linux-kernel - Re: [PATCH v2 4/5] mm, kswapd: replace kswapd compaction with waking up kcompactd

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Date:	Wed, 2 Mar 2016 15:09:33 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <js1304@...il.com>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	David Rientjes <rientjes@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v2 4/5] mm, kswapd: replace kswapd compaction with waking
 up kcompactd

On 03/02/2016 02:57 PM, Joonsoo Kim wrote:
> 2016-03-02 19:04 GMT+09:00 Vlastimil Babka <vbabka@...e.cz>:
>> On 03/02/2016 07:33 AM, Joonsoo Kim wrote:
>>>
>>>
>>> Why you did the test with THP? THP interferes result of main test so
>>> it would be better not to enable it.
>>
>>
>> Hmm I've always left it enabled. It makes for a more realistic interference
>> and would also show unintended regressions in that closely related area.
>
> But, it makes review hard because complex analysis is needed to
> understand the result.
>
> Following is the example.
>
> "The compaction stalls
> (direct compaction) in the interfering kernel builds (probably THP's) also
> decreased somewhat to kcompactd activity, yet THP alloc successes improved a
> bit."
>
> So, why do we need this comment to understand effect of this patch? If you did
> a test without THP, it would not be necessary.

I see. Next time I'll do a run with THP disabled.

>>> And, this patch increased compaction activity (10 times for migrate
>>> scanned)
>>> may be due to resetting skip block information.
>>
>>
>> Note that kswapd compaction activity was completely non-existent for reasons
>> outlined in the changelog.
>>> Isn't is better to disable it
>>> for this patch to work as similar as possible that kswapd does and
>>> re-enable it
>>> on next patch? If something goes bad, it can simply be reverted.
>>>
>>> Look like it is even not mentioned in the description.
>>
>>
>> Yeah skip block information is discussed in the next patch, which mentions
>> that it's being reset and why. I think it makes more sense, as when kswapd
>
> Yes, I know.
> What I'd like to say here is that you need to care current_is_kswapd() in
> this patch. This patch unintentionally change the back ground compaction thread
> behaviour to restart compaction by every 64 trials because calling
> curret_is_kswapd()
 > by kcompactd would return false and is treated as direct reclaim.

Oh, you mean this path to reset the skip bits. I see. But if skip bits 
are already reset by kswapd when waking kcompactd, then effect of 
another (rare) reset in kcompactd itself will be minimal?

> Result of patch 4
> and patch 5 would be same.

It's certainly possible to fold patch 5 into 4. I posted them separately 
mainly to make review more feasible. But the differences in results are 
already quite small.