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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Fri, 21 May 2021 07:57:35 -0400
Message-ID: <CAEXW_YRxOqQCF2FgXAjL3xkZhRD4rdFuxvyPd-ESXYQQ78cyfQ@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 3:53 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, May 20, 2021 at 08:06:07PM -0700, Hugh Dickins wrote:
> > Hi Peter,
> >
> > make oldconfig gave me no help at all on how to decide whether to choose
> > SCHED_CORE Y or n, beyond it recommending Y.  Maybe you'll delete that
> > option later, or maybe removing the prompt string would silence it.
>
> Ah, you're quite right. I never seem to have gotten around to actually
> writing anything useful there :/ Similarly the documentation for all
> this seems to have gone missing too.
>
> Joel, could I ask you to refresh the document to match the current state
> of things and repost? I still whole hartedly despise this RST crud, it
> makes it so hard to read / modify the files.
>
> ( I think the latest version is here:
>   https://lkml.kernel.org/r/20210324214020.34142-8-joel@joelfernandes.org
> )
>
> Anyway, how is something like the below, Joel can add a reference to the
> document once it's there.
>
> ---
>  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?