[PATCH v2 1/1] mm: count time in drain_all_pages during direct reclaim as memory pressure

[Suren Baghdasaryan <surenb@google.com>]

When page allocation in direct reclaim path fails, the system will
make one attempt to shrink per-cpu page lists and free pages from
high alloc reserves. Draining per-cpu pages into buddy allocator can
be a very slow operation because it's done using workqueues and the
task in direct reclaim waits for all of them to finish before
proceeding. Currently this time is not accounted as psi memory stall.

While testing mobile devices under extreme memory pressure, when
allocations are failing during direct reclaim, we notices that psi
events which would be expected in such conditions were not triggered.
After profiling these cases it was determined that the reason for
missing psi events was that a big chunk of time spent in direct
reclaim is not accounted as memory stall, therefore psi would not
reach the levels at which an event is generated. Further investigation
revealed that the bulk of that unaccounted time was spent inside
drain_all_pages call.

A typical captured case when drain_all_pages path gets activated:

__alloc_pages_slowpath  took 44.644.613ns
    __perform_reclaim   took    751.668ns (1.7%)
    drain_all_pages     took 43.887.167ns (98.3%)

PSI in this case records the time spent in __perform_reclaim but
ignores drain_all_pages, IOW it misses 98.3% of the time spent in
__alloc_pages_slowpath.

Annotate __alloc_pages_direct_reclaim in its entirety so that delays
from handling page allocation failure in the direct reclaim path are
accounted as memory stall.

Reported-by: Tim Murray <timmurray@google.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
---
changes in v2:
- Added captured sample case to show the delay numbers, per Michal Hocko
- Moved annotation from __perform_reclaim into __alloc_pages_direct_reclaim,
per Minchan Kim

 mm/page_alloc.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3589febc6d31..2e9fbf28938f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4595,13 +4595,12 @@ __perform_reclaim(gfp_t gfp_mask, unsigned int order,
 					const struct alloc_context *ac)
 {
 	unsigned int noreclaim_flag;
-	unsigned long pflags, progress;
+	unsigned long progress;
 
 	cond_resched();
 
 	/* We now go into synchronous reclaim */
 	cpuset_memory_pressure_bump();
-	psi_memstall_enter(&pflags);
 	fs_reclaim_acquire(gfp_mask);
 	noreclaim_flag = memalloc_noreclaim_save();
 
@@ -4610,7 +4609,6 @@ __perform_reclaim(gfp_t gfp_mask, unsigned int order,
 
 	memalloc_noreclaim_restore(noreclaim_flag);
 	fs_reclaim_release(gfp_mask);
-	psi_memstall_leave(&pflags);
 
 	cond_resched();
 
@@ -4624,11 +4622,13 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
 		unsigned long *did_some_progress)
 {
 	struct page *page = NULL;
+	unsigned long pflags;
 	bool drained = false;
 
+	psi_memstall_enter(&pflags);
 	*did_some_progress = __perform_reclaim(gfp_mask, order, ac);
 	if (unlikely(!(*did_some_progress)))
-		return NULL;
+		goto out;
 
 retry:
 	page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
@@ -4644,7 +4644,8 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
 		drained = true;
 		goto retry;
 	}
-
+	psi_memstall_leave(&pflags);
+out:
 	return page;
 }
 
-- 
2.35.1.473.g83b2b277ed-goog