linux-kernel - Re: [PATCH v7 01/13] vmstat: allow_direct_reclaim should use zone_page_state

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Date:   Mon, 20 Mar 2023 19:21:04 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Russell King <linux@...linux.org.uk>,
        Huacai Chen <chenhuacai@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>, x86@...nel.org,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v7 01/13] vmstat: allow_direct_reclaim should use
 zone_page_state_snapshot

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@...e.com>
> Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>

The patch makes sense regardless but a note about testing should be
added.

Acked-by: Michal Hocko <mhocko@...e.com>

> 
> ---
> 
> Index: linux-vmstat-remote/mm/vmscan.c
> ===================================================================
> --- linux-vmstat-remote.orig/mm/vmscan.c
> +++ linux-vmstat-remote/mm/vmscan.c
> @@ -6861,7 +6861,7 @@ 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