linux-kernel - Re: [PATCH v2 4/7] sched/fair: Introduce sched_smt_siblings

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Date:   Thu, 29 Dec 2022 16:17:23 -0800
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     Valentin Schneider <vschneid@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, "Tim C . Chen" <tim.c.chen@...el.com>
Subject: Re: [PATCH v2 4/7] sched/fair: Introduce sched_smt_siblings_idle()

On Wed, Dec 28, 2022 at 11:29:52PM +0800, Chen Yu wrote:
> On 2022-12-23 at 21:28:50 -0800, Ricardo Neri wrote:
> > On Thu, Dec 22, 2022 at 04:56:22PM +0000, Valentin Schneider wrote:
> > > On 22/11/22 12:35, Ricardo Neri wrote:
> > > > Architectures that implement arch_asym_cpu_priority() may need to know the
> > > > idle state of the SMT siblings of a CPU. The scheduler has this information
> > > > and functionality. Expose it.
> > > >
> > > > Move the existing functionality outside of the NUMA code.
> > > >
> > > 
> > > test_idle_cores() does something similar without an iteration, did you
> > > consider using that instead?
> > 
> > IIUC, test_idle_cores() returns true if there is at least one idle core in
> > the package. In my case, I need to know the idle state of only the SMT
> > siblings of a specific CPU. Am I missing something?
> >
> I guess a similar one is select_idle_core(), but it also consider the CPU with
> SCHED_IDLE task as idle. Is CPU with SCHED_IDLE task a candidate in your
> scenario?

However, we are not looking for an idle CPU. We want to know the idle state of
the siblings of a CPU. I see that select_idle_core() uses available_idle_cpu(),
which in turn uses idle_cpu(). is_core_idle() also uses it.

As per 943d355d7fee ("sched/core: Distinguish between idle_cpu() calls based on
desired effect, introduce available_idle_cpu()") the load balancer can just
call idle_cpu().

Thanks and BR,
Ricardo