From: Ingo Molnar <mingo@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Mel Gorman <mgorman@suse.de>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Andrew Morton <akpm@linux-foundation.org>
Subject: [PATCH, -v2] sched/core: Disable CONFIG_SCHED_CORE by default
Date: Mon, 28 Jun 2021 22:46:39 +0200 [thread overview]
Message-ID: <YNo1L2j/AWPmrtHS@gmail.com> (raw)
In-Reply-To: <CAHk-=wjNP8Oi4nve=uu=Q3+rGar3CY9fBUQFuJK-WYCK3F198w@mail.gmail.com>
* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Mon, Jun 28, 2021 at 12:57 PM Ingo Molnar <mingo@kernel.org> wrote:
> >
> > You are completely right, I missed this. Find below the patch to fix it -
>
> I think you should update the helptext too, which currently says that
> it's enabled if SCHED_SMT is enabled.
>
> And yes, it may be that there is no measurable performance downside,
> but this feature has been discussed and under development for a long
> time now, and I'd like the default to try to limit the impact since it
> makes little sense to most people.
Updated patch attached, which updates the help text and clarifies what
overhead there is.
While the worst of the overhead is behind the __sched_core_enabled static
key, there's a notable increase in pick_next_task(), which might be
measurable with the right microbenchmark.
So the 'no overhead' claim is probably not entirely true, and comes from
the context of *horrible* overhead of early iterations of the core
scheduling feature ...
Thanks,
Ingo
===================================>
From: Ingo Molnar <mingo@kernel.org>
Date: Mon, 28 Jun 2021 21:55:16 +0200
Subject: [PATCH] sched/core: Disable CONFIG_SCHED_CORE by default
This option at minimum adds extra code to the scheduler - even if
it's default unused - and most users wouldn't want it.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
kernel/Kconfig.preempt | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index bd7c4147b9a8..5876e30c5740 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -102,7 +102,6 @@ config PREEMPT_DYNAMIC
config SCHED_CORE
bool "Core Scheduling for SMT"
- default y
depends on SCHED_SMT
help
This option permits Core Scheduling, a means of coordinated task
@@ -115,7 +114,8 @@ config SCHED_CORE
- mitigation of some (not all) SMT side channels;
- limiting SMT interference to improve determinism and/or performance.
- SCHED_CORE is default enabled when SCHED_SMT is enabled -- when
- unused there should be no impact on performance.
+ SCHED_CORE is default disabled. When it is enabled and unused,
+ which is the likely usage by Linux distributions, there should
+ be no measurable impact on performance.
next prev parent reply other threads:[~2021-06-28 20:46 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-28 6:51 [GIT PULL] scheduler changes for v5.14 Ingo Molnar
2021-06-28 19:32 ` Linus Torvalds
2021-06-28 19:57 ` [PATCH] sched/core: Disable CONFIG_SCHED_CORE by default Ingo Molnar
2021-06-28 20:10 ` Linus Torvalds
2021-06-28 20:46 ` Ingo Molnar [this message]
2021-06-28 19:34 ` [GIT PULL] scheduler changes for v5.14 pr-tracker-bot
2021-06-30 10:53 ` [GIT PULL] scheduler fixes " Ingo Molnar
2021-06-30 23:14 ` pr-tracker-bot
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