Messages in this thread |  | | Date | Sun, 23 May 2021 13:30:22 -0700 (PDT) | From | Hugh Dickins <> | Subject | Re: config SCHED_CORE |
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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@infradead.org> 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
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