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

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Message-Id: <20220825225529.26465-1-ricardo.neri-calderon@linux.intel.com>
Date:   Thu, 25 Aug 2022 15:55:25 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     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,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Subject: [PATCH 0/4] sched/fair: Avoid unnecessary migrations within SMT domains

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.

Thanks and BR,
Ricardo

Ricardo Neri (4):
  sched/fair: Simplify asym_packing logic for SMT sched groups
  sched/fair: Do not disqualify either runqueues of SMT sched groups
  sched/fair: Let lower-priority CPUs do active balancing
  x86/sched: Avoid unnecessary migrations within SMT domains

 arch/x86/kernel/itmt.c | 23 +++++-----------------
 kernel/sched/fair.c    | 44 ++++++++++++++++++++----------------------
 2 files changed, 26 insertions(+), 41 deletions(-)

-- 
2.25.1