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

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Date:   Sun, 02 May 2021 05:25:35 +0200
From:   Mike Galbraith <efault@....de>
To:     Scott Wood <swood@...hat.com>,
        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 Sat, 2021-05-01 at 17:03 -0500, Scott Wood wrote:
> 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.

If NEWIDLE balancing migrates one task, how does that manage to consume
a full *millisecond*, and why would that only be a problem for RT?

	-Mike

(rt tasks don't play !rt balancer here, if CPU goes idle, tough titty)