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Date: Tue, 2 May 2017 10:02:47 +0200
From: Michal Hocko <mhocko@kernel.org>
To: David Rientjes <rientjes@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Minchan Kim <minchan@kernel.org>, Johannes Weiner <hannes@cmpxchg.org>,
        Mel Gorman <mgorman@techsingularity.net>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [patch v2] mm, vmscan: avoid thrashing anon lru when free + file
 is low
Message-ID: <20170502080246.GD14593@dhcp22.suse.cz>
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On Mon 01-05-17 14:34:21, David Rientjes wrote:
[...]
> @@ -2204,8 +2204,17 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
>  		}
>  
>  		if (unlikely(pgdatfile + pgdatfree <= total_high_wmark)) {
> -			scan_balance = SCAN_ANON;
> -			goto out;
> +			/*
> +			 * Force SCAN_ANON if there are enough inactive
> +			 * anonymous pages on the LRU in eligible zones.
> +			 * Otherwise, the small LRU gets thrashed.
> +			 */
> +			if (!inactive_list_is_low(lruvec, false, sc, false) &&
> +			    lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, sc->reclaim_idx)
> +					>> sc->priority) {
> +				scan_balance = SCAN_ANON;
> +				goto out;
> +			}

I have already asked and my questions were ignored. So let me ask again
and hopefuly not get ignored this time. So Why do we need a different
criterion on anon pages than file pages? I do agree that blindly
scanning anon pages when file pages are low is very suboptimal but this
adds yet another heuristic without _any_ numbers. Why cannot we simply
treat anon and file pages equally? Something like the following

	if (pgdatfile + pgdatanon + pgdatfree > 2*total_high_wmark) {
		scan_balance = SCAN_FILE;
		if (pgdatfile < pgdatanon)
			scan_balance = SCAN_ANON;
		goto out;
	}

Also it would help to describe the workload which can trigger this
behavior so that we can compare numbers before and after this patch.
-- 
Michal Hocko
SUSE Labs