Re: 4.2.2: NR_CPUS effectively being 1 bug

[Mike Galbraith]

On Sat, 2015-10-03 at 09:38 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxx> 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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