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

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Date:   Thu, 14 Oct 2021 17:45:17 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Linux-MM <linux-mm@...ck.org>, 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 10/14/21 15:03, Mel Gorman wrote:
> On Thu, Oct 14, 2021 at 02:31:17PM +0200, Vlastimil Babka wrote:
>> 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.
>> 
> 
> Yes, you're right, consider_reclaim_throttle not only throttles on the
> appropriate pgdat but takes priority into account.
> 
> Well spotted!

So with that part removed
Acked-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!