Subject: Re: [PATCH v2 4/5] mm, kswapd: replace kswapd compaction with waking
 up kcompactd
To: Joonsoo Kim <js1304@gmail.com>
References: <1454938691-2197-1-git-send-email-vbabka@suse.cz>
 <1454938691-2197-5-git-send-email-vbabka@suse.cz>
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 <56D6F41D.9080107@suse.cz>
 <CAAmzW4PGgYkL9xnCXgSQ=8kW0sJkaYyrxenb_XKHcW1wDGMEyw@mail.gmail.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Rik van Riel <riel@redhat.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        David Rientjes <rientjes@google.com>, Michal Hocko <mhocko@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <56D6FB77.2090801@suse.cz>
Date: Wed, 2 Mar 2016 15:40:55 +0100
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On 03/02/2016 03:22 PM, Joonsoo Kim wrote:
> 2016-03-02 23:09 GMT+09:00 Vlastimil Babka <vbabka@suse.cz>:
>> On 03/02/2016 02:57 PM, Joonsoo Kim wrote:
>>>
>>>
>>> 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.

So I understand that patch 5 would be just about this?

-	if (compaction_restarting(zone, cc->order) && !current_is_kcompactd())
+	if (compaction_restarting(zone, cc->order))
 		__reset_isolation_suitable(zone);

I'm more inclined to fold it in that case. 

> 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.
>>