` clause to the `if (is_par_gc())`.

The par_* counters do not need to be updated in the sequential case
because they must be 0.

2 files changed, 13 insertions, 0 deletions

diff --git a/docs/users_guide/9.6.1-notes.rst b/docs/users_guide/9.6.1-notes.rst
index 04ea0e3bd2..d1eb93aef9 100644
--- a/docs/users_guide/9.6.1-notes.rst
+++ b/docs/users_guide/9.6.1-notes.rst
@@ -66,6 +66,12 @@ Compiler
 - The :extension:`TypeInType` is now marked as deprecated. Its meaning has been included 
   in :extension:`PolyKinds` and :extension:`DataKinds`.
 
+Runtime System
+~~~~~~~~~~~~~~
+
+- Summary statistics, i.e. the output of :rts-flag:`-s [⟨file⟩]`, now correctly
+  accounts for bytes copied during sequential collections.
+
 ``base`` library
 ~~~~~~~~~~~~~~~~
 
diff --git a/rts/sm/GC.c b/rts/sm/GC.c
index 22b29cf48d..e60fb314d8 100644
--- a/rts/sm/GC.c
+++ b/rts/sm/GC.c
@@ -170,6 +170,9 @@ is requested it takes 1, when a SYNC_GC_PAR is requested it takes n_capabilities
 
 Clearly this is in need of some tidying up, but for now we tread carefully. We
 call is_par_gc() to see whether we are in a parallel or sequential collection.
+If we are in a parallel collection we iterate over gc_threads, being careful to
+account for idle caps. If we are in a sequential collection we deal only with
+the thread local gct.
 Of course this is valid only inside GarbageCollect ().
 
 Omitting this check has led to issues such as #19147.
@@ -652,7 +655,11 @@ GarbageCollect (uint32_t collect_gen,
           par_balanced_copied =
               (par_balanced_copied_acc - copied + other_active_threads / 2) /
               other_active_threads;
+      } else {
           copied += gct->copied;
+          any_work += gct->any_work;
+          scav_find_work += gct->scav_find_work;
+          max_n_todo_overflow += gct->max_n_todo_overflow;
       }
   }