linux-kernel - Re: [PATCH 0/4] sched/fair: Avoid unnecessary migrations within SMT domains

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Message-ID: <Y050e5XQkaUrwr5j@hirez.programming.kicks-ass.net>
Date:   Tue, 18 Oct 2022 11:40:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     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>,
        Dietmar Eggemann <dietmar.eggemann@....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
Subject: Re: [PATCH 0/4] sched/fair: Avoid unnecessary migrations within SMT
 domains

On Mon, Oct 17, 2022 at 07:35:27PM -0700, Ricardo Neri wrote:
> On Thu, Aug 25, 2022 at 03:55:25PM -0700, Ricardo Neri wrote:
> > Intel processors that support Intel Turbo Boost Max 3.0 use asym_packing
> > to assign higher priorities to CPUs with higher maximum frequencies. It
> > artificially assigns, however, a lower priority to the higher-numbered
> > SMT siblings to ensure that they are used last.
> > 
> > This results in unnecessary task migrations within the SMT domains.
> > 
> > On processors with a mixture of higher-frequency SMT cores and lower-
> > frequency non-SMT cores (such as Intel hybrid processors), a lower-
> > priority CPU pulls tasks from the higher-priority cores if more than one
> > SMT sibling is busy.
> > 
> > Do not use different priorities for each SMT sibling. Instead, tweak the
> > asym_packing load balancer to recognize SMT cores with more than one
> > busy sibling and let lower-priority CPUs pull tasks.
> > 
> > Removing these artificial priorities avoids superfluous migrations and
> > lets lower-priority cores inspect all SMT siblings for the busiest queue.
> 
> Hello. I'd like to know if there are any comments on these patches. This
> patchset is a requisite for the IPC classes of tasks patchset [1].

Urgh.. so I'm not liking this, afaict you're sprinkling SMT2
assumptions.

Why can't we make arch_asym_cpu_priority() depend on CPU state? Doesn't
it then magically work?