linux-kernel - Re: [PATCH 3/8] mm/vmscan: Throttle reclaim when no progress is being made

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Message-ID: <63336163-e709-65de-6d53-8764facd3924@suse.cz>
Date:   Thu, 14 Oct 2021 14:31:17 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Linux-MM <linux-mm@...ck.org>
Cc:     NeilBrown <neilb@...e.de>, Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Dave Chinner <david@...morbit.com>,
        Rik van Riel <riel@...riel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/8] mm/vmscan: Throttle reclaim when no progress is being
 made

On 10/8/21 15:53, Mel Gorman wrote:
> Memcg reclaim throttles on congestion if no reclaim progress is made.
> This makes little sense, it might be due to writeback or a host of
> other factors.
> 
> For !memcg reclaim, it's messy. Direct reclaim primarily is throttled
> in the page allocator if it is failing to make progress. Kswapd
> throttles if too many pages are under writeback and marked for
> immediate reclaim.
> 
> This patch explicitly throttles if reclaim is failing to make progress.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
...
> @@ -3769,6 +3797,16 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
>  	trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
>  	set_task_reclaim_state(current, NULL);
>  
> +	if (!nr_reclaimed) {
> +		struct zoneref *z;
> +		pg_data_t *pgdat;
> +
> +		z = first_zones_zonelist(zonelist, sc.reclaim_idx, sc.nodemask);
> +		pgdat = zonelist_zone(z)->zone_pgdat;
> +
> +		reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS, HZ/10);
> +	}

Is this necessary? AFAICS here we just returned from:

do_try_to_free_pages()
  shrink_zones()
   for_each_zone()...
     consider_reclaim_throttle()

Which already throttles when needed and using the appropriate pgdat, while
here we have to somewhat awkwardly assume the preferred one.

> +
>  	return nr_reclaimed;
>  }
>  #endif
>