Re: [PATCH 1/1] mm: vmscan: Reduce throttling due to a failure to make progress

[Vlastimil Babka]

On 11/25/21 16:18, Mel Gorman wrote:
> Mike Galbraith, Alexey Avramov and Darrick Wong all reported similar
> problems due to reclaim throttling for excessive lengths of time.
> In Alexey's case, a memory hog that should go OOM quickly stalls for
> several minutes before stalling. In Mike and Darrick's cases, a small
> memcg environment stalled excessively even though the system had enough
> memory overall.
> 
> Commit 69392a403f49 ("mm/vmscan: throttle reclaim when no progress is being
> made") introduced the problem although commit a19594ca4a8b ("mm/vmscan:
> increase the timeout if page reclaim is not making progress") made it
> worse. Systems at or near an OOM state that cannot be recovered must
> reach OOM quickly and memcg should kill tasks if a memcg is near OOM.
> 
> To address this, only stall for the first zone in the zonelist, reduce
> the timeout to 1 tick for VMSCAN_THROTTLE_NOPROGRESS and only stall if
> the scan control nr_reclaimed is 0 and kswapd is still active.  If kswapd
> has stopped reclaiming due to excessive failures, do not stall at all so
> that OOM triggers relatively quickly.
> 
> Alexey's test case was the most straight forward
> 
> 	for i in {1..3}; do tail /dev/zero; done
> 
> On vanilla 5.16-rc1, this test stalled and was reset after 10 minutes.
> After the patch, the test gets killed after roughly 15 seconds which is
> the same length of time taken in 5.15.
> 
> Link: https://lore.kernel.org/r/99e779783d6c7fce96448a3402061b9dc1b3b602.camel@xxxxxx
> Link: https://lore.kernel.org/r/20211124011954.7cab9bb4@xxxxxxxxxxxxx
> Link: https://lore.kernel.org/r/20211022144651.19914-1-mgorman@xxxxxxxxxxxxxxxxxxx

Should probably include Reported-by: tags too?

> Fixes: 69392a403f49 ("mm/vmscan: throttle reclaim when no progress is being made")
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Tested-by: Darrick J. Wong <djwong@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/vmscan.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index fb9584641ac7..176ddd28df21 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1057,7 +1057,17 @@ void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason)
>  
>  		break;
>  	case VMSCAN_THROTTLE_NOPROGRESS:
> -		timeout = HZ/2;
> +		timeout = 1;
> +
> +		/*
> +		 * If kswapd is disabled, reschedule if necessary but do not
> +		 * throttle as the system is likely near OOM.
> +		 */
> +		if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES) {
> +			cond_resched();
> +			return;
> +		}
> +
>  		break;
>  	case VMSCAN_THROTTLE_ISOLATED:
>  		timeout = HZ/50;
> @@ -3395,7 +3405,7 @@ static void consider_reclaim_throttle(pg_data_t *pgdat, struct scan_control *sc)
>  		return;
>  
>  	/* Throttle if making no progress at high prioities. */
> -	if (sc->priority < DEF_PRIORITY - 2)
> +	if (sc->priority < DEF_PRIORITY - 2 && !sc->nr_reclaimed)
>  		reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS);
>  }
>  
> @@ -3415,6 +3425,7 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
>  	unsigned long nr_soft_scanned;
>  	gfp_t orig_mask;
>  	pg_data_t *last_pgdat = NULL;
> +	pg_data_t *first_pgdat = NULL;
>  
>  	/*
>  	 * If the number of buffer_heads in the machine exceeds the maximum
> @@ -3478,14 +3489,18 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
>  			/* need some check for avoid more shrink_zone() */
>  		}
>  
> +		if (!first_pgdat)
> +			first_pgdat = zone->zone_pgdat;
> +
>  		/* See comment about same check for global reclaim above */
>  		if (zone->zone_pgdat == last_pgdat)
>  			continue;
>  		last_pgdat = zone->zone_pgdat;
>  		shrink_node(zone->zone_pgdat, sc);
> -		consider_reclaim_throttle(zone->zone_pgdat, sc);
>  	}
>  
> +	consider_reclaim_throttle(first_pgdat, sc);
> +
>  	/*
>  	 * Restore to original mask to avoid the impact on the caller if we
>  	 * promoted it to __GFP_HIGHMEM.
>