Re: [PATCH v2 2/5] locking/pvqspinlock: Fix missed PV wakeup problem

[Pan Xinhui]

å 16/7/15 16:47, Peter Zijlstra åé:
> So the reason I never get around to this is because the patch stinks.
>
> It simply doesn't make sense... Remember, the harder you make a reviewer
> work the less likely the review will be done.
>
> Present things in clear concise language and draw a picture.
>
> On Tue, May 31, 2016 at 12:53:48PM -0400, Waiman Long wrote:
> > Currently, calling pv_hash() and setting _Q_SLOW_VAL is only
> > done once for any pv_node. It is either in pv_kick_node() or in
> > pv_wait_head_or_lock().
>
> So far so good....
>
> > Because of lock stealing, a pv_kick'ed node is
> > not guaranteed to get the lock before the spinning threshold expires
> > and has to call pv_wait() again. As a result, the new lock holder
> > won't see _Q_SLOW_VAL and so won't wake up the sleeping vCPU.
waiman, it might be "as a result, the head node will not really enter wait state because ->locked is not_Q_SLOW_VAL, the pv_wait will return directly."

> *brain melts* what!? pv_kick'ed node reads like pv_kick_node() and that
> doesn't make any kind of sense.
>
> I'm thinking you're trying to say this:
>
>
> CPU0			CPU1			CPU2
>
> __pv_queued_spin_unlock_slowpath()
>   ...
>   smp_store_release(&l->locked, 0);
> 			__pv_queued_spin_lock_slowpath()
> 			  ...
> 			  pv_queued_spin_steal_lock()
> 			    cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0
>
>
> 						pv_wait_head_or_lock()
>
>   pv_kick(node->cpu);  ---------------------->	  pv_wait(&l->locked, _Q_SLOW_VAL);
>
> 			__pv_queued_spin_unlock()
> 			  cmpxchg(&l->locked, _Q_LOCKED_VAL, 0) == _Q_LOCKED_VAL
>
> 						  for () {
> 						    trylock_clear_pending();
> 						    cpu_relax();
> 						  }
>
> 						  pv_wait(&l->locked, _Q_SLOW_VAL);
>
>
> Which is indeed 'bad', but not fatal, note that the later pv_wait() will
> not in fact go wait, since l->locked will _not_ be _Q_SLOW_VAL.
hi, Peter

the problem is that "this later pv_wait will do nothing as l->locked is not _Q_SLOW_VAL",
So it is not paravirt friendly then. we will go into the trylock loop again and again until the lock is unlocked.

So if we are kicked by the unlock_slowpath, and the lock is stealed by someone else,  we need hash its node again and set l->locked to _Q_SLOW_VAL, then enter pv_wait.

but I am worried about lock stealing. could the node in the queue starve for a long time? I notice the latency of pv_wait on an over-commited guest can be bigger than 300us. I have not seen such starving case, but I think it is possible to happen.

thanks
xinhui

> Is this indeed the 3 CPU scenario you tried to describe in a scant 4
> lines of text, or is there more to it?