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Subject: Re: [PATCH 1/2] sched/wait: Break up long wake list walk
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From: Tim Chen <tim.c.chen@linux.intel.com>
Message-ID: <6e8b81de-e985-9222-29c5-594c6849c351@linux.intel.com>
Date: Wed, 23 Aug 2017 08:58:37 -0700
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To: "Liang, Kan" <kan.liang@intel.com>, Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@techsingularity.net>, Mel Gorman <mgorman@suse.de>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>, Andi Kleen <ak@linux.intel.com>, Andrew Morton <akpm@linux-foundation.org>, Johannes Weiner <hannes@cmpxchg.org>, Jan Kara <jack@suse.cz>, linux-mm <linux-mm@kvack.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>

On 08/23/2017 07:49 AM, Liang, Kan wrote:
>> On Tue, Aug 22, 2017 at 12:55 PM, Liang, Kan <kan.liang@intel.com> wrote:
>>>
>>>> So I propose testing the attached trivial patch.
>>>
>>> It doesna??t work.
>>> The call stack is the same.
>>
>> So I would have expected the stack trace to be the same, and I would even
>> expect the CPU usage to be fairly similar, because you'd see repeating from
>> the callers (taking the fault again if the page is - once again - being migrated).
>>
>> But I was hoping that the wait queues would be shorter because the loop for
>> the retry would be bigger.
>>
>> Oh well.
>>
>> I'm slightly out of ideas. Apparently the yield() worked ok (apart from not
>> catching all cases), and maybe we could do a version that waits on the page
>> bit in the non-contended case, but yields under contention?
>>
>> IOW, maybe this is the best we can do for now? Introducing that
>> "wait_on_page_migration()" helper might allow us to tweak this a bit as
>> people come up with better ideas..
> 
> The "wait_on_page_migration()" helper works well in the overnight testing.
> 


Linus,

Will you still consider the original patch as a fail safe mechanism?

Thanks.

Tim

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