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

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Message-ID: <1441197197.4163.35.camel@gmail.com>
Date:	Wed, 02 Sep 2015 14:33:17 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.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 Wed, 2015-09-02 at 11:38 +0200, Mike Galbraith wrote:
> On Tue, 2015-09-01 at 22:47 +0200, Frederic Weisbecker 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.
> 
> IMHO, nohz_full -> cpu_isolated_map removal really wants to happen.
> NO_HZ_FULL_ALL currently means "Woohoo, next stop NR_CPUS=0".

(which surprises folks who have [had] it enabled [and may even have been
using and/or testing it])

https://lkml.org/lkml/2015/9/2/145

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