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Date:   Fri, 21 May 2021 15:33:49 +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:

> > ---
> >  kernel/Kconfig.preempt | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> > index ea1e3331c0ba..3c4566cd20ef 100644
> > --- a/kernel/Kconfig.preempt
> > +++ b/kernel/Kconfig.preempt
> > @@ -104,4 +104,16 @@ 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 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.
> 
> Thoughts?

You're right. And there's this whole class of people who want to use
this to eliminate SMT interference. I'll see if I can work that in
without turning the whole thing into a novella or so ;-/