Re: [PATCH 3/3] mm: when handling percpu_pagelist_fraction, use on_each_cpu() to set percpu pageset fields.

[Cody P Schafer <cody@linux.vnet.ibm.com>]

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@linux.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?

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>