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

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Date:	Mon, 08 Apr 2013 10:28:45 -0700
From:	Cody P Schafer <cody@...ux.vnet.ibm.com>
To:	Gilad Ben-Yossef <gilad@...yossef.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] mm: when handling percpu_pagelist_fraction, use on_each_cpu()
 to set percpu pageset fields.

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.

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