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

[Peter Zijlstra]

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 :-)

It doens't allow tasks for form their own groups (by for example setting
the key to that of another task).

It is also horribly ill defined what it means to 'enable', with whoem
is it allows to share a core.

> Special cases:

> (1)
> The core-scheduling patchset contains a CGroup interface as well. In
> order for us to respect users of that interface, we avoid overriding the
> tag if a task was CGroup-tagged because the task becomes inconsistent
> with the CGroup tag. Instead return -EBUSY.
> 
> (2)
> If a task is prctl-tagged, allow the CGroup interface to override
> the task's tag.

OK, so cgroup always wins; is why is that a good thing?

> 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.

That also means we should very much default to disable.