On Sat, 2015-10-03 at 09:38 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <pet...@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 <adobri...@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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