vmstat: allow_direct_reclaim should use zone_page_state_snapshot

Patch series "fold per-CPU vmstats remotely", v7.

This patch series addresses the following two problems:

1. 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.

 2. With a task that busy loops on a given CPU,
    the kworker interruption to execute vmstat_update
    is undesired and may exceed latency thresholds
    for certain applications.

By having vmstat_shepherd flush the per-CPU counters to the
global counters from remote CPUs.

This is done using cmpxchg to manipulate the counters,
both CPU locally (via the account functions),
and remotely (via cpu_vm_stats_fold).

Thanks to Aaron Tomlin for diagnosing issue 1 and writing
the initial patch series.

Performance details for the kworker interruption:

oslat   1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000)
oslat   1094.456971: workqueue_queue_work: ... function=vmstat_update ...
oslat   1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ...
kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ...

The example above shows an additional 7us for the

        oslat -> kworker -> oslat

switches. In the case of a virtualized CPU, and the vmstat_update
interruption in the host (of a qemu-kvm vcpu), the latency penalty
observed in the guest is higher than 50us, violating the acceptable
latency threshold for certain applications.

This patch (of 13):

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.

Link: https://lkml.kernel.org/r/20230320180332.102837832@redhat.com
Link: https://lkml.kernel.org/r/20230320180745.556821285@redhat.com
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Suggested-by: Michal Hocko <mhocko@suse.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Aaron Tomlin <atomlin@atomlin.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 98719e72b5e2d48c1fc14b10af8ffd67f5f8a26e..ebbc2265871d5adbf16965eabc6088ff292e5074 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -6873,7 +6873,7 @@ static bool allow_direct_reclaim(pg_data_t *pgdat)
                        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 */