Re: [PATCH RFC] sched: Add a per-thread core scheduling interface

Peter Zijlstra
2020-05-22T12:59:25Z

On Thu, May 21, 2020 at 09:47:05AM -0400, Joel Fernandes wrote:
> Hi Peter,
> Thanks for the comments.
>
> On Thu, May 21, 2020 at 10:51:22AM +0200, Peter Zijlstra wrote:
> > On Wed, May 20, 2020 at 06:26:42PM -0400, Joel Fernandes (Google) wrote:
> > > Add a per-thread core scheduling interface which allows a thread to tag
> > > itself and enable core scheduling. Based on discussion at OSPM with
> > > maintainers, we propose a prctl(2) interface accepting values of 0 or 1.
> > > 1 - enable core scheduling for the task.
> > > 0 - disable core scheduling for the task.
> >
> > Yeah, so this is a terrible interface :-)
>
> I tried to keep it simple. You are right, lets make it better.
>
> > It doens't allow tasks for form their own groups (by for example setting
> > the key to that of another task).
>
> So for this, I was thinking of making the prctl pass in an integer. And 0
> would mean untagged. Does that sound good to you?

A TID, I think. If you pass your own TID, you tag yourself as
not-sharing. If you tag yourself with another tasks's TID, you can do
ptrace tests to see if you're allowed to observe their junk.

> > It is also horribly ill defined what it means to 'enable', with whoem
> > is it allows to share a core.
>
> I couldn't parse this. Do you mean "enabling coresched does not make sense if
> we don't specify whom to share the core with?"

As a corrolary yes. I mostly meant that a blanket 'enable' doesn't
specify a 'who' you're sharing your bits with.

> > OK, so cgroup always wins; is why is that a good thing?
>
> I was just trying to respect the functionality of the CGroup patch in the
> coresched series, after all a gentleman named Peter Zijlstra wrote that
> patch ;-) ;-).

Yeah, but I think that same guy said that that was a shit interface and
only hacked up because it was easy :-)

> More seriously, the reason I did it this way is the prctl-tagging is a bit
> incompatible with CGroup tagging:
>
> 1. What happens if 2 tasks are in a tagged CGroup and one of them changes
> their cookie through prctl? Do they still remain in the tagged CGroup but are
> now going to not trust each other? Do they get removed from the CGroup? This
> is why I made the prctl fail with -EBUSY in such cases.
>
> 2. What happens if 2 tagged tasks with different cookies are added to a
> tagged CGroup? Do we fail the addition of the tasks to the group, or do we
> override their cookie (like I'm doing)?

For #2 I think I prefer failure.

But having the rationale spelled out in documentation (man-pages for
example) is important.

> > > ChromeOS will use core-scheduling to securely enable hyperthreading.
> > > This cuts down the keypress latency in Google docs from 150ms to 50ms
> > > while improving the camera streaming frame rate by ~3%.
> >
> > It doesn't consider permissions.
> >
> > Basically, with the way you guys use it, it should be a CAP_SYS_ADMIN
> > only to enable core-sched.
>
> True, we were relying on the seccomp sandboxing in ChromeOS to protect the
> prctl but you're right and I fixed it for next revision.

With the TID idea above you get the ptrace tests.