Re: [PATCH 1/4] vmstat: allow_direct_reclaim should use zone_page_state_snapshot

[Michal Hocko]

Andrew, as I've stated in previously, this sounds like a reasonable
standalone fix worth taking independently of the rest.

On Tue 30-05-23 11:52:35, 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.
> 
> Testing: Due to difficulties accessing the system, it has not been
> possible for the reproducer to test the patch (however its
> clear from available data and analysis that it should fix it).
> 
> Reviewed-by: Michal Hocko <mhocko@xxxxxxxx>
> Reviewed-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>
> Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> 
> ---
> 
> Index: linux-vmstat-remote/mm/vmscan.c
> ===================================================================
> --- linux-vmstat-remote.orig/mm/vmscan.c
> +++ linux-vmstat-remote/mm/vmscan.c
> @@ -6887,7 +6887,7 @@ 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 */
> 

-- 
Michal Hocko
SUSE Labs