Re: [RESEND PATCH v5 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path

From: Peter Zijlstra <peterz@infradead.org>
To: Barry Song <21cnbao@gmail.com>
Cc: Yicong Yang <yangyicong@hisilicon.com>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	"Gautham R. Shenoy" <gautham.shenoy@amd.com>,
	LKML <linux-kernel@vger.kernel.org>,
	LAK <linux-arm-kernel@lists.infradead.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	prime.zeng@huawei.com,
	Jonathan Cameron <jonathan.cameron@huawei.com>,
	ego@linux.vnet.ibm.com,
	Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
	Linuxarm <linuxarm@huawei.com>,
	Guodong Xu <guodong.xu@linaro.org>,
	hesham.almatary@huawei.com, john.garry@huawei.com,
	Yang Shen <shenyang39@huawei.com>,
	kprateek.nayak@amd.com, Chen Yu <yu.c.chen@intel.com>,
	wuyun.abel@bytedance.com
Subject: Re: [RESEND PATCH v5 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path
Date: Thu, 21 Jul 2022 12:33:57 +0200	[thread overview]
Message-ID: <YtkrlVmqnx9VTbLl@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <CAGsJ_4yrZnPbhBb4wd3x1h994tv6Jfri+y2xrPSJv7yF+Mj7tg@mail.gmail.com>

On Thu, Jul 21, 2022 at 09:38:04PM +1200, Barry Song wrote:
> On Wed, Jul 20, 2022 at 11:15 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, Jul 20, 2022 at 04:11:50PM +0800, Yicong Yang wrote:
> > > +     /* TODO: Support SMT system with cluster topology */
> > > +     if (!sched_smt_active() && sd) {
> > > +             for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
> >
> > So that's no SMT and no wrap iteration..
> >
> > Does something like this work?
> >
> > ---
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6437,6 +6437,30 @@ static int select_idle_cpu(struct task_s
> >                 }
> >         }
> >
> > +       if (IS_ENABLED(CONFIG_SCHED_CLUSTER) &&
> > +           static_branch_unlikely(&sched_cluster_active)) {
> > +               struct sched_domain *sdc = rcu_dereference(per_cpu(sd_cluster, target));
> > +               if (sdc) {
> > +                       for_each_cpu_wrap(cpu, sched_domain_span(sdc), target + 1) {
> > +                               if (!cpumask_test_cpu(cpu, cpus))
> > +                                       continue;
> > +
> > +                               if (has_idle_core) {
> > +                                       i = select_idle_core(p, cpu, cpus, &idle_cpu);
> > +                                       if ((unsigned int)i < nr_cpumask_bits)
> > +                                               return i;
> > +                               } else {
> > +                                       if (--nr <= 0)
> > +                                               return -1;
> > +                                       idle_cpu = __select_idle_cpu(cpu, p);
> > +                                       if ((unsigned int)idle_cpu < nr_cpumask_bits)
> > +                                               break;
> 
> Guess here it should be "return idle_cpu", but not "break". as "break"
> will make us scan more
> other cpus outside the cluster if we have found idle_cpu within the cluster.
> 
> Yicong,
> Please test Peter's code with the above change.

Indeed. Sorry for that.

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