From: Michal Hocko <mhocko@suse.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Christoph Lameter <cl@linux.com>,
Aaron Tomlin <atomlin@atomlin.com>,
Frederic Weisbecker <frederic@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Russell King <linux@armlinux.org.uk>,
Huacai Chen <chenhuacai@kernel.org>,
Heiko Carstens <hca@linux.ibm.com>,
x86@kernel.org, Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v7 01/13] vmstat: allow_direct_reclaim should use zone_page_state_snapshot
Date: Wed, 22 Mar 2023 11:03:47 +0100 [thread overview]
Message-ID: <ZBrSg3y7Hu8Bfspc@dhcp22.suse.cz> (raw)
In-Reply-To: <ZBimr/DeUhcf7I5D@tpad>
On Mon 20-03-23 15:32:15, Marcelo Tosatti wrote:
> On Mon, Mar 20, 2023 at 07:21:04PM +0100, Michal Hocko wrote:
> > On Mon 20-03-23 15:03:33, Marcelo Tosatti wrote:
> > > A customer provided evidence indicating that a process
> > > was stalled in direct reclaim:
> > >
> > > - The process was trapped in throttle_direct_reclaim().
> > > The function wait_event_killable() was called to wait condition
> > > allow_direct_reclaim(pgdat) for current node to be true.
> > > The allow_direct_reclaim(pgdat) examined the number of free pages
> > > on the node by zone_page_state() which just returns value in
> > > zone->vm_stat[NR_FREE_PAGES].
> > >
> > > - On node #1, zone->vm_stat[NR_FREE_PAGES] was 0.
> > > However, the freelist on this node was not empty.
> > >
> > > - This inconsistent of vmstat value was caused by percpu vmstat on
> > > nohz_full cpus. Every increment/decrement of vmstat is performed
> > > on percpu vmstat counter at first, then pooled diffs are cumulated
> > > to the zone's vmstat counter in timely manner. However, on nohz_full
> > > cpus (in case of this customer's system, 48 of 52 cpus) these pooled
> > > diffs were not cumulated once the cpu had no event on it so that
> > > the cpu started sleeping infinitely.
> > > I checked percpu vmstat and found there were total 69 counts not
> > > cumulated to the zone's vmstat counter yet.
> > >
> > > - In this situation, kswapd did not help the trapped process.
> > > In pgdat_balanced(), zone_wakermark_ok_safe() examined the number
> > > of free pages on the node by zone_page_state_snapshot() which
> > > checks pending counts on percpu vmstat.
> > > Therefore kswapd could know there were 69 free pages correctly.
> > > Since zone->_watermark = {8, 20, 32}, kswapd did not work because
> > > 69 was greater than 32 as high watermark.
> > >
> > > Change allow_direct_reclaim to use zone_page_state_snapshot, which
> > > allows a more precise version of the vmstat counters to be used.
> > >
> > > allow_direct_reclaim will only be called from try_to_free_pages,
> > > which is not a hot path.
> >
> > Have you managed to test this patch to confirm it addresses the above
> > issue? It should but better double check that.
> >
> > > Suggested-by: Michal Hocko <mhocko@suse.com>
> > > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> >
> > The patch makes sense regardless but a note about testing should be
> > added.
> >
> > Acked-by: Michal Hocko <mhocko@suse.com>
>
> Michal,
>
> The patch has not been tested in the original setup where the problem
> was found, however i don't think its easy to do that validation
> (checking with the reporter anyway).
This is a fair point and I would just add it to the changelog for the
future reference.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2023-03-22 10:04 UTC|newest]
Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-20 18:03 [PATCH v7 00/13] fold per-CPU vmstats remotely Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 01/13] vmstat: allow_direct_reclaim should use zone_page_state_snapshot Marcelo Tosatti
2023-03-20 18:21 ` Michal Hocko
2023-03-20 18:32 ` Marcelo Tosatti
2023-03-22 10:03 ` Michal Hocko [this message]
2023-03-20 18:03 ` [PATCH v7 02/13] this_cpu_cmpxchg: ARM64: switch this_cpu_cmpxchg to locked, add _local function Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 03/13] this_cpu_cmpxchg: loongarch: " Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 04/13] this_cpu_cmpxchg: S390: " Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 05/13] this_cpu_cmpxchg: x86: " Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 06/13] add this_cpu_cmpxchg_local and asm-generic definitions Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 07/13] convert this_cpu_cmpxchg users to this_cpu_cmpxchg_local Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 08/13] mm/vmstat: switch counter modification to cmpxchg Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 09/13] vmstat: switch per-cpu vmstat counters to 32-bits Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 10/13] mm/vmstat: use xchg in cpu_vm_stats_fold Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 11/13] mm/vmstat: switch vmstat shepherd to flush per-CPU counters remotely Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 12/13] mm/vmstat: refresh stats remotely instead of via work item Marcelo Tosatti
2023-03-20 18:03 ` [PATCH v7 13/13] vmstat: add pcp remote node draining via cpu_vm_stats_fold Marcelo Tosatti
2023-03-20 20:43 ` Tim Chen
2023-03-22 1:20 ` Marcelo Tosatti
2023-03-20 18:25 ` [PATCH v7 00/13] fold per-CPU vmstats remotely Michal Hocko
2023-03-20 19:07 ` Marcelo Tosatti
2023-03-22 10:13 ` Michal Hocko
2023-03-22 11:23 ` Marcelo Tosatti
2023-03-22 13:35 ` Michal Hocko
2023-03-22 14:20 ` Marcelo Tosatti
2023-03-23 7:51 ` Michal Hocko
2023-03-23 10:52 ` Marcelo Tosatti
2023-03-23 10:59 ` Marcelo Tosatti
2023-03-23 12:17 ` Michal Hocko
2023-03-23 13:30 ` Marcelo Tosatti
2023-03-23 13:32 ` Marcelo Tosatti
2023-04-18 22:02 ` Andrew Morton
2023-04-19 11:14 ` Marcelo Tosatti
2023-04-19 11:15 ` Marcelo Tosatti
2023-04-19 13:44 ` Andrew Theurer
2023-04-20 7:55 ` Michal Hocko
2023-04-23 1:25 ` Marcelo Tosatti
2023-04-19 11:29 ` Marcelo Tosatti
2023-04-19 11:59 ` Marcelo Tosatti
2023-04-19 12:24 ` Frederic Weisbecker
2023-04-19 13:48 ` Marcelo Tosatti
2023-04-19 14:35 ` Michal Hocko
2023-04-19 16:35 ` Marcelo Tosatti
2023-04-20 8:40 ` Michal Hocko
2023-04-23 1:10 ` Marcelo Tosatti
2023-04-20 13:45 ` Marcelo Tosatti
2023-04-26 14:34 ` Marcelo Tosatti
2023-04-27 8:31 ` Michal Hocko
2023-04-27 14:59 ` Marcelo Tosatti
2023-04-26 15:04 ` Vlastimil Babka
2023-04-26 16:10 ` Marcelo Tosatti
2023-04-27 8:39 ` Michal Hocko
2023-04-27 16:25 ` Marcelo Tosatti
2023-04-19 16:47 ` Vlastimil Babka
2023-04-19 19:15 ` Marcelo Tosatti
2023-05-03 13:51 ` Marcelo Tosatti
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