linux-kernel - Re: [PATCH] mm: vmstat: fix /proc/sys/vm/stat

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Message-ID: <20200714174235.GA484245@carbon.dhcp.thefacebook.com>
Date:   Tue, 14 Jul 2020 10:42:35 -0700
From:   Roman Gushchin <guro@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, <linux-mm@...ck.org>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] mm: vmstat: fix /proc/sys/vm/stat_refresh generating
 false warnings

On Tue, Jul 14, 2020 at 10:37:47AM -0700, Roman Gushchin wrote:
> I've noticed a number of warnings like "vmstat_refresh: nr_free_cma
> -5" or "vmstat_refresh: nr_zone_write_pending -11" on our production
> hosts. The numbers of these warnings were relatively low and stable,
> so it didn't look like we are systematically leaking the counters.
> The corresponding vmstat counters also looked sane.
> 
> These warnings are generated by the vmstat_refresh() function, which
> assumes that atomic zone and numa counters can't go below zero.
> However, on a SMP machine it's not quite right: due to per-cpu
> caching it can in theory be as low as -(zone threshold) * NR_CPUs.
> 
> For instance, let's say all cma pages are in use and NR_FREE_CMA_PAGES
> reached 0. Then we've reclaimed a small number of cma pages on each
> CPU except CPU0, so that most percpu NR_FREE_CMA_PAGES counters are
> slightly positive (the atomic counter is still 0). Then somebody on
> CPU0 consumes all these pages. The number of pages can easily exceed
> the threshold and a negative value will be committed to the atomic
> counter.
> 
> To fix the problem and avoid generating false warnings, let's just
> relax the condition and warn only if the value is less than minus
> the maximum theoretically possible drift value, which is 125 *
> number of online CPUs. It will still allow to catch systematic leaks,
> but will not generate bogus warnings.
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Roman Gushchin <guro@...com>

Spotted a double sign-off a second after sending. Fixed in v2.
Please, ignore this version.

Thanks!