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SubjectRe: [PATCH 19/25] mm, compaction: Do not consider a need to reschedule as contention
On Thu, Jan 17, 2019 at 06:33:37PM +0100, Vlastimil Babka wrote:
> 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.
>

I didn't dig into the history but one motivating factor around all the
logic would have been reducing the time IRQs were disabled. With changes
like scanning COMPACT_CLUSTER_MAX and dropping locks, it's less of a
factor. Then again, the retry loops around in the page allocator would
also have changed the problem. Things just changed enough that the
original motivation no longer applies.

> > 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.
>

I guess so. I liked having the helper to remind that the contention points
mattered at some point and wasn't a random sprinkling of cond_resched
but I'll remove it.

--
Mel Gorman
SUSE Labs

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