linux-kernel - Re: mmotm woes, mainly compaction

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Date:	Mon, 18 Apr 2016 15:14:13 -0400
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Hugh Dickins <hughd@...gle.com>, Michal Hocko <mhocko@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: mmotm woes, mainly compaction

On 04/14/2016 07:24 PM, Vlastimil Babka wrote:
>> > @@ -1459,8 +1459,8 @@ static enum compact_result compact_zone(
>> >   		zone->compact_cached_migrate_pfn[1] = cc->migrate_pfn;
>> >   	}
>> >
>> > -	if (cc->migrate_pfn == start_pfn)
>> > -		cc->whole_zone = true;
>> > +	cc->whole_zone = cc->migrate_pfn == start_pfn &&
>> > +			cc->free_pfn == pageblock_start_pfn(end_pfn - 1);
>> >
>> >   	cc->last_migrated_pfn = 0;
> This would be for Michal, but I agree.

So there's an alternative here that wouldn't have the danger of missing
cc->whole_zone multiple time due to races. When resetting either one
scanner to zone boundary, reset the other as well (and set
cc->whole_zone). I think the situations, where not doing that have any
(performance) advantage, are rare.