Re: [PATCH v7 01/13] vmstat: allow_direct_reclaim should use zone_page_state_snapshot

Marcelo Tosatti
2023-03-21T12:58:58Z

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).

Perhaps one could find a synthetic reproducer.

It is pretty easy to note that, on an isolated nohz_full CPU, the
deferrable timer that is queued on it (timer which should queue vmstat_update
on that CPU) does not execute for long periods.
This makes the global stats stale (since per-CPU free pages can become
stale for as long as the CPU has tick processing stopped).
Which matches the data available.

Thanks!