linux-kernel - Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file is low

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Date:   Thu, 4 May 2017 13:43:58 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file
 is low

On Wed 03-05-17 15:52:04, David Rientjes wrote:
> On Wed, 3 May 2017, Michal Hocko wrote:
[...]
> >  	/*
> > -	 * If there is enough inactive page cache, i.e. if the size of the
> > -	 * inactive list is greater than that of the active list *and* the
> > -	 * inactive list actually has some pages to scan on this priority, we
> > -	 * do not reclaim anything from the anonymous working set right now.
> > -	 * Without the second condition we could end up never scanning an
> > -	 * lruvec even if it has plenty of old anonymous pages unless the
> > -	 * system is under heavy pressure.
> > +	 * Make sure there are enough pages on the biased LRU before we go
> > +	 * and do an exclusive reclaim from that list, i.e. if the
> > +	 * size of the inactive list is greater than that of the active list
> > +	 * *and* the inactive list actually has some pages to scan on this
> > +	 * priority.
> > +	 * Without the second condition we could end up never scanning other
> > +	 * lruvecs even if they have plenty of old pages unless the system is
> > +	 * under heavy pressure.
> >  	 */
> > -	if (!inactive_list_is_low(lruvec, true, memcg, sc, false) &&
> > -	    lruvec_lru_size(lruvec, LRU_INACTIVE_FILE, sc->reclaim_idx) >> sc->priority) {
> > -		scan_balance = SCAN_FILE;
> > +	lru = LRU_INACTIVE_ANON + LRU_FILE * (scan_balance == SCAN_FILE);
> 
> This part seems to complicate the logic since it determines the lru under 
> test based on the current setting of scan_balance.  I think I prefer 
> individual heuristics with well written comments, but others may feel 
> differently about this.

I do not claim the code would more obvious than before but it gets rid
of the duplication which is usually a good thing. This size check has
the same reasoning regardless of the type of the LRU. But I am not going
to insist...
 
> > +	if (!inactive_list_is_low(lruvec, is_file_lru(lru), memcg, sc, false) &&
> > +	    lruvec_lru_size(lruvec, lru, sc->reclaim_idx) >> sc->priority)
> >  		goto out;
> > -	}
> >  
> >  	scan_balance = SCAN_FRACT;
> >  

-- 
Michal Hocko
SUSE Labs