linux-kernel - Re: CONFIG_NO_HZ_FULL restricts cpu usage to the equivalent of one in 4.2

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Date:	Sat, 18 Jul 2015 15:26:14 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ken Moffat <zarniwhoop@...world.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Jeff Epler <jepler@...ythonic.net>
Subject: Re: CONFIG_NO_HZ_FULL restricts cpu usage to the equivalent of one
 in 4.2

On Thu, Jul 16, 2015 at 01:32:18AM +0100, Ken Moffat wrote:
> On Wed, Jul 15, 2015 at 09:11:46PM +0200, Frederic Weisbecker wrote:
> > 2015-07-15 18:27 GMT+02:00 Ken Moffat <zarniwhoop@...world.com>:
> > >
> > > The config differences follow.  Perhaps it is actually one of the
> > > subsequent choices that is the problem.  And I guess it could still
> > > be a gcc-5.1 issue.
> > >
> > > --- config-4.2-initial  2015-07-15 16:25:12.548005751 +0100
> > > +++ config-4.2-speed-ok 2015-07-15 17:00:50.919998703 +0100
> > > @@ -104,11 +104,8 @@
> > >  CONFIG_TICK_ONESHOT=y
> > >  CONFIG_NO_HZ_COMMON=y
> > >  # CONFIG_HZ_PERIODIC is not set
> > > -# CONFIG_NO_HZ_IDLE is not set
> > > -CONFIG_NO_HZ_FULL=y
> > > -CONFIG_NO_HZ_FULL_ALL=y
> > 
> > You had CONFIG_NO_HZ_FULL_ALL enabled? Because that would indeed
> > produce that effect since it isolates all CPUs but 0 off sched
> > domains.
> > 
> > Which means that basically only CPU 0 runs user tasks unless you
> > forces these otherwise.
> 
> Thanks.  I'll put it down to a bad .config choice, although it was
> fine on early 4.1.

Yeah we decided to include the nohz_full on cpu_isolated_map recently,
except CPU 0.

> While I was starting to bisect, I noticed that
> on the A10 everything was happening on CPU 0 - not sure if that was
> happening on the original box, but for the moment it sounds likely.
> 
> ĸen
> -- 
> This one goes up to eleven!
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