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

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Date:   Tue, 11 May 2021 21:57:07 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Parth Shah <parth@...ux.ibm.com>
Subject: Re: [PATCH v2 2/8] sched/fair: Maintain the identity of idle-core

* Valentin Schneider <valentin.schneider@....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