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

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Date:   Wed, 21 Dec 2022 14:03:15 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.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 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 `If @sg
does not have SMT siblings` still reflect the old code layout.