From: Aaron Tomlin <atomlin@atomlin.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Christoph Lameter <cl@linux.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>,
Michal Hocko <mhocko@suse.com>,
Aaron Tomlin <atomlin@atomlin.com>
Subject: Re: [PATCH v8 01/13] vmstat: allow_direct_reclaim should use zone_page_state_snapshot
Date: Thu, 25 May 2023 08:05:26 +0100 [thread overview]
Message-ID: <20230525070526.5uhmh6zku5gzxyny@atomlin.usersys.com> (raw)
In-Reply-To: <20230515180138.442505633@redhat.com>
On Mon, May 15, 2023 at 03:00:16PM -0300, 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.
>
> Suggested-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>
> ---
>
> Index: linux-vmstat-remote/mm/vmscan.c
> ===================================================================
> --- linux-vmstat-remote.orig/mm/vmscan.c
> +++ linux-vmstat-remote/mm/vmscan.c
> @@ -6886,7 +6886,7 @@ static bool allow_direct_reclaim(pg_data
> continue;
>
> pfmemalloc_reserve += min_wmark_pages(zone);
> - free_pages += zone_page_state(zone, NR_FREE_PAGES);
> + free_pages += zone_page_state_snapshot(zone, NR_FREE_PAGES);
> }
>
> /* If there are no reserves (unexpected config) then do not throttle */
>
>
Reviewed-by: Aaron Tomlin <atomlin@atomlin.com>
--
Aaron Tomlin
next prev parent reply other threads:[~2023-05-25 7:07 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-15 18:00 [PATCH v8 00/13] fold per-CPU vmstats remotely Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 01/13] vmstat: allow_direct_reclaim should use zone_page_state_snapshot Marcelo Tosatti
2023-05-24 12:34 ` Michal Hocko
2023-05-25 7:05 ` Aaron Tomlin [this message]
2023-05-15 18:00 ` [PATCH v8 02/13] this_cpu_cmpxchg: ARM64: switch this_cpu_cmpxchg to locked, add _local function Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 03/13] this_cpu_cmpxchg: loongarch: " Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 04/13] this_cpu_cmpxchg: S390: " Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 05/13] this_cpu_cmpxchg: x86: " Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 06/13] add this_cpu_cmpxchg_local and asm-generic definitions Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 07/13] convert this_cpu_cmpxchg users to this_cpu_cmpxchg_local Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 08/13] mm/vmstat: switch counter modification to cmpxchg Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 09/13] vmstat: switch per-cpu vmstat counters to 32-bits Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 10/13] mm/vmstat: use xchg in cpu_vm_stats_fold Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 11/13] mm/vmstat: switch vmstat shepherd to flush per-CPU counters remotely Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 12/13] mm/vmstat: refresh stats remotely instead of via work item Marcelo Tosatti
2023-05-15 18:00 ` [PATCH v8 13/13] vmstat: add pcp remote node draining via cpu_vm_stats_fold Marcelo Tosatti
2023-05-16 8:09 ` [PATCH v8 00/13] fold per-CPU vmstats remotely Christoph Lameter
2023-05-16 18:02 ` Marcelo Tosatti
2023-05-24 12:51 ` Michal Hocko
2023-05-24 13:53 ` Marcelo Tosatti
2023-05-25 6:47 ` Michal Hocko
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