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: <xhsmhtu1ne5sl.mognet@vschneid.remote.csb>
Date:   Thu, 22 Dec 2022 16:56:10 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        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>, 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 21/12/22 20:55, Ricardo Neri wrote:
> On Wed, Dec 21, 2022 at 06:12:52PM +0100, Dietmar Eggemann wrote:
>> 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.
>
> Agreed. I was hoping I could get away with this. x86 would not have the
> the SD_ASYM_PACKING flag at the SMT level and Power7 would ignore
> `check_smt`. :)
>
> Callers of sched_asym_prefer() could use the flags of the sched domain to
> check if we are at the SMT level.
>
> I rescanned the code again and it looks like the needed sched domain flags
> are available in all the places sched_asym_prefer() is called. The only
> exception is asym_smt_can_pull_tasks(), but we already know that we don't
> need such check. (We are looking for the sched group priority, regardless
> of the idle state of the SMT siblings).
>

Given this is fed back to arch code, another option here would be to feed
the topology level this is happening at. You get it via sd->level and it
maps back into the arch's sched_domain_topology_level array.

Though the SD flags are probably the more generic solution.