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

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Date:   Mon, 03 May 2021 11:33:45 -0500
From:   Scott Wood <swood@...hat.com>
To:     Mike Galbraith <efault@....de>,
        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 Sun, 2021-05-02 at 05:25 +0200, Mike Galbraith wrote:
> 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)

Determining which task to pull is apparently taking that long (again, this
is on a 128-cpu system).  RT is singled out because that is the config that
makes significant tradeoffs to keep latencies down (I expect this would be
far from the only possible 1ms+ latency on a non-RT kernel), and there was
concern about the overhead of a double context switch when pulling a task to
a newidle cpu.

-Scott