linux-kernel - Re: [PATCH 1/3] sched: Add new API wake_up_if

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Message-ID: <20140903095410.GJ4783@worktop.ger.corp.intel.com>
Date:	Wed, 3 Sep 2014 11:54:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Chuansheng Liu <chuansheng.liu@...el.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"Wang, Xiaoming" <xiaoming.wang@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	"Chakravarty, Souvik K" <souvik.k.chakravarty@...el.com>,
	"Liu, Changcheng" <changcheng.liu@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] sched: Add new API wake_up_if_idle() to wake up the
 idle cpu

On Thu, Aug 21, 2014 at 02:21:51PM -0700, Andy Lutomirski wrote:
> On Aug 18, 2014 3:52 AM, "Chuansheng Liu" <chuansheng.liu@...el.com> wrote:

> > +void wake_up_if_idle(int cpu)
> > +{
> > +       struct rq *rq = cpu_rq(cpu);
> > +       unsigned long flags;
> > +
> > +       if (set_nr_if_polling(rq->idle)) {
> > +               trace_sched_wake_idle_without_ipi(cpu);
> > +       } else {
> > +
> 
> FWIW, adding:
> 
> if (rq->curr != rq->idle)
>     return;
> 
> Right here could improve performance on large, mostly non-idle
> systems.  It would skip the spinlock in most cases.
> 

 !is_idle_task() :-)
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