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 23:22:11 +0900
From:	Joonsoo Kim <js1304@...il.com>
To:	Vlastimil Babka <vbabka@...e.cz>
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

2016-03-02 23:09 GMT+09:00 Vlastimil Babka <vbabka@...e.cz>:
> 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?

If you care current_is_kswapd() in this patch properly (properly means change
like "current_is_kcompactd()), reset in kswapd would not
happen because, compact_blockskip_flush would not be set by kcompactd.

In this case, patch 5 would have it's own meaning so cannot be folded.

Thanks.

>> 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.
>