linux-kernel - Re: [PATCH RT 5/8] sched/deadline: Reclaim cpuset bandwidth in .migrate_task

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Date:   Thu, 10 Oct 2019 10:18:00 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Scott Wood <swood@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Clark Williams <williams@...hat.com>,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH RT 5/8] sched/deadline: Reclaim cpuset bandwidth in
 .migrate_task_rq()

On 09/10/19 14:12, Scott Wood wrote:
> 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.

I'm OK with dropping it for now (as we also have other possible issues
as you point out below), but I really wonder what would be a solution
here. Problem is that if a domain(s) reconfiguration happened while the
task was migrate disabled, and we let the reconf destroy/rebuild
domains, the old rd could be gone by the time the task gets migrate
enabled again and the task could continue running, w/o its bandwidth
been accounted for, in a new rd since the migrate enable instant, no?

:-/

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

Oh, yeah, that doesn't look good. :-(

Maybe we can use cancel_attach() to fix things up?

Thanks,

Juri