linux-kernel - Re: [RFC][PATCH 4/4] futex: Rewrite FUTEX_UNLOCK

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Date:   Wed, 5 Oct 2016 10:32:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     mingo@...nel.org, tglx@...utronix.de, juri.lelli@....com,
        rostedt@...dmis.org, xlpang@...hat.com,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        jdesfossez@...icios.com, bristot@...hat.com
Subject: Re: [RFC][PATCH 4/4] futex: Rewrite FUTEX_UNLOCK_PI

On Wed, Oct 05, 2016 at 10:21:02AM +0200, Sebastian Andrzej Siewior wrote:
> On 2016-10-05 10:09:12 [+0200], Peter Zijlstra wrote:
> > On Wed, Oct 05, 2016 at 09:41:47AM +0200, Sebastian Andrzej Siewior wrote:
> > > are those problems DL related?
> > 
> > One of them, the other is that PI thing you did that ugly nodeboost
> > thing for, right?
> 
> this no-de-boost yes. This is probably a problem since we have this
> "delayed" wake-up. I've been thinking about a marked in PI state to
> ignore a de-boost so the spin_unlock() won't be a problem. But if I
> understand it right, then this won't solve the DL problem since you
> can't have two tasks at the same priority.

The primary concern for DL right now is being able to have a stable
pointer to the top waiter. We do this by having  rt_mutex_setprio()
update the pointer while holding both rq->lock and tsk->pi_lock.

This means the pointer is stable when holding either lock, which is
sufficient.

But this means, we need to deboost _before_ we wake. Otherwise the task
could've continued running and called do_exit() on us.

Secondary, once we start looking at BWI (bandwidth inheritance), where a
blocked DL task donates its runtime budget along with its deadline, we
also very much need this, since a task cannot be running of its own
budget while at the same time the boosted task is also running off that
same budget.

(having the 'blocked' DL task spin-waiting, as per optimistic spinning,
makes all that rather 'interesting').

In any case, this is two problems:

  - your inversion issue
  - my pointer stability (and eventually bandwidth issue)

that are caused by this hb->lock being in the way.