linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
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.
 
 

  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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=YNo1L2j/AWPmrtHS@gmail.com \
    --to=mingo@kernel.org \
    --cc=akpm@linux-foundation.org \
    --cc=bristot@redhat.com \
    --cc=dietmar.eggemann@arm.com \
    --cc=juri.lelli@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mgorman@suse.de \
    --cc=peterz@infradead.org \
    --cc=rostedt@goodmis.org \
    --cc=tglx@linutronix.de \
    --cc=torvalds@linux-foundation.org \
    --cc=vincent.guittot@linaro.org \
    --subject='Re: [PATCH, -v2] sched/core: Disable CONFIG_SCHED_CORE by default' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).