From: Vincent Guittot <vincent.guittot@linaro.org>
To: Scott Wood <swood@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Rik van Riel <riel@surriel.com>, Mel Gorman <mgorman@suse.de>,
linux-kernel <linux-kernel@vger.kernel.org>,
linux-rt-users <linux-rt-users@vger.kernel.org>
Subject: Re: [RFC PATCH 0/3] newidle_balance() latency mitigation
Date: Thu, 30 Apr 2020 14:48:11 +0200 [thread overview]
Message-ID: <CAKfTPtDfk-aq7PTVmMgJcZSOupXb0SGHS8Fc2k+qTZTsnfiV=A@mail.gmail.com> (raw)
In-Reply-To: <20200428050242.17717-1-swood@redhat.com>
On Tue, 28 Apr 2020 at 07:02, Scott Wood <swood@redhat.com> wrote:
>
> These patches mitigate latency caused by newidle_balance() on large
> systems, by enabling interrupts when the lock is dropped, and exiting
> early at various points if an RT task is runnable on the current CPU.
>
> When applied to an RT kernel on a 72-core machine (2 threads per core), I
> saw significant reductions in latency as reported by rteval -- from
> over 500us to around 160us with hyperthreading disabled, and from
> over 1400us to around 380us with hyperthreading enabled.
Do you know how each patch contributes to the decrease ? Because patch
3 not only impacts newly idle lb but each and every lb. So most of the
decrease might come from aborting the busy or idle lb at the highest
sched_domai level which scan all cpus and moving newly idle load
balance is not a major part.
>
> This isn't the first time something like this has been tried:
> https://lore.kernel.org/lkml/20121222003019.433916240@goodmis.org/
> That attempt ended up being reverted:
> https://lore.kernel.org/lkml/5122CD9C.9070702@oracle.com/
>
> The problem in that case was the failure to keep BH disabled, and the
> difficulty of fixing that when called from the post_schedule() hook.
> This patchset uses finish_task_switch() to call newidle_balance(), which
> enters in non-atomic context so we have full control over what we disable
> and when.
>
> There was a note at the end about wanting further discussion on the matter --
> does anyone remember if that ever happened and what the conclusion was?
> Are there any other issues with enabling interrupts here and/or moving
> the newidle_balance() call?
>
> Rik van Riel (1):
> sched,rt: break out of load balancing if an RT task appears
>
> Scott Wood (2):
> sched/fair: Call newidle_balance() from finish_task_switch()
> sched/fair: Enable interrupts when dropping lock in newidle_balance()
>
> kernel/sched/core.c | 7 +++--
> kernel/sched/fair.c | 72 +++++++++++++++++++++++---------------------
> kernel/sched/sched.h | 12 +++++---
> 3 files changed, 49 insertions(+), 42 deletions(-)
>
> --
> 2.18.2
>
prev parent reply other threads:[~2020-04-30 12:48 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-28 5:02 [RFC PATCH 0/3] newidle_balance() latency mitigation Scott Wood
2020-04-28 5:02 ` [RFC PATCH 1/3] sched/fair: Call newidle_balance() from finish_task_switch() Scott Wood
2020-04-28 21:37 ` Valentin Schneider
2020-04-28 22:09 ` Peter Zijlstra
2020-04-28 22:55 ` Scott Wood
2020-04-28 23:02 ` Peter Zijlstra
2020-04-28 23:20 ` Scott Wood
2020-04-29 9:05 ` Peter Zijlstra
2020-04-30 1:31 ` Scott Wood
2020-05-11 10:58 ` Peter Zijlstra
2020-05-11 12:13 ` Peter Zijlstra
2020-04-28 22:33 ` Scott Wood
2020-04-29 12:00 ` Valentin Schneider
2020-04-29 8:27 ` Vincent Guittot
2020-04-30 1:36 ` Scott Wood
2020-04-28 5:02 ` [RFC PATCH 2/3] sched/fair: Enable interrupts when dropping lock in newidle_balance() Scott Wood
2020-04-28 5:02 ` [RFC PATCH 3/3] sched,rt: break out of load balancing if an RT task appears Scott Wood
2020-04-28 21:56 ` Valentin Schneider
2020-04-28 22:33 ` Scott Wood
2020-04-28 22:52 ` Scott Wood
2020-04-29 12:01 ` Valentin Schneider
2020-04-28 13:27 ` [RFC PATCH 0/3] newidle_balance() latency mitigation Steven Rostedt
2020-04-29 23:13 ` Valentin Schneider
2020-04-30 7:44 ` Vincent Guittot
2020-04-30 10:14 ` Valentin Schneider
2020-04-30 12:42 ` Vincent Guittot
2020-04-30 13:56 ` Valentin Schneider
2020-04-30 12:48 ` Vincent Guittot [this message]
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