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Subject[PATCH 19/25] mm, compaction: Do not consider a need to reschedule as contention
Date
Scanning on large machines can take a considerable length of time and
eventually need to be rescheduled. This is treated as an abort event but
that's not appropriate as the attempt is likely to be retried after making
numerous checks and taking another cycle through the page allocator.
This patch will check the need to reschedule if necessary but continue
the scanning.

The main benefit is reduced scanning when compaction is taking a long time
or the machine is over-saturated. It also avoids an unnecessary exit of
compaction that ends up being retried by the page allocator in the outer
loop.

4.20.0 4.20.0
synccached-v2r15 noresched-v2r15
Amean fault-both-3 2655.55 ( 0.00%) 2736.50 ( -3.05%)
Amean fault-both-5 4580.67 ( 0.00%) 4133.70 ( 9.76%)
Amean fault-both-7 5740.50 ( 0.00%) 5738.61 ( 0.03%)
Amean fault-both-12 9237.55 ( 0.00%) 9392.82 ( -1.68%)
Amean fault-both-18 12899.51 ( 0.00%) 13257.15 ( -2.77%)
Amean fault-both-24 16342.47 ( 0.00%) 16859.44 ( -3.16%)
Amean fault-both-30 20394.26 ( 0.00%) 16249.30 * 20.32%*
Amean fault-both-32 17450.76 ( 0.00%) 14904.71 * 14.59%*

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
mm/compaction.c | 12 ++----------
1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 1a41a2dbff24..75eb0d40d4d7 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -398,19 +398,11 @@ static bool compact_lock_irqsave(spinlock_t *lock, unsigned long *flags,
return true;
}

-/*
- * Aside from avoiding lock contention, compaction also periodically checks
- * need_resched() and records async compaction as contended if necessary.
- */
+/* Avoid soft-lockups due to long scan times */
static inline void compact_check_resched(struct compact_control *cc)
{
- /* async compaction aborts if contended */
- if (need_resched()) {
- if (cc->mode == MIGRATE_ASYNC)
- cc->contended = true;
-
+ if (need_resched())
cond_resched();
- }
}

/*
--
2.16.4
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