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

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Message-ID: <20151116160910.3811c8ca@icelake>
Date:	Mon, 16 Nov 2015 16:09:10 -0800
From:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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 Mon, 16 Nov 2015 23:01:12 +0100 (CET)
Thomas Gleixner <tglx@...utronix.de> wrote:

> > For the second case, which is much more rare, I think we do have
> > next timer exactly one tick away. Just don't know why tick will
> > continue into idle loop.  
> 
> Well, it should not be hard to figure that out. There are not so many
> checks involved when tick_nohz_irq_exit() is called.
Thanks for the tip, I found the cause is in 
int idle_cpu(int cpu)
{
	if (rq->nr_running)
		return 0;

Since we only take into account of cfs_rq runnable taking over
cfs_rq->nr_running when forced_idle is set.


Jacob
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