Messages in this thread | | | Subject | Re: 4.2.2: NR_CPUS effectively being 1 bug | From | Mike Galbraith <> | Date | Sat, 03 Oct 2015 10:15:48 +0200 |
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On Sat, 2015-10-03 at 09:38 +0200, Ingo Molnar wrote: > * Peter Zijlstra <peterz@infradead.org> wrote: > > > On Fri, Oct 02, 2015 at 02:00:33PM -0700, Andrew Morton wrote: > > > On Fri, 2 Oct 2015 23:46:59 +0300 Alexey Dobriyan <adobriyan@gmail.com> wrote: > > > > > > > FYI, I've upgraded from 4.1.7 to 4.2.1 (and retested with 4.2.2) and > > > > everything is scheduled on 1 CPU out of 4 (i5 760). > > > > > > > > $ sudo cat /proc/1/status | grep cpu -i > > > > Cpus_allowed: 1 > > > > Cpus_allowed_list: 0 > > > > > > > > Every process inherits this tiny cpumask. > > > > > > Sell the other CPUs on ebay? > > > > > > I haven't seen such a report before - maybe it rings a bell with Peter > > > & Ingo? > > > > I think this is related to some NO_HZ_FULL quackery. People seem to have > > enabled stuff they've really no sane reason for. > > So the question is, is CONFIG_NO_HZ_FULL_ALL=y enabled? If yes then please disable > it. > > Frederic, is there a fix for that? The Kconfig help text for CONFIG_NO_HZ_FULL_ALL > says:: > > CONFIG_NO_HZ_FULL_ALL: > > If the user doesn't pass the nohz_full boot option to > define the range of full dynticks CPUs, consider that all > CPUs in the system are full dynticks by default. > Note the boot CPU will still be kept outside the range to > handle the timekeeping duty. > > I can see people enabling that. Why are all CPUs lost if it's done?
Simple. Rik made it such that cpu_isolated_map is immune to cpusets, and Chris made tick_nohz_full_mask automatically set cpu_isolated_map, so now if nohz_full is ever turned on, that CPU is gone from the generic pool forever, with obvious consequences for CONFIG_NO_HZ_FULL_ALL.
Lovely eh?
-Mike
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