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

Phil Auld
2020-05-28T18:34:36Z

On Thu, May 28, 2020 at 02:17:19PM -0400 Phil Auld wrote:
> On Thu, May 28, 2020 at 07:01:28PM +0200 Peter Zijlstra wrote:
> > 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.
> >
> > Well, don't do that then ;-)
> >
>
> That was a poorly worded example :)
>

Heh, sorry, I thought that was my statement. I do not mean to belittle Joel's
example... That's a fine example of a totally different problem than I
was thinking of :)

Cheers,
Phil

> The point I was trying to make was more that one TID of a group (not cgroup!)
> of tasks is just an arbitrary value.
>
> At a single process (or pair rather) level, sure, you can see it as an
> identifier of whom you want to share with, but even then you have to tag
> both processes with this. And it has less meaning when the whom you want to
> share with is mutltiple tasks.
>
> > > > 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.
> >
> > So the actual permission model can be different depending on how broken
> > the hardware is.
> >
> > > > 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.
> >
> > How do you avoid reuse in such a huge space? That just creates yet
> > another problem for the kernel to keep track of who is who.
> >
>
> The kernel doesn't care or have to track anything. The admin does.
> At the kernel level it's just matching cookies.
>
> Tasks A,B,C all can share core so you give them each A's TID as a cookie.
> Task A then exits. Now B and C are using essentially a random value.
> Task D comes along and want to share with B and C. You have to tag it
> with A's old TID, which has no meaning at this point.
>
> And if A's TID ever gets reused. The new A` gets to share too. At some
> level aren't those still 32bits?
>
> > With random u64 numbers, it even becomes hard to determine if you're
> > sharing at all or not.
> >
> > Now, with the current SMT+MDS trainwreck, any sharing is bad because it
> > allows leaking kernel privates. But under a less severe thread scenario,
> > say where only user data would be at risk, the ptrace() tests make
> > sense, but those become really hard with random u64 numbers too.
> >
> > What would the purpose of random u64 values be for cgroups? That only
> > replicates the problem of determining uniqueness there. Then you can get
> > two cgroups unintentionally sharing because you got lucky.
> >
>
> Seems that would be more flexible for the admin.
>
> What if you had two cgroups you wanted to allow to run together? Or a
> cgroup and a few processes from a different one (say with different
> quotas or something).
>
> I don't have such use cases so I don't feel that strongly but it seemed
> more flexible and followed the mechanism-in-kernel/policy-in-userspace
> dictum rather than basing the functionality on the implementation details.
>
>
> Cheers,
> Phil
>
>
> > Also, fundamentally, we cannot have more threads than TID space, it's a
> > natural identifier.
> >
>
> --

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