linux-kernel - Re: [PATCH v2 1/7] sched/fair: Generalize asym

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Date:   Wed, 21 Dec 2022 20:32:49 -0800
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     "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>,
        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>,
        Valentin Schneider <vschneid@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, "Tim C . Chen" <tim.c.chen@...el.com>
Subject: Re: [PATCH v2 1/7] sched/fair: Generalize asym_packing logic for SMT
 local sched group

On Wed, Dec 21, 2022 at 02:03:15PM +0100, Dietmar Eggemann wrote:
> On 12/12/2022 18:53, Ricardo Neri wrote:
> > On Tue, Dec 06, 2022 at 06:22:41PM +0100, Dietmar Eggemann wrote:
> >> On 22/11/2022 21:35, Ricardo Neri wrote:
> 
> [...]
> 
> >> I'm not sure why you change asym_smt_can_pull_tasks() together with
> >> removing SD_ASYM_PACKING from SMT level (patch 5/7)?
> > 
> > In x86 we have SD_ASYM_PACKING at the MC, CLS* and, before my patches, SMT
> > sched domains.
> > 
> >>
> >> update_sg_lb_stats()
> >>
> >>   ... && env->sd->flags & SD_ASYM_PACKING && .. && sched_asym()
> >>                                                    ^^^^^^^^^^^^
> >>     sched_asym()
> >>
> >>       if ((sds->local->flags & SD_SHARE_CPUCAPACITY) ||
> >>           (group->flags & SD_SHARE_CPUCAPACITY))
> >>         return asym_smt_can_pull_tasks()
> >>                ^^^^^^^^^^^^^^^^^^^^^^^^^
> >>
> >> So x86 won't have a sched domain with SD_SHARE_CPUCAPACITY and
> >> SD_ASYM_PACKING anymore. So sched_asym() would call sched_asym_prefer()
> >> directly on MC. What do I miss here?
> > 
> > asym_smt_can_pull_tasks() is used above the SMT level *and* when either the
> > local or sg sched groups are composed of CPUs that are SMT siblings.
> 
> OK.
> 
> > In fact, asym_smt_can_pull_tasks() can only be called above the SMT level.
> > This is because the flags of a sched_group in a sched_domain are equal to
> > the flags of the child sched_domain. Since SMT is the lowest sched_domain,
> > its groups' flags are 0.
> 
> I see. I forgot about `[PATCH v5 0/6] sched/fair: Fix load balancing of
> SMT siblings with ASYM_PACKING` from Sept 21 (specifically [PATCH v5
> 2/6] sched/topology: Introduce sched_group::flags).
> 
> > sched_asym() calls sched_asym_prefer() directly if balancing at the
> > SMT level and, at higher domains, if the child domain is not SMT.
> 
> OK.
> 
> > This meets the requirement of Power7, where SMT siblings have different
> > priorities; and of x86, where physical cores have different priorities.
> > 
> > Thanks and BR,
> > Ricardo
> > 
> > * The target of these patches is Intel hybrid processors, on which cluster
> >   scheduling is disabled - cabdc3a8475b ("sched,x86: Don't use cluster
> >   topology for x86 hybrid CPUs"). Also, I have not observed topologies in
> >   which CPUs of the same cluster have different priorities.
> 
> OK.
> 
> IMHO, the function header of asym_smt_can_pull_tasks() (3rd and 4th
> paragraph ...  `If both @dst_cpu and @sg have SMT siblings` and

Agreed. I changed the behavior of the function. I will update the
description.

>`If @sg does not have SMT siblings` still reflect the old code layout.

But this behavior did not change. The check covers both SMT and non-SMT
cases:

	 /*
	  * non-SMT @sg can only have 1 busy CPU. We only care SMT @sg
	  * has exactly one busy sibling
	  */
	if (sg_busy_cpus == 1 && 
	    /* local group is fully idle, SMT and non-SMT. */
	    !sds->local_stat.sum_nr_running)

Perhaps I can collapse the two paragraphs into one.

Thanks and BR,
Ricardo