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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Mon, 24 May 2021 16:50:43 -0400
Message-ID: <CAEXW_YSC_Ee=EnX7csVkhoEF6KFYbusuNx=mQcyxfdu2R7a3Cg@mail.gmail.com>
Subject: Re: config SCHED_CORE
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Hugh Dickins <hughd@google.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, May 21, 2021 at 9:36 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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.

That's good, even more core scheduling users ;-)

> I'll see if I can work that in
> without turning the whole thing into a novella or so ;-/

Your patch looked good to me (delta Randy/Hugh's comments). I still
owe you the documentation patch refresh, I'll send it out soon.
Thanks,

- Joel