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.
