linux-kernel - Re: [PATCH 19/25] mm, compaction: Do not consider a need to reschedule as contention

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 17 Jan 2019 18:05:32 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Linux-MM <linux-mm@...ck.org>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>, ying.huang@...el.com,
        kirill@...temov.name, Andrew Morton <akpm@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [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@...hsingularity.net>
> 
> Acked-by: Vlastimil Babka <vbabka@...e.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