Re: [PATCH v2 2/8] sched/fair: Maintain the identity of idle-core

From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Mel Gorman <mgorman@techsingularity.net>,
	Rik van Riel <riel@surriel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Gautham R Shenoy <ego@linux.vnet.ibm.com>,
	Parth Shah <parth@linux.ibm.com>
Subject: Re: [PATCH v2 2/8] sched/fair: Maintain the identity of idle-core
Date: Tue, 11 May 2021 21:57:07 +0530	[thread overview]
Message-ID: <20210511162707.GT2633526@linux.vnet.ibm.com> (raw)
In-Reply-To: <87v97p1oap.mognet@arm.com>

* Valentin Schneider <valentin.schneider@arm.com> [2021-05-11 12:51:26]:

> On 06/05/21 22:15, Srikar Dronamraju wrote:
> > Scheduler maintains a per LLC info which tells if there is any idle core
> > in that LLC. However this information doesn't provide which core is idle.
> >
> 

<snip>

> > On the converse, if a task is pinned, and has a restricted
> > cpus_allowed_list and LLC has multiple idle-cores, but select_idle_core
> > cannot find a idle-core, LLC will no more maintain that it has an
> > idle-core.
> >
> 
> This however does sound icky.
> 

<snip>

> > @@ -6127,7 +6129,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> >  {
> >       struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
> >       int i, cpu, idle_cpu = -1, nr = INT_MAX;
> > -	bool smt = test_idle_cores(target, false);
> > +	int idle_core = get_idle_core(target, -1);
> > +	bool smt = (idle_core != -1);
> 
> test_idle_cores() tells you if there's at least one idle core in the
> target's LLC. AFAICT get_idle_core() only tells you whether the target's
> core is idle, which is not the same thing.
> 
> Note that this code has recently been changed by Rik in
> 
>   c722f35b513f ("sched/fair: Bring back select_idle_smt(), but differently")
> 
> so as annoying as it is you should probably go try this out / rebase your
> series on top of it (as a rule of thumb for core scheduler stuff you should
> use https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git -b
> tip/sched/core as a base).

Yes I did notice it, I will rebase the next version on top of tip/sched/core

-- 
Thanks and Regards
Srikar Dronamraju