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Date:	Tue, 21 Dec 2010 02:55:07 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 13/15] sched: Protect nohz task cpu affinity

On Mon, Dec 20, 2010 at 12:05:30PM -0500, Steven Rostedt wrote:
> On Mon, 2010-12-20 at 17:28 +0100, Peter Zijlstra wrote:
> > On Mon, 2010-12-20 at 16:24 +0100, Frederic Weisbecker wrote:
> > > Don't allow to change a nohz task cpu affinity as we want them
> > > to be bound to a single CPU and we want this affinity not to
> > > change.
> > > 
> > > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > > Cc: Thomas Gleixner <tglx@...utronix.de>
> > > Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > Cc: Ingo Molnar <mingo@...e.hu>
> > > Cc: Steven Rostedt <rostedt@...dmis.org>
> > > Cc: Lai Jiangshan <laijs@...fujitsu.com>
> > > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > > Cc: Anton Blanchard <anton@....ibm.com>
> > > Cc: Tim Pepper <lnxninja@...ux.vnet.ibm.com>
> > > ---
> > >  kernel/sched.c |    7 +++++++
> > >  1 files changed, 7 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/kernel/sched.c b/kernel/sched.c
> > > index 4412493..bd0a41f 100644
> > > --- a/kernel/sched.c
> > > +++ b/kernel/sched.c
> > > @@ -5712,6 +5712,13 @@ again:
> > >  		goto out;
> > >  	}
> > >  
> > > +	/* Nohz tasks must keep their affinity */
> > > +	if (test_tsk_thread_flag(p, TIF_NOHZ) &&
> > > +	    !cpumask_equal(&p->cpus_allowed, new_mask)) {
> > > +		ret = -EBUSY;
> > > +		goto out;
> > > +	}
> > > +
> > >  	if (p->sched_class->set_cpus_allowed)
> > >  		p->sched_class->set_cpus_allowed(p, new_mask);
> > >  	else {
> > 
> > NAK, this is really way too restrictive.
> 
> Agreed, the better solution is to disable the nohz from the task. If the
> use just changed its affinity (or something else did), disable the nohz.
> Maybe you can add a printk or warning, but I'm not sure about that
> either.

Right. Or even better: don't force the nohz task to be affine to
a single cpu.
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