linux-kernel - Re: [PATCH] sched/core: fix formatting issues in sched_can_stop

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Date:	Tue, 15 Apr 2014 00:48:18 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com
Subject: Re: [PATCH] sched/core: fix formatting issues in
 sched_can_stop_tick()

On Mon, Apr 14, 2014 at 08:38:38PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 14, 2014 at 09:47:41PM +0530, Viresh Kumar wrote:
> > sched_can_stop_tick() was using 7 spaces instead of 8 spaces or a 'tab' at the
> > beginning of each line. Which doesn't align with the Coding Guidelines.
> > 
> > Also it removes the *rq variable as it was used at only one place and hence we
> > can directly use this_rq() instead.
> > 
> > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> > ---
> > I don't think rq = tihs_rq() has to be done before smp_mb(), in case yes sorry
> > for this patch :(
> > 
> >  kernel/sched/core.c | 16 ++++++----------
> >  1 file changed, 6 insertions(+), 10 deletions(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 268a45e..13299c5 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -666,18 +666,14 @@ static inline bool got_nohz_idle_kick(void)
> >  #ifdef CONFIG_NO_HZ_FULL
> >  bool sched_can_stop_tick(void)
> >  {
> > +	/* Make sure rq->nr_running update is visible after the IPI */
> > +	smp_rmb();
> >  
> > +	/* More than one running task need preemption */
> > +	if (this_rq()->nr_running > 1)
> > +		return false;
> >  
> > +	return true;
> >  }
> >  #endif /* CONFIG_NO_HZ_FULL */
> 
> AFAICT the smp_rmb() is entirely spurious, arch interrupts should ensure
> consistency on their own. That is:
> 
>   CPU 0		  CPU 1
> 
> [w] X = 1
>     IPI 1	    <int>
> 		[r] r = X
> 
> Should act as if there was a full memory barrier, making it so that the
> read on CPU1 observes the write on CPU0.

Right, I have a pending patch for that:
https://git.kernel.org/cgit/linux/kernel/git/frederic/linux-dynticks.git/commit/?h=nohz/ipi&id=ca981d9f87fe0f113ad972098cfe181180b3675a
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