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From: Juri Lelli <juri.lelli@redhat.com>
To: Scott Wood <swood@redhat.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Clark Williams <williams@redhat.com>,
	linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org
Subject: Re: [PATCH RT 5/8] sched/deadline: Reclaim cpuset bandwidth in .migrate_task_rq()
Date: Thu, 10 Oct 2019 10:18:00 +0200
Message-ID: <20191010081800.GK19588@localhost.localdomain> (raw)
In-Reply-To: <9c12342ed1e6d180fae3348409fabb9fc045361d.camel@redhat.com>

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

  reply index

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-27  5:56 [RT PATCH 0/8] migrate disable fixes and performance Scott Wood
2019-07-27  5:56 ` [PATCH RT 1/8] sched: migrate_enable: Use sleeping_lock to indicate involuntary sleep Scott Wood
2019-07-29 13:26   ` Steven Rostedt
2019-07-27  5:56 ` [PATCH RT 2/8] sched: __set_cpus_allowed_ptr: Check cpus_mask, not cpus_ptr Scott Wood
2019-09-17 14:57   ` Sebastian Andrzej Siewior
2019-09-17 15:23     ` Scott Wood
2019-07-27  5:56 ` [PATCH RT 3/8] sched: Remove dead __migrate_disabled() check Scott Wood
2019-07-27  5:56 ` [PATCH RT 4/8] sched: migrate disable: Protect cpus_ptr with lock Scott Wood
2019-09-26 16:39   ` Sebastian Andrzej Siewior
2019-09-26 16:52     ` Scott Wood
2019-09-27 12:19       ` Sebastian Andrzej Siewior
2019-09-27 20:02         ` Scott Wood
2019-07-27  5:56 ` [PATCH RT 5/8] sched/deadline: Reclaim cpuset bandwidth in .migrate_task_rq() Scott Wood
2019-09-17 15:10   ` Sebastian Andrzej Siewior
2019-09-27  8:11   ` Juri Lelli
2019-09-27 16:40     ` Scott Wood
2019-09-30  7:12       ` Juri Lelli
2019-09-30 16:24         ` Scott Wood
2019-10-01  8:52           ` Juri Lelli
2019-10-09  6:25             ` Scott Wood
2019-10-09  7:27               ` Juri Lelli
2019-10-09 19:12                 ` Scott Wood
2019-10-10  8:18                   ` Juri Lelli [this message]
2019-07-27  5:56 ` [PATCH RT 6/8] sched: migrate_enable: Set state to TASK_RUNNING Scott Wood
2019-09-17 15:31   ` Sebastian Andrzej Siewior
2019-09-17 17:54     ` Scott Wood
2019-07-27  5:56 ` [PATCH RT 7/8] sched: migrate_enable: Use select_fallback_rq() Scott Wood
2019-09-17 16:00   ` Sebastian Andrzej Siewior
2019-09-24 18:05     ` Scott Wood
2019-07-27  5:56 ` [PATCH RT 8/8] sched: Lazy migrate_disable processing Scott Wood
2019-09-17 16:50   ` Sebastian Andrzej Siewior
2019-09-17 17:06     ` Scott Wood

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