linux-kernel - Re: [PATCH 4/4] nohz: Convert a few places to use local per cpu accesses

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Date:	Fri, 16 Aug 2013 18:34:39 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH 4/4] nohz: Convert a few places to use local per cpu
 accesses

On Fri, Aug 16, 2013 at 06:19:54PM +0200, Oleg Nesterov wrote:
> On 08/16, Peter Zijlstra wrote:
> >
> > OK so these patches look ok to me -- didn't read in detail though.
> >
> > On Fri, Aug 16, 2013 at 05:42:33PM +0200, Frederic Weisbecker wrote:
> > > A few functions use remote per CPU access APIs when they
> > > deal with local values.
> > >
> > > Just to the right conversion to improve performance, code
> > > readability and debug checks.
> >
> > > @@ -410,7 +408,7 @@ static void tick_nohz_update_jiffies(ktime_t now)
> > >
> > >  static void tick_nohz_stop_idle(int cpu, ktime_t now)
> > >  {
> > > -	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> > > +	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
> >
> > What's there from stopping someone from calling this with cpu !=
> > smp_processor_id() ?
> 
> I _guess_ this should not happen, but in this case we should probably
> remove the "cpu" argument. And smp_processor_id() in tick_nohz_idle_exit().
> 
> tick_check_idle/tick_check_nohz doesn't need "int cpu" too, it seems.

Yep, indeed it seems right to remove the cpu argument to prevent from further
calls to these APIs on remote CPUs, which would otherwise result in random disaster.

> 
> Oleg.
> 
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