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

[Joel Fernandes <joel@joelfernandes.org>]

On Sun, May 24, 2020 at 10:00:46AM -0400, Phil Auld wrote:
> On Fri, May 22, 2020 at 05:35:24PM -0400 Joel Fernandes wrote:
> > On Fri, May 22, 2020 at 02:59:05PM +0200, Peter Zijlstra wrote:
> > [..]
> > > > > 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.
> > 
> > But that would require a bunch of tasks agreeing on which TID to tag with.
> > For example, if 2 tasks tag with each other's TID, then they would have
> > different tags and not share.
> > 
> > What's wrong with passing in an integer instead? In any case, we would do the
> > CAP_SYS_ADMIN check to limit who can do it.
> > 
> > Also, one thing CGroup interface allows is an external process to set the
> > cookie, so I am wondering if we should use sched_setattr(2) instead of, or in
> > addition to, the prctl(2). That way, we can drop the CGroup interface
> > completely. How do you feel about that?
> >
> 
> I think it should be an arbitrary 64bit value, in both interfaces to avoid
> any potential reuse security issues. 
> 
> I think the cgroup interface could be extended not to be a boolean but take
> the value. With 0 being untagged as now.
> 
> And sched_setattr could be used to set it on a per task basis.

Yeah, something like this will be needed.

> > > > 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.

In util-clamp's design (which has task-specific attribute and task-group
attribute), it seems for that the priority is task-specific value first, then
the group one, then the system-wide one.

Perhaps a similar design can be adopted for this interface. So probably we
should let the per-task interface not fail if the task was already in CGroup
and rather prioritize its value first before looking at the group one?

Uclamp's comments:

 * The effective clamp bucket index of a task depends on, by increasing
 * priority:
 * - the task specific clamp value, when explicitly requested from userspace
 * - the task group effective clamp value, for tasks not either in the root
 *   group or in an autogroup
 * - the system default clamp value, defined by the sysadmin

> > > > 
> > > > 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.
> > 
> > If we drop the CGroup interface, this would avoid both #1 and #2.
> >
> 
> I believe both are useful.  Personally, I think the per-task setting should
> win over the cgroup tagging. In that case #1 just falls out.

Cool, this is similar to what I mentioned above.

> And #2 pretty
> much as well. Nothing would happen to the tagged task as they were added
> to the cgroup. They'd keep their explicitly assigned tags and everything
> should "just work". There are other reasons to be in a cpu cgroup together
> than just the core scheduling tag.

Well ok, so there's no reason to fail them the addition to CGroup of a
prctl-tagged task then, we can let it succeed but prioritize the
task-specific attribute over the group-specific one.

> There are a few other edge cases, like if you are in a cgroup, but have
> been tagged explicitly with sched_setattr and then get untagged (presumably
> by setting 0) do you get the cgroup tag or just stay untagged? I think based
> on per-task winning you'd stay untagged. I supposed you could move out and
> back in the cgroup to get the tag reapplied (Or maybe the cgroup interface
> could just be reused with the same value to re-tag everyone who's untagged).

If we maintain a task-specific tag and a group-specific tag, then I think
both tags can coexist and the final tag is decided on priority basis
mentioned above.

So before getting into CGroup, I think first we develop the task-specific
tagging mechanism like Peter was suggesting. So let us talk about that. I
will reply to the other thread Vineeth started while CC'ing you. In
particular, I like Peter's idea about user land passing a TID to share a core
with.

thanks,

 - Joel


> 
> 
> 
> Cheers,
> Phil
> 
> 
> > thanks,
> > 
> >  - Joel
> > 
> 
> -- 
>