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Date:   Fri, 21 Feb 2020 09:07:37 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Sultan Alsawaf <sultan@kerneltoast.com>
Cc:     Mel Gorman <mgorman@suse.de>, Dave Hansen <dave.hansen@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH] mm: Stop kswapd early when nothing's waiting for it to
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On Thu 20-02-20 20:22:32, Sultan Alsawaf wrote:
> On Thu, Feb 20, 2020 at 10:19:45AM +0000, Mel Gorman wrote:
> > I'm not entirely convinced. The reason the high watermark exists is to have
> > kswapd work long enough to make progress without a process having to direct
> > reclaim. The most straight-forward example would be a streaming reader of
> > a large file. It'll keep pushing the zone towards the low watermark and
> > kswapd has to keep ahead of the reader. If we cut kswapd off too quickly,
> > the min watermark is hit and stalls occur. While kswapd could stop at the
> > min watermark, it leaves a very short window for kswapd to make enough
> > progress before the min watermark is hit.
> > 
> > At minimum, any change in this area would need to include the /proc/vmstats
> > on allocstat and pg*direct* to ensure that direct reclaim stalls are
> > not worse.
> > 
> > I'm not a fan of the patch in question because kswapd can be woken between
> > the low and min watermark without stalling but we really do expect kswapd
> > to make progress and continue to make progress to avoid future stalls. The
> > changelog had no information on the before/after impact of the patch and
> > this is an area where intuition can disagree with real behaviour.
> > 
> > -- 
> > Mel Gorman
> > SUSE Labs
> 
> Okay, then let's test real behavior.
> 
> I fired up my i5-8265U laptop with vanilla linux-next and passed mem=2G on the
> command line. After boot up, I opened up chromium and played a video on YouTube.
> Immediately after the video started, my laptop completely froze for a few
> seconds; then, a few seconds later, my cursor began to respond again, but moving
> it around was very laggy. The audio from the video playing was choppy during
> this time. About 15-20 seconds after I had started the YouTube video, my system
> finally stopped lagging.

Could you provide regular (e.g. each second) snapshots of /proc/vmstat,
ideally started before and finished after the observed behavior?
Something like
while true
do
	cp /proc/vmstat vmstat.$(date +%s)
done

If you can perf record and see where the kernel spends time during that
time period then it would be really helpful as well.

> Then I tried again with my patch applied (albeit a correct version that doesn't
> use the refcount API). Upon starting the same YouTube video in chromium, my
> laptop didn't freeze or stutter at all. The cursor was responsive and there was
> no stuttering, or choppy audio.
> 
> I tested this multiple times with reproducible results each time.

Your patch might be simply papering over a real problem.

> I will attach a functional v2 of the patch that I used.
> 
> Sultan

-- 
Michal Hocko
SUSE Labs