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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Thu, 29 Apr 2021 09:12:32 +0200
Message-ID: <CAKfTPtBrJNBg3847R_b8A-1c5rb9Fb5FFNMX+z11QGAiO0ofkw@mail.gmail.com>
Subject: Re: [PATCH v2 0/3] newidle_balance() PREEMPT_RT latency mitigations
To:     Scott Wood <swood@redhat.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Thomas Gleixner <tglx@linutronix.de>
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Hi Scott,

On Thu, 29 Apr 2021 at 01:28, Scott Wood <swood@redhat.com> wrote:
>
> 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).

The patch below has been queued for v5.13 and removed the update of
blocked load what seemed to be the major reason for long preempt/irq
off during newly idle balance:
https://lore.kernel.org/lkml/20210224133007.28644-1-vincent.guittot@linaro.org/

I would be curious to see how it impacts your cases

>
> 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
>