archive mirror
 help / color / mirror / Atom feed
From: Ingo Molnar <>
To: Linus Torvalds <>
Cc: Linux Kernel Mailing List <>,
	Peter Zijlstra <>,
	Juri Lelli <>,
	Vincent Guittot <>,
	Dietmar Eggemann <>,
	Steven Rostedt <>,
	Mel Gorman <>,
	Daniel Bristot de Oliveira <>,
	Thomas Gleixner <>,
	Andrew Morton <>
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/> (raw)
In-Reply-To: <>

* Linus Torvalds <> wrote:

> On Mon, Jun 28, 2021 at 12:57 PM Ingo Molnar <> 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 ...



From: Ingo Molnar <>
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 <>
Signed-off-by: Ingo Molnar <>
 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
 	  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:

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

  git send-email \
    --in-reply-to=YNo1L2j/ \ \ \ \ \ \ \ \ \ \ \ \ \
    --subject='Re: [PATCH, -v2] sched/core: Disable CONFIG_SCHED_CORE by default' \

* 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).