Re: config SCHED_CORE

[Hugh Dickins]

On Sun, 23 May 2021, Peter Zijlstra wrote:
> 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@xxxxxxxxxxxxx> 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?

Much more helpful, thanks. And I agree that you have to keep it fairly
brief here: I think you've struck the right balance.  Some nits below.

> 
> ---
>  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

Maybe s/scheduling/Scheduling/ to match the title?

I think I got the picture once I reached the end, but was confused here
by the stages of enablement.  s/This option enables/This option permits/
would be clearer, I think.


> +	  selection across SMT siblings. When enabled -- see
> +	  prctl(PR_SCHED_CORE) -- task selection will ensure all SMT siblings

s/will ensure/ensures that/ (it felt like too many "will"s before)

> +	  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

"SCHED_CORE is default enabled when SCHED_SMT is enabled - when unused there"
would be better.

> +	  should be no impact on performance.
> +

Thanks,
Hugh