linux-kernel - Re: [PATCH 11/31] mm: vmscan: do not reclaim from kswapd if there is any eligible zone

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Message-ID: <20160705103806.GH11498@techsingularity.net>
Date:	Tue, 5 Jul 2016 11:38:06 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...riel.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/31] mm: vmscan: do not reclaim from kswapd if there is
 any eligible zone

On Tue, Jul 05, 2016 at 03:11:17PM +0900, Minchan Kim wrote:
> > -		if (i < 0)
> > -			goto out;
> > +		/*
> > +		 * Only reclaim if there are no eligible zones. Check from
> > +		 * high to low zone to avoid prematurely clearing pgdat
> > +		 * congested state.
> 
> I cannot understand "prematurely clearing pgdat congested state".
> Could you add more words to clear it out?
> 

It's surprisingly difficult to concisely explain. Is this any better?

                /*
                 * Only reclaim if there are no eligible zones. Check from
                 * high to low zone as allocations prefer higher zones.
                 * Scanning from low to high zone would allow congestion to be
                 * cleared during a very small window when a small low
                 * zone was balanced even under extreme pressure when the
                 * overall node may be congested.
                 */
> > +		 */
> > +		for (i = classzone_idx; i >= 0; i--) {
> > +			zone = pgdat->node_zones + i;
> > +			if (!populated_zone(zone))
> > +				continue;
> > +
> > +			if (zone_balanced(zone, sc.order, classzone_idx))
> 
> If buffer_head is over limit, old logic force to reclaim highmem but
> this zone_balanced logic will prevent it.
> 

The old logic was always busted on 64-bit because is_highmem would always
be 0. The original intent appears to be that buffer_heads_over_limit
would release the buffers when pages went inactive. There are a number
of things we treated inconsistently that get fixed up in the series and
buffer_heads_over_limit is one of them.

-- 
Mel Gorman
SUSE Labs