linux-kernel - Re: [PATCH 3/3] mm: when handling percpu_pagelist_fraction, use on_each

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Date:	Mon, 08 Apr 2013 18:23:49 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Cody P Schafer <cody@...ux.vnet.ibm.com>
CC:	Gilad Ben-Yossef <gilad@...yossef.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, kosaki.motohiro@...il.com
Subject: Re: [PATCH 3/3] mm: when handling percpu_pagelist_fraction, use on_each_cpu()
 to set percpu pageset fields.

(4/8/13 3:50 PM), Cody P Schafer wrote:
> On 04/08/2013 10:28 AM, Cody P Schafer wrote:
>> On 04/08/2013 05:20 AM, Gilad Ben-Yossef wrote:
>>> On Fri, Apr 5, 2013 at 11:33 PM, Cody P Schafer
>>> <cody@...ux.vnet.ibm.com> wrote:
>>>> In free_hot_cold_page(), we rely on pcp->batch remaining stable.
>>>> Updating it without being on the cpu owning the percpu pageset
>>>> potentially destroys this stability.
>>>>
>>>> Change for_each_cpu() to on_each_cpu() to fix.
>>>
>>> Are you referring to this? -
>>
>> This was the case I noticed.
>>
>>>
>>> 1329         if (pcp->count >= pcp->high) {
>>> 1330                 free_pcppages_bulk(zone, pcp->batch, pcp);
>>> 1331                 pcp->count -= pcp->batch;
>>> 1332         }
>>>
>>> I'm probably missing the obvious but won't it be simpler to do this in
>>>   free_hot_cold_page() -
>>>
>>> 1329         if (pcp->count >= pcp->high) {
>>> 1330                  unsigned int batch = ACCESS_ONCE(pcp->batch);
>>> 1331                 free_pcppages_bulk(zone, batch, pcp);
>>> 1332                 pcp->count -= batch;
>>> 1333         }
>>>
>>
>> Potentially, yes. Note that this was simply the one case I noticed,
>> rather than certainly the only case.
>>
>> I also wonder whether there could be unexpected interactions between
>> ->high and ->batch not changing together atomically. For example, could
>> adjusting this knob cause ->batch to rise enough that it is greater than
>> the previous ->high? If the code above then runs with the previous
>> ->high, ->count wouldn't be correct (checking this inside
>> free_pcppages_bulk() might help on this one issue).
>>
>>> Now the batch value used is stable and you don't have to IPI every CPU
>>> in the system just to change a config knob...
>>
>> Is this really considered an issue? I wouldn't have expected someone to
>> adjust the config knob often enough (or even more than once) to cause
>> problems. Of course as a "It'd be nice" thing, I completely agree.
> 
> Would using schedule_on_each_cpu() instead of on_each_cpu() be an 
> improvement, in your opinion?

No. As far as lightweight solusion work, we shouldn't introduce heavyweight
code never. on_each_cpu() is really heavy weight especially when number of 
cpus are much than a thousand.


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