Re: [RFC][PATCH 4/4] futex: Rewrite FUTEX_UNLOCK_PI
From: Peter Zijlstra
Date: Wed Oct 05 2016 - 04:33:12 EST
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
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
(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.