linux-kernel - Re: [PATCH v2 3/7] sched: Teach arch_asym_cpu_priority() the idle state of SMT siblings

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Message-ID: <72ed59b5-c7e1-c425-d1b6-e8d703d11d7a@arm.com>
Date:   Wed, 21 Dec 2022 18:12:52 +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 3/7] sched: Teach arch_asym_cpu_priority() the idle
 state of SMT siblings

On 12/12/2022 18:54, Ricardo Neri wrote:
> On Tue, Dec 06, 2022 at 06:54:39PM +0100, Dietmar Eggemann wrote:
>> On 22/11/2022 21:35, Ricardo Neri wrote:

[...]

>>> + * want to check the idle state of the SMT siblngs of @cpu.
>>
>> s/siblngs/siblings
>>
>> The scheduler calls sched_asym_prefer(..., true) from
>> find_busiest_queue(), asym_active_balance() and nohz_balancer_kick()
> 
> In these places we are comparing two specific CPUs, of which the idle
> state of its siblings impact their throughput and, in consequence, the
> decision of attempt to balance load.  
> 
> In the places were sched_asym_prefer(...., false) is called we compare the
> destination CPU with a CPU that bears the priority of a sched group,
> regardless of the idle state of its siblings.

OK.

>> even from SMT layer on !x86.
> 
> This is true, but the default arch_asym_cpu_priority ignores check_smt.

True.

>>  So I guess a `bool check_smt` wouldn't be
>> sufficient to distinguish whether sched_smt_siblings_idle() should be
>> called or not.
> 
> But it is the caller who determines whether the idle state of the SMT
> siblings of @cpu may be relevant.

I assume caller being the task scheduler here. Callers with
`check_smt=true` can be called from any SD level with SD_ASYM_PACKING.

Imagine an arch w/ SD_ASYM_PACKING on SMT & MC overwriting
arch_asym_cpu_priority(). `bool check_smt` wouldn't be sufficient to
know whether a call to something like sched_smt_siblings_idle()
(is_core_idle()) which iterates over cpu_smt_mask(cpu) would make sense.

>> To me this comment is a little bit misleading. Not an
>> issue currently since there is only the x86 overwrite right now.
> 
> If my justification make sense to you, I can expand the comment to state
> that the caller decides whether check_smt is needed but arch-specific
> implementations are free to ignore it.

Not a big issue but to me if the task scheduler asks for `bool
check_smt` then archs would have to check to guarantee common behaviour.
And the meaning of `bool check_smt` on SMT is unclear to me.
Since only x86 would use this so far it can be adapted later for others
if needed.

[...]