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Date: Tue, 1 Sep 2015 22:47:24 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: "Jiang, Yunhong" <yunhong.jiang@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Vatika Harlalka <vatikaharlalka@gmail.com>,
        Chris Metcalf <cmetcalf@ezchip.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Preeti U Murthy <preeti@linux.vnet.ibm.com>,
        Christoph Lameter <cl@linux.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] nohz: Affine unpinned timers to housekeepers
Message-ID: <20150901204723.GA6590@lerouge>
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On Tue, Sep 01, 2015 at 07:14:13PM +0000, Jiang, Yunhong wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 8b864ec..0902e4d 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -623,18 +623,21 @@ int get_nohz_timer_target(void)
> >  	int i, cpu = smp_processor_id();
> >  	struct sched_domain *sd;
> > 
> > -	if (!idle_cpu(cpu))
> > +	if (!idle_cpu(cpu) && is_housekeeping_cpu(cpu))
> >  		return cpu;
> > 
> >  	rcu_read_lock();
> >  	for_each_domain(cpu, sd) {
> >  		for_each_cpu(i, sched_domain_span(sd)) {
> > -			if (!idle_cpu(i)) {
> > +			if (!idle_cpu(i) && is_housekeeping_cpu(cpu)) {
> 
> Hi, Frederic, sorry for a naive question. Per my understanding, the tick_nohz_full_mask is added to cpu_isolated_map in
> sched_init_smp(), and the cpu_isolated_map is excluded from sched_domain in init_sched_domains(), so why check here?

Very good observation! But it's better to keep this check in the domain loop in
case things change in the future such as removing that cpu_isolated_map inclusion
or other suprises.