Re: [PATCH] mm: page_alloc: update the COMPACT[STALL|FAIL] events properly

Thanks David for the review!!

On 2/2/2021 2:54 AM, David Rientjes wrote:

On Mon, 1 Feb 2021, Charan Teja Reddy wrote:

quoted

By defination, COMPACT[STALL|FAIL] events needs to be counted when there

s/defination/definition/\

Done.

quoted

is 'At least in one zone compaction wasn't deferred or skipped from the
direct compaction'. And when compaction is skipped or deferred,
COMPACT_SKIPPED will be returned but it will still go and update these
compaction events which is wrong in the sense that COMPACT[STALL|FAIL]
is counted without even trying the compaction.

Correct this by skipping the counting of these events when
COMPACT_SKIPPED is returned for compaction. This indirectly also avoid
the unnecessary try into the get_page_from_freelist() when compaction is
not even tried.

Signed-off-by: Charan Teja Reddy <redacted>
---
 mm/page_alloc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 519a60d..531f244 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -4152,6 +4152,8 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 	memalloc_noreclaim_restore(noreclaim_flag);
 	psi_memstall_leave(&pflags);
 
+	if (*compact_result == COMPACT_SKIPPED)
+		return NULL;
 	/*
 	 * At least in one zone compaction wasn't deferred or skipped, so let's
 	 * count a compaction stall

This makes sense, I wonder if it would also be useful to check that 
page == NULL, either in try_to_compact_pages() or here for 
COMPACT_SKIPPED?

In the code, when COMPACT_SKIPPED is being returned, the page will
always be NULL. So, I'm not sure how much useful it is for the page ==
NULL check here. Or I failed to understand your point here?

As, till now, COMPACTFAIL also presents page allocation failures because
of the direct compaction is skipped, creating a separate COMPACTSKIPFAIL
event which indicates that 'failed to get the free page as direct
compaction is skipped' is useful?

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