Date:   Tue, 30 Nov 2021 11:22:44 +0000
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Mike Galbraith <efault@gmx.de>
Cc:     Alexey Avramov <hakavlad@inbox.lv>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Rik van Riel <riel@surriel.com>,
        Darrick Wong <djwong@kernel.org>, regressions@lists.linux.dev,
        Linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/1] mm: vmscan: Reduce throttling due to a failure to
 make progress
Message-ID: <20211130112244.GQ3366@techsingularity.net>
References: <20211125151853.8540-1-mgorman@techsingularity.net>
 <20211127011246.7a8ac7b8@mail.inbox.lv>
 <20211129150117.GO3366@techsingularity.net>
 <a20f17c4b1b5fdfade3f48375d148e97bd162dd6.camel@gmx.de>
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On Tue, Nov 30, 2021 at 11:14:32AM +0100, Mike Galbraith wrote:
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index fb9584641ac7..1af12072f40e 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1021,6 +1021,39 @@ static void handle_write_error(struct address_space *mapping,
> > ????????unlock_page(page);
> > ?}
> > ?
> > +bool skip_throttle_noprogress(pg_data_t *pgdat)
> > +{
> > +???????int reclaimable = 0, write_pending = 0;
> > +???????int i;
> > +
> > +???????/*
> > +??????? * 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)
> > +???????????????return true;
> > +
> > +???????/*
> > +??????? * If there are a lot of dirty/writeback pages then do not
> > +??????? * throttle as throttling will occur when the pages cycle
> > +??????? * towards the end of the LRU if still under writeback.
> > +??????? */
> > +???????for (i = 0; i < MAX_NR_ZONES; i++) {
> > +???????????????struct zone *zone = pgdat->node_zones + i;
> > +
> > +???????????????if (!populated_zone(zone))
> > +???????????????????????continue;
> > +
> > +???????????????reclaimable += zone_reclaimable_pages(zone);
> > +???????????????write_pending += zone_page_state_snapshot(zone,
> > +???????????????????????????????????????????????? NR_ZONE_WRITE_PENDING);
> > +???????}
> > +???????if (2 * write_pending <= reclaimable)
> 
> That is always true here...
> 

Always true for you or always true in general?

The intent of the check is "are a majority of reclaimable pages
marked WRITE_PENDING?". It's similar to the check that existed prior
to 132b0d21d21f ("mm/page_alloc: remove the throttling logic from the
page allocator").

-- 
Mel Gorman
SUSE Labs