Messages in this thread | | | Date | Mon, 08 Apr 2013 12:50:33 -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 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?
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