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

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Date:   Sat, 01 May 2021 17:03:47 -0500
From:   Scott Wood <swood@...hat.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 0/3] newidle_balance() PREEMPT_RT latency mitigations

On Thu, 2021-04-29 at 09:12 +0200, Vincent Guittot wrote:
> Hi Scott,
> 
> On Thu, 29 Apr 2021 at 01:28, Scott Wood <swood@...hat.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 still get 1000+ ms latencies with those patches applied.

-Scott