linux-kernel - Re: 4.2.2: NR_CPUS effectively being 1 bug

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Date:	Sat, 3 Oct 2015 09:38:13 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>,
	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: 4.2.2: NR_CPUS effectively being 1 bug


* Peter Zijlstra <peterz@...radead.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@...il.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?

Thanks,

	Ingo
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