From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 1/3] sched: Add new API wake_up_if_idle() to wake up the
 idle cpu
Date: Wed, 3 Sep 2014 11:54:10 +0200
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To: Andy Lutomirski <luto@amacapital.net>
Cc: Chuansheng Liu <chuansheng.liu@intel.com>, "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>, "Rafael J. Wysocki" <rjw@rjwysocki.net>, Daniel Lezcano <daniel.lezcano@linaro.org>, "Wang, Xiaoming" <xiaoming.wang@intel.com>, Ingo Molnar <mingo@redhat.com>, "Chakravarty, Souvik K" <souvik.k.chakravarty@intel.com>, "Liu, Changcheng" <changcheng.liu@intel.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>

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