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

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Message-ID: <20151119195905.GR3816@twins.programming.kicks-ass.net>
Date:	Thu, 19 Nov 2015 20:59:05 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
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>
Subject: Re: [PATCH 2/4] timer: relax tick stop in idle entry

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.

---
 kernel/softirq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 479e4436f787..3bd30404f2ee 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -367,7 +367,7 @@ static inline void tick_irq_exit(void)
 	int cpu = smp_processor_id();
 
 	/* Make sure that timer wheel updates are propagated */
-	if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
+	if ((is_idle_task(current) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
 		if (!in_interrupt())
 			tick_nohz_irq_exit();
 	}
--
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