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Date:   Sun, 23 May 2021 12:57:04 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Hugh Dickins <hughd@google.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: config SCHED_CORE
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On Fri, May 21, 2021 at 07:57:35AM -0400, Joel Fernandes wrote:
> On Fri, May 21, 2021 at 3:53 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > +       help
> > +         This option enables Core scheduling, a means of coordinated task
> > +         selection across SMT siblings with the express purpose of creating a
> > +         Core wide privilidge boundary. When enabled -- see prctl(PR_SCHED_CORE)
> > +         -- task selection will ensure all SMT siblings will execute a task
> > +         from the same 'core group', forcing idle when no matching task is found.
> > +
> > +         This provides means of mitigation against a number of SMT side-channels;
> > +         but is, on its own, insufficient to mitigate all known side-channels.
> > +         Notable: the MDS class of attacks require more.
> > +
> > +         Default enabled for anything that has SCHED_SMT, when unused there should
> > +         be no impact on performance.
> 
> This description sort of makes it sound like security is the only
> usecase. Perhaps we can also add here that core-scheduling can help
> performance of workloads where hyperthreading is undesired, such as
> when VM providers don't want to share hyperthreads.

Something like so then?

---
 kernel/Kconfig.preempt | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index ea1e3331c0ba..cd497fecfd43 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -104,4 +104,18 @@ config SCHED_CORE
 	bool "Core Scheduling for SMT"
 	default y
 	depends on SCHED_SMT
+	help
+	  This option enables Core scheduling, a means of coordinated task
+	  selection across SMT siblings. When enabled -- see
+	  prctl(PR_SCHED_CORE) -- task selection will ensure all SMT siblings
+	  will execute a task from the same 'core group', forcing idle when no
+	  matching task is found.
+
+	  Use of this feature includes:
+	   - mitigation of some (not all) SMT side channels;
+	   - limiting SMT interference to improve determinism and/or performance.
+
+	  Default enabled for anything that has SCHED_SMT, when unused there
+	  should be no impact on performance.
+