From: Scott Wood <swood@redhat.com>
To: Mike Galbraith <efault@gmx.de>,
Vincent Guittot <vincent.guittot@linaro.org>
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>
Subject: Re: [PATCH v2 0/3] newidle_balance() PREEMPT_RT latency mitigations
Date: Mon, 03 May 2021 11:33:45 -0500 [thread overview]
Message-ID: <4170501b7c4f19ba66d870b671dc90ffbf4623d6.camel@redhat.com> (raw)
In-Reply-To: <a46f9b6c719666357e568eadd1d615c05c4171ac.camel@gmx.de>
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@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 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
next prev parent reply other threads:[~2021-05-03 16:33 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-28 23:28 [PATCH v2 0/3] newidle_balance() PREEMPT_RT latency mitigations Scott Wood
2021-04-28 23:28 ` [PATCH v2 1/3] sched/fair: Call newidle_balance() from balance_callback on PREEMPT_RT Scott Wood
2021-05-05 12:13 ` Vincent Guittot
2021-05-07 15:19 ` Valentin Schneider
2021-04-28 23:28 ` [PATCH v2 2/3] sched/fair: Enable interrupts when dropping lock in newidle_balance() Scott Wood
2021-04-28 23:28 ` [PATCH v2 3/3] sched/fair: break out of newidle balancing if an RT task appears Scott Wood
2021-04-29 4:11 ` kernel test robot
2021-04-29 6:37 ` kernel test robot
2021-05-07 11:03 ` Peter Zijlstra
2021-05-15 7:29 ` Mike Galbraith
2021-05-15 8:36 ` Mike Galbraith
2021-04-29 7:12 ` [PATCH v2 0/3] newidle_balance() PREEMPT_RT latency mitigations Vincent Guittot
2021-05-01 22:03 ` Scott Wood
2021-05-02 3:25 ` Mike Galbraith
2021-05-03 16:33 ` Scott Wood [this message]
2021-05-03 18:52 ` Mike Galbraith
2021-05-03 21:57 ` Scott Wood
2021-05-04 4:07 ` Mike Galbraith
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