linux-kernel - Re: [PATCH v4 1/1] mm: vmscan: Reduce throttling due to a failure to make progress

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Date:   Wed, 8 Dec 2021 22:20:47 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Alexey Avramov <hakavlad@...ox.lv>,
        Rik van Riel <riel@...riel.com>,
        Mike Galbraith <efault@....de>,
        Darrick Wong <djwong@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        regressions@...ts.linux.dev,
        Linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/1] mm: vmscan: Reduce throttling due to a failure
 to make progress

On Thu, 2 Dec 2021, Mel Gorman wrote:
...
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
...
> @@ -3478,14 +3520,18 @@ 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);

My tmpfs swapping load (tweaked to use huge pages more heavily than
in real life) is far from being a realistic load: but it was notably
slowed down by your throttling mods in 5.16-rc, and this patch makes
it well again - thanks.

But: it very quickly hit NULL pointer until I changed that last line to

	if (first_pgdat)
		consider_reclaim_throttle(first_pgdat, sc);

I've given no thought as to whether that is the correct fix,
or if first_pgdat should be set earlier in the loop above.

Hugh