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SubjectRe: config SCHED_CORE
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 ;-/

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