[PATCH] mm: throttle LRU pages skipping on rmap_lock contention

[Minchan Kim <minchan@kernel.org>]

On Thu, May 12, 2022 at 12:55:16PM -0700, Minchan Kim wrote:
> On Wed, May 11, 2022 at 07:05:23PM -0700, Andrew Morton wrote:
> > On Wed, 11 May 2022 15:57:09 -0700 Minchan Kim <minchan@kernel.org> wrote:
> > 
> > > > 
> > > > Could we burn much CPU time pointlessly churning though the LRU?  Could
> > > > it mess up aging decisions enough to be performance-affecting in any
> > > > workload?
> > > 
> > > Yes, correct. However, we are already churning LRUs by several
> > > ways. For example, isolate and putback from LRU list for page
> > > migration from several sources(typical example is compaction)
> > > and trylock_page and sc->gfp_mask not allowing page to be
> > > reclaimed in shrink_page_list.
> > 
> > Well.  "we're already doing a risky thing so it's OK to do more of that
> > thing"?
> 
> I meant the aging is not rocket science.
> 
> 
> > 
> > > > 
> > > > Something else?
> > > 
> > > One thing I am worry about was the granularity of the churning.
> > > Example above was page granuarity churning so might be execuse
> > > but this one is address space's churning, especically for file LRU
> > > (i_mmap_rwsem) which might cause too many rotating and live-lock
> > > in the end(keey rotating in small LRU with heavy memory pressure).
> > > 
> > > If it could be a problem, maybe we use sc->priority to stop
> > > the skipping on a certain level of memory pressure.
> > > 
> > > Any thought? Do we really need it?
> > 
> > Are we able to think of a test which might demonstrate any worst case? 
> > Whip that up and see what the numbers say?
> 
> Yeah, let me create a worst test case to see how it goes.
> 
> A thread keep reading a file-backed vma with 2xRAM file but other threads
> keep changing other vmas mapped at the same file so heavy i_mmap_rwsem
> contention in aging path.

Forking new thread

I checked what happens the worst case. I am not sure how the worst
case is realistic but would be great to have safety net.

From 5ccc8b170af5496f803243732e96b131419d7462 Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@kernel.org>
Date: Thu, 19 May 2022 19:48:12 -0700
Subject: [PATCH] mm: throttle LRU pages skipping on rmap_lock contention

On heavy contention on rmap_lock(e.g., i_mmap_rwsem), VM can keep
skipping LRU pages so reclaim efficiency(steal/scanning) would drop
from 48% to 27% and workingset would be reclaimed faster than old
so workingset_refault rate increased to 240%.

We need a safe net to throttle the skipping LRU pages. This patch
throttle the skipping policy using (DEF_PRIRORITY - 2) magic value
VM has used for indicating non-light memory pressure.
IOW, let's skip rmap_lock contendeded pages only when
only when sc->priority >= (DEF_PRIRORITY - 2).

The test scenario to see the worst case:

1. A thread mmap a big file(e.g., 2x times of RAM) and keep touching
   the address space up to three times.
2. B thread keeps doing mmap/munmap with the same file to cause
   heavy lock contention in i_mmap_rwsem until the A thread finish
   the job.
3. measure vmstat and thread A's elapsed time.

Thread's elapsed time:

1. vanilla
24.64sec(5.04%)

2. rmap_skip(i.e., mm-dont-be-stuck-to-rmap-lock-on-reclaim-path.patch)
25.20sec(4.16%)

3. priority(2 + this patch)
23.62sec(6.61%)

Vmstat Comparison:
				     vanilla    rmap_skip    priority
	     allocstall_movable          582         9772       14643
		     pgactivate          232        25865        4906
      		   pgdeactivate           78        17265         651
        	     pgmajfault           58        10639        1376
    		 pgsteal_kswapd     15947857     15133195    15095445
    		 pgsteal_direct       105439       583092      943195
     	          pgscan_kswapd     24647536     52768898    28103170
     		  pgscan_direct      8398139      3767100     7966353
	workingset_refault_file     12582926     12248353    12565934

B test scenario

1. A thread mmap a big file(e.g., 2x times of RAM) and keep touching
   the address space up to three times.
2. B thread keeps doing mmap/munmap with the same file to cause
   heavy lock contention in i_mmap_rwsem until the A thread finish
   the job.
3. C thread keep reading other big file using read(2) syscall
4. measure vmstat and thread A's elapsed time.

1. vanilla
27.24sec(5.29%)

2. rmap_skip
33.54sec(3.20%)

3. priority
28.68sec(1.26%)

Vmstat Comparison:
				     vanilla    rmap_skip    priority
	     allocstall_movable        15262        81258       21644
        	     pgactivate      3042004      3086906     3502959
      		   pgdeactivate      2307849      8959162     3605768
        	     pgmajfault          566         1059	  557
    		 pgsteal_kswapd     17557735     30861283    18385674
    		 pgsteal_direct       955389      6353527     1233605
     		  pgscan_kswapd     31622695     59670433    35372575
		  pgscan_direct      4924052     13939254     4310247
	workingset_refault_file     13466538     32193161    14588019

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 include/linux/rmap.h | 5 +++--
 mm/rmap.c            | 6 ++++--
 mm/vmscan.c          | 6 ++++--
 3 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 9ec23138e410..2893da3f1cd3 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -296,7 +296,8 @@ static inline int page_try_share_anon_rmap(struct page *page)
  * Called from mm/vmscan.c to handle paging out
  */
 int folio_referenced(struct folio *, int is_locked,
-			struct mem_cgroup *memcg, unsigned long *vm_flags);
+			struct mem_cgroup *memcg, unsigned long *vm_flags,
+			bool rmap_try_lock);
 
 void try_to_migrate(struct folio *folio, enum ttu_flags flags);
 void try_to_unmap(struct folio *, enum ttu_flags flags);
@@ -418,7 +419,7 @@ void page_unlock_anon_vma_read(struct anon_vma *anon_vma);
 
 static inline int folio_referenced(struct folio *folio, int is_locked,
 				  struct mem_cgroup *memcg,
-				  unsigned long *vm_flags)
+				  unsigned long *vm_flags, bool rmap_try_lock)
 {
 	*vm_flags = 0;
 	return 0;
diff --git a/mm/rmap.c b/mm/rmap.c
index d4cf3ea1b616..a75c7f7a0392 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -888,6 +888,7 @@ static bool invalid_folio_referenced_vma(struct vm_area_struct *vma, void *arg)
  * @is_locked: Caller holds lock on the folio.
  * @memcg: target memory cgroup
  * @vm_flags: A combination of all the vma->vm_flags which referenced the folio.
+ * @rmap_try_lock: bail out if the rmap lock is contended
  *
  * Quick test_and_clear_referenced for all mappings of a folio,
  *
@@ -895,7 +896,8 @@ static bool invalid_folio_referenced_vma(struct vm_area_struct *vma, void *arg)
  * the function bailed out due to rmap lock contention.
  */
 int folio_referenced(struct folio *folio, int is_locked,
-		     struct mem_cgroup *memcg, unsigned long *vm_flags)
+		     struct mem_cgroup *memcg, unsigned long *vm_flags,
+		     bool rmap_try_lock)
 {
 	int we_locked = 0;
 	struct folio_referenced_arg pra = {
@@ -906,7 +908,7 @@ int folio_referenced(struct folio *folio, int is_locked,
 		.rmap_one = folio_referenced_one,
 		.arg = (void *)&pra,
 		.anon_lock = folio_lock_anon_vma_read,
-		.try_lock = true,
+		.try_lock = rmap_try_lock,
 	};
 
 	*vm_flags = 0;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index ac168f4b0492..f0987e027aba 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1381,7 +1381,8 @@ static enum page_references folio_check_references(struct folio *folio,
 	unsigned long vm_flags;
 
 	referenced_ptes = folio_referenced(folio, 1, sc->target_mem_cgroup,
-					   &vm_flags);
+					   &vm_flags,
+					   sc->priority >= DEF_PRIORITY - 2);
 	referenced_folio = folio_test_clear_referenced(folio);
 
 	/*
@@ -2497,7 +2498,8 @@ static void shrink_active_list(unsigned long nr_to_scan,
 
 		/* Referenced or rmap lock contention: rotate */
 		if (folio_referenced(folio, 0, sc->target_mem_cgroup,
-				     &vm_flags) != 0) {
+				     &vm_flags,
+				     sc->priority >= DEF_PRIORITY - 2) != 0) {
 			/*
 			 * Identify referenced, file-backed active pages and
 			 * give them one more trip around the active list. So
-- 
2.36.1.124.g0e6072fb45-goog