Messages in this thread | | | Date | Mon, 08 Apr 2013 10:28:45 -0700 | From | Cody P Schafer <> | Subject | Re: [PATCH 3/3] mm: when handling percpu_pagelist_fraction, use on_each_cpu() to set percpu pageset fields. |
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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.
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