linux-kernel - [PATCH v2 0/3] newidle_balance() PREEMPT

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Message-Id: <20210428232821.2506201-1-swood@redhat.com>
Date:   Wed, 28 Apr 2021 18:28:18 -0500
From:   Scott Wood <swood@...hat.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH v2 0/3] newidle_balance() PREEMPT_RT latency mitigations

These patches mitigate latency caused by newidle_balance() on large
systems when PREEMPT_RT is enabled, by enabling interrupts when the lock
is dropped, and exiting early at various points if an RT task is runnable
on the current CPU.

On a system with 128 CPUs, these patches dropped latency (as measured by
a 12 hour rteval run) from 1045us to 317us (when applied to
5.12.0-rc3-rt3).

I tried a couple scheduler benchmarks (perf bench sched pipe, and
sysbench threads) to try to determine whether the overhead is measurable
on non-RT, but the results varied widely enough (with or without the patches)
that I couldn't draw any conclusions from them.  So at least for now, I
limited the balance callback change to when PREEMPT_RT is enabled.

Link to v1 RFC patches:
https://lore.kernel.org/lkml/20200428050242.17717-1-swood@redhat.com/

Scott Wood (3):
  sched/fair: Call newidle_balance() from balance_callback on PREEMPT_RT
  sched/fair: Enable interrupts when dropping lock in newidle_balance()
  sched/fair: break out of newidle balancing if an RT task appears

 kernel/sched/fair.c  | 66 ++++++++++++++++++++++++++++++++++++++------
 kernel/sched/sched.h |  6 ++++
 2 files changed, 64 insertions(+), 8 deletions(-)

-- 
2.27.0