mm: convert mm's rss stats into percpu_counter

Currently mm_struct maintains rss_stats which are updated on page fault
and the unmapping codepaths.  For page fault codepath the updates are
cached per thread with the batch of TASK_RSS_EVENTS_THRESH which is 64. 
The reason for caching is performance for multithreaded applications
otherwise the rss_stats updates may become hotspot for such applications.

However this optimization comes with the cost of error margin in the rss
stats.  The rss_stats for applications with large number of threads can be
very skewed.  At worst the error margin is (nr_threads * 64) and we have a
lot of applications with 100s of threads, so the error margin can be very
high.  Internally we had to reduce TASK_RSS_EVENTS_THRESH to 32.

Recently we started seeing the unbounded errors for rss_stats for specific
applications which use TCP rx0cp.  It seems like vm_insert_pages()
codepath does not sync rss_stats at all.

This patch converts the rss_stats into percpu_counter to convert the error
margin from (nr_threads * 64) to approximately (nr_cpus ^ 2).  However
this conversion enable us to get the accurate stats for situations where
accuracy is more important than the cpu cost.

This patch does not make such tradeoffs - we can just use
percpu_counter_add_local() for the updates and percpu_counter_sum() (or
percpu_counter_sync() + percpu_counter_read) for the readers.  At the
moment the readers are either procfs interface, oom_killer and memory
reclaim which I think are not performance critical and should be ok with
slow read.  However I think we can make that change in a separate patch.

Link: https://lkml.kernel.org/r/20221024052841.3291983-1-shakeelb@google.com
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 8 insertions, 18 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f919befc8fac..0cb4e196d60b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2052,40 +2052,30 @@ static inline bool get_user_page_fast_only(unsigned long addr,
  */
 static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
 {
-	long val = atomic_long_read(&mm->rss_stat.count[member]);
-
-#ifdef SPLIT_RSS_COUNTING
-	/*
-	 * counter is updated in asynchronous manner and may go to minus.
-	 * But it's never be expected number for users.
-	 */
-	if (val < 0)
-		val = 0;
-#endif
-	return (unsigned long)val;
+	return percpu_counter_read_positive(&mm->rss_stat[member]);
 }
 
-void mm_trace_rss_stat(struct mm_struct *mm, int member, long count);
+void mm_trace_rss_stat(struct mm_struct *mm, int member);
 
 static inline void add_mm_counter(struct mm_struct *mm, int member, long value)
 {
-	long count = atomic_long_add_return(value, &mm->rss_stat.count[member]);
+	percpu_counter_add(&mm->rss_stat[member], value);
 
-	mm_trace_rss_stat(mm, member, count);
+	mm_trace_rss_stat(mm, member);
 }
 
 static inline void inc_mm_counter(struct mm_struct *mm, int member)
 {
-	long count = atomic_long_inc_return(&mm->rss_stat.count[member]);
+	percpu_counter_inc(&mm->rss_stat[member]);
 
-	mm_trace_rss_stat(mm, member, count);
+	mm_trace_rss_stat(mm, member);
 }
 
 static inline void dec_mm_counter(struct mm_struct *mm, int member)
 {
-	long count = atomic_long_dec_return(&mm->rss_stat.count[member]);
+	percpu_counter_dec(&mm->rss_stat[member]);
 
-	mm_trace_rss_stat(mm, member, count);
+	mm_trace_rss_stat(mm, member);
 }
 
 /* Optimized variant when page is already known not to be PageAnon */