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SubjectRe: [PATCH 19/25] mm, compaction: Do not consider a need to reschedule as contention
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On 1/4/19 1:50 PM, Mel Gorman wrote:
> 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%*

I always assumed that this was the main factor that (clumsily) limited THP fault
latencies. Seems like it's (no longer?) the case, or the lock contention
detection alone works as well.

> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> 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();

Seems like plain "cond_resched()" is sufficient at this point, and probably
doesn't need a wrapper anymore.

> - }
> }
>
> /*
>

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