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

[Marcelo Tosatti <mtosatti@redhat.com>]

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!