linux-kernel - Re: [PATCH 2/4] timer: relax tick stop in idle entry

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Message-ID: <20151119154124.6ffa0cec@icelake>
Date:	Thu, 19 Nov 2015 15:41:24 -0800
From:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Len Brown <len.brown@...el.com>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	Eduardo Valentin <edubezval@...il.com>,
	Paul Turner <pjt@...gle.com>,
	Josh Triplett <josh@...htriplett.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH 2/4] timer: relax tick stop in idle entry

On Thu, 19 Nov 2015 20:59:05 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Nov 19, 2015 at 11:21:25AM -0800, Jacob Pan wrote:
> > On Thu, 19 Nov 2015 20:06:30 +0100
> > Peter Zijlstra <peterz@...radead.org> wrote:
> >   
> > > Which one? That is, I cannot find a idle_cpu() call from
> > > tick_nohz_irq_exit().  
> > it is  tick_irq_exit(void), i will not call tick_nohz_irq_exit() if
> > !idle_cpu()  
> 
> Ah, but I think we really only need to test for the idle task there,
> the && need_resched() part of that function takes care of the rest.
> 
> Once we have need_resched() set, we'll be on our way to
> tick_nohz_idle_exit() to restart the tick again.
Tested, it did the trick also. has less impact than changing idle_cpu().

My other point is the callers of idle_cpu() will see it returns false
but in forced idle. Can't see anything bad since we skip load balance.

Thanks, 

Jacob
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