linux-kernel - Re: [PATCH v3 5/6] sched/fair: Consider SMT in ASYM

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Date:   Wed, 19 May 2021 21:09:14 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Aubrey Li <aubrey.li@...el.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Ben Segall <bsegall@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Len Brown <len.brown@...el.com>, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, Mel Gorman <mgorman@...e.de>,
        Ingo Molnar <mingo@...nel.org>,
        Quentin Perret <qperret@...gle.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v3 5/6] sched/fair: Consider SMT in ASYM_PACKING load
 balance

Excerpts from Peter Zijlstra's message of May 19, 2021 7:59 pm:
> On Tue, May 18, 2021 at 12:07:40PM -0700, Ricardo Neri wrote:
>> On Fri, May 14, 2021 at 07:14:15PM -0700, Ricardo Neri wrote:
>> > On Fri, May 14, 2021 at 11:47:45AM +0200, Peter Zijlstra wrote:
> 
>> > > So I'm thinking that this is a property of having ASYM_PACKING at a core
>> > > level, rather than some arch special. Wouldn't something like this be
>> > > more appropriate?
> 
>> > Thanks Peter for the quick review! This makes sense to me. The only
>> > reason we proposed arch_asym_check_smt_siblings() is because we were
>> > about breaking powerpc (I need to study how they set priorities for SMT,
>> > if applicable). If you think this is not an issue I can post a
>> > v4 with this update.
>> 
>> As far as I can see, priorities in powerpc are set by the CPU number.
>> However, I am not sure how CPUs are enumerated? If CPUs in brackets are
>> SMT sibling, Does an enumeration looks like A) [0, 1], [2, 3] or B) [0, 2],
>> [1, 3]? I guess B is the right answer. Otherwise, both SMT siblings of a
>> core would need to be busy before a new core is used.
>> 
>> Still, I think the issue described in the cover letter may be
>> reproducible in powerpc as well. If CPU3 is offlined, and [0, 2] pulled
>> tasks from [1, -] so that both CPU0 and CPU2 become busy, CPU1 would not be
>> able to help since CPU0 has the highest priority.
>> 
>> I am cc'ing the linuxppc list to get some feedback.
> 
> IIRC the concern with Power is that their Cores can go faster if the
> higher SMT siblings are unused.
> 
> That is, suppose you have an SMT4 Core with only a single active task,
> then if only SMT0 is used it can reach max performance, but if the
> active sibling is SMT1 it can not reach max performance, and if the only
> active sibling is SMT2 it goes slower still.
> 
> So they need to pack the tasks to the lowest SMT siblings, and have the
> highest SMT siblings idle (where possible) in order to increase
> performance.

That's correct.

Thanks,
Nick