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

[Michal Hocko]

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@xxxxxxxx>
> > > Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> > 
> > The patch makes sense regardless but a note about testing should be
> > added.
> > 
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> 
> 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