linux-kernel - Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to helpers

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Date:	Tue, 12 Apr 2016 13:53:47 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Joonsoo Kim <js1304@...il.com>,
	Hillf Danton <hillf.zj@...baba-inc.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/11] mm, compaction: Abstract compaction feedback to
 helpers

On 04/11/2016 05:14 PM, Michal Hocko wrote:
> On Mon 11-04-16 16:39:21, Vlastimil Babka wrote:
>> On 04/05/2016 01:25 PM, Michal Hocko wrote:
> [...]
>>> +/* Compaction has failed and it doesn't make much sense to keep retrying. */
>>> +static inline bool compaction_failed(enum compact_result result)
>>> +{
>>> +	/* All zones where scanned completely and still not result. */
>>
>> Hmm given that try_to_compact_pages() uses a max() on results, then in fact
>> it takes only one zone to get this. Others could have been also SKIPPED or
>> DEFERRED. Is that what you want?
>
> In short I didn't find any better way and still guarantee a some
> guarantee of convergence. COMPACT_COMPLETE means that at least one zone
> was completely scanned and led to no result. That zone would be
> compact_suitable by definition. If I made DEFERRED or SKIPPED more
> priorite (aka higher in the enum) then I could easily end up in a state
> where all zones would return COMPACT_COMPLETE and few remaining would
> just alternate returning their DEFFERED resp. SKIPPED. So while this
> might sound like giving up too early I couldn't come up with anything
> more specific that would lead to reliable results.
>
> I am open to any suggestions of course.

I guess you would have to track each zone separately and make sure 
you've seen COMPACT_COMPLETE in all of them, although not necessary 
during the same zonelist attempt. But then do the same for reclaim, as 
you would also have to match COMPAT_SKIPPED and inability of reclaim... 
and that gets uglier and uglier, and also against the move to node-based 
reclaim...

So there's a danger that you'll see COMPACT_COMPLETE on a small ZONE_DMA 
early on, before the larger zones even stop being deferred, but I don't 
see an easy solution.