Re: [PATCH RT 5/8] sched/deadline: Reclaim cpuset bandwidth in .migrate_task_rq()

[Scott Wood <swood@redhat.com>]

On Wed, 2019-10-09 at 09:27 +0200, Juri Lelli wrote:
> On 09/10/19 01:25, Scott Wood wrote:
> > On Tue, 2019-10-01 at 10:52 +0200, Juri Lelli wrote:
> > > On 30/09/19 11:24, Scott Wood wrote:
> > > > On Mon, 2019-09-30 at 09:12 +0200, Juri Lelli wrote:
> > > 
> > > [...]
> > > 
> > > > > Hummm, I was actually more worried about the fact that we call
> > > > > free_old_
> > > > > cpuset_bw_dl() only if p->state != TASK_WAKING.
> > > > 
> > > > Oh, right. :-P  Not sure what I had in mind there; we want to call
> > > > it
> > > > regardless.
> > > > 
> > > > I assume we need rq->lock in free_old_cpuset_bw_dl()?  So something
> > > > like
> > > 
> > > I think we can do with rcu_read_lock_sched() (see
> > > dl_task_can_attach()).
> > 
> > RCU will keep dl_bw from being freed under us (we're implicitly in an
> > RCU
> > sched read section due to atomic context).  It won't stop rq->rd from
> > changing, but that could have happened before we took rq->lock.  If the
> > cpu
> > the task was running on was removed from the cpuset, and that raced with
> > the
> > task being moved to a different cpuset, couldn't we end up erroneously
> > subtracting from the cpu's new root domain (or failing to subtract at
> > all if
> > the old cpu's new cpuset happens to be the task's new cpuset)?  I don't
> > see
> > anything that forces tasks off of the cpu when a cpu is removed from a
> > cpuset (though maybe I'm not looking in the right place), so the race
> > window
> > could be quite large.  In any case, that's an existing problem that's
> > not
> > going to get solved in this patchset.
> 
> OK. So, mainline has got cpuset_read_lock() which should be enough to
> guard against changes to rd(s).
> 
> I agree that rq->lock is needed here.

My point was that rq->lock isn't actually helping, because rq->rd could have
changed before rq->lock is acquired (and it's still the old rd that needs
the bandwidth subtraction).  cpuset_mutex/cpuset_rwsem helps somewhat,
though there's still a problem due to the subtraction not happening until
the task is woken up (by which time cpuset_mutex may have been released and
further reconfiguration could have happened).  This would be an issue even
without lazy migrate, since in that case ->set_cpus_allowed() can get
deferred, but this patch makes the window much bigger.

The right solution is probably to explicitly track the rd for which a task
has a pending bandwidth subtraction (if any), and to continue doing it from
set_cpus_allowed() if the task is not migrate-disabled.  In the meantime, I
think we should drop this patch from the patchset -- without it, lazy
migrate disable doesn't really make the race situation any worse than it
already was.

BTW, what happens to the bw addition in dl_task_can_attach() if a subsequent
can_attach fails and the whole operation is cancelled?

-Scott