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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!
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