linux-kernel - Re: [PATCH v2 3/4] mm: /proc/sys/vm/stat_refresh skip checking known negative stats

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Message-ID: <YEBFDtz9F5U3G1Su@carbon.dhcp.thefacebook.com>
Date:   Wed, 3 Mar 2021 18:25:18 -0800
From:   Roman Gushchin <guro@...com>
To:     Hugh Dickins <hughd@...gle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/4] mm: /proc/sys/vm/stat_refresh skip checking known
 negative stats

On Mon, Mar 01, 2021 at 10:03:26PM -0800, Hugh Dickins wrote:
> vmstat_refresh() can occasionally catch nr_zone_write_pending and
> nr_writeback when they are transiently negative.  The reason is partly
> that the interrupt which decrements them in test_clear_page_writeback()
> can come in before __test_set_page_writeback() got to increment them;
> but transient negatives are still seen even when that is prevented, and
> I am not yet certain why (but see Roman's note below).  Those stats are
> not buggy, they have never been seen to drift away from 0 permanently:
> so just avoid the annoyance of showing a warning on them.
> 
> Similarly avoid showing a warning on nr_free_cma: CMA users have seen
> that one reported negative from /proc/sys/vm/stat_refresh too, but it
> does drift away permanently: I believe that's because its incrementation
> and decrementation are decided by page migratetype, but the migratetype
> of a pageblock is not guaranteed to be constant.
> 
> Roman Gushchin points out:
> For performance reasons, vmstat counters are incremented and decremented
> using per-cpu batches.  vmstat_refresh() flushes the per-cpu batches on
> all CPUs, to get values as accurate as possible; but this method is not
> atomic, so the resulting value is not always precise.  As a consequence,
> for those counters whose actual value is close to 0, a small negative
> value may occasionally be reported.  If the value is small and the state
> is transient, it is not an indication of an error.
> 
> Link: https://lore.kernel.org/linux-mm/20200714173747.3315771-1-guro@fb.com/
> Reported-by: Roman Gushchin <guro@...com>
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---

Oh, sorry, it looks like I missed to ack it. Thank you for updating
the commit log!

Acked-by: Roman Gushchin <guro@...com>