linux-kernel - Re: [PATCH 1/2] nohz: Affine unpinned timers to housekeepers

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Date:	Fri, 4 Sep 2015 11:56:33 -0400
From:	Luiz Capitulino <lcapitulino@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	"Jiang, Yunhong" <yunhong.jiang@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Vatika Harlalka <vatikaharlalka@...il.com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] nohz: Affine unpinned timers to housekeepers

On Tue, 1 Sep 2015 22:47:24 +0200
Frederic Weisbecker <fweisbec@...il.com> wrote:

> 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.

I have another observation.

As nohz_full cores are already excluded from the sched domains, this patch
boils down to migrating timers which are queued by the nohz_full core
itself. However, it only fully works for lowres timers. hrtimers may
still fire on nohz_full cores, because hrtimers are only migrated if
the core returned by get_nohz_timer_target() has a queued timer expiring
before than the timer being migrated.
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