[RFC PATCH] mm: use compaction feedback for thp backoff conditions

[Michal Hocko <mhocko@kernel.org>]

On Mon 11-04-16 17:40:36, Michal Hocko wrote:
> Hi Andrew,
> Vlastimil has pointed out[1] that using compaction_withdrawn() for THP
> allocations has some non-trivial consequences. While I still think that
> the check is OK it is true we shouldn't sneak in a potential behavior
> change into something that basically provides an API. So can you fold
> the following partial revert into the original patch please?
> 
> [1] http://lkml.kernel.org/r/570BB719.2030007@suse.cz

This would be an RFC on top.
---
>From 6cfed80ad41f3f1506930b9a3254fe135bf90d4c Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Mon, 11 Apr 2016 17:51:28 +0200
Subject: [PATCH] mm: use compaction feedback for thp backoff conditions

THP requests skip the direct reclaim if the compaction is either
deferred or contended to reduce stalls which wouldn't help the
allocation success anyway. These checks are ignoring other potential
feedback modes which we have available now.

It clearly doesn't make much sense to go and reclaim few pages if the
previous compaction has failed.

We can also simplify the check by using compaction_withdrawn which
checks for both COMPACT_CONTENDED and COMPACT_DEFERRED. This check
is however covering more reasons why the compaction was withdrawn.
None of them should be a problem for the THP case though.

It is safe to back of if we see COMPACT_SKIPPED because that means
that compaction_suitable failed and a single round of the reclaim is
unlikely to make any difference here. We would have to be close to
the low watermark to reclaim enough and even then there is no guarantee
that the compaction would make any progress while the direct reclaim
would have caused the stall.

COMPACT_PARTIAL_SKIPPED is slightly different because that means that we
have only seen a part of the zone so a retry would make some sense. But
it would be a compaction retry not a reclaim retry to perform. We are
not doing that and that might indeed lead to situations where THP fails
but this should happen only rarely and it would be really hard to
measure.

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 mm/page_alloc.c | 27 ++++++++-------------------
 1 file changed, 8 insertions(+), 19 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5eae9e0555ed..6d1da0ceaf1e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3420,25 +3420,14 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	if (order && compaction_made_progress(compact_result))
 		compaction_retries++;
 
-	/* Checks for THP-specific high-order allocations */
-	if (is_thp_gfp_mask(gfp_mask)) {
-		/*
-		 * If compaction is deferred for high-order allocations, it is
-		 * because sync compaction recently failed. If this is the case
-		 * and the caller requested a THP allocation, we do not want
-		 * to heavily disrupt the system, so we fail the allocation
-		 * instead of entering direct reclaim.
-		 */
-		if (compact_result == COMPACT_DEFERRED)
-			goto nopage;
-
-		/*
-		 * Compaction is contended so rather back off than cause
-		 * excessive stalls.
-		 */
-		if(compact_result == COMPACT_CONTENDED)
-			goto nopage;
-	}
+	/*
+	 * Checks for THP-specific high-order allocations and back off
+	 * if the the compaction backed off or failed
+	 */
+	if (is_thp_gfp_mask(gfp_mask) &&
+			(compaction_withdrawn(compact_result) ||
+			 compaction_failed(compact_result)))
+		goto nopage;
 
 	/* Try direct reclaim and then allocating */
 	page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, ac,
-- 
2.8.0.rc3

-- 
Michal Hocko
SUSE Labs