Re: [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word

[Waiman Long]

On 04/18/2014 01:53 PM, Peter Zijlstra wrote:
> On Fri, Apr 18, 2014 at 01:32:47PM -0400, Waiman Long wrote:
> > On 04/18/2014 04:15 AM, Peter Zijlstra wrote:
> > > On Thu, Apr 17, 2014 at 05:28:17PM -0400, Waiman Long wrote:
> > > > On 04/17/2014 11:49 AM, Peter Zijlstra wrote:
> > > > > On Thu, Apr 17, 2014 at 11:03:56AM -0400, Waiman Long wrote:
> > > > > > @@ -192,36 +220,25 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> > > > > >   	node->next = NULL;
> > > > > >
> > > > > >   	/*
> > > > > > +	 * We touched a (possibly) cold cacheline; attempt the trylock once
> > > > > > +	 * more in the hope someone let go while we weren't watching as long
> > > > > > +	 * as no one was queuing.
> > > > > >   	 */
> > > > > > +	if (!(val&    _Q_TAIL_MASK)&&    queue_spin_trylock(lock))
> > > > > > +		goto release;
> > > > > But you just did a potentially very expensive op; @val isn't
> > > > > representative anymore!
> > > > That is not true. I pass in a pointer to val to trylock_pending() (the
> > > > pointer thing) so that it will store the latest value that it reads from the
> > > > lock back into val. I did miss one in the PV qspinlock exit loop. I will add
> > > > it back when I do the next version.
> > > But you did that read _before_ you touched a cold cacheline, that's 100s
> > > of cycles. Whatever value you read back then is now complete nonsense.
> > For spin_lock(), the lock cacheline is touched by a cmpxchg(). It can takes
> > 100s of cycles whether it is hot or cold.
> Its not the lock cacheline, you just touched the per-cpu node cacheline
> for the first time, setting up the node.

Thank for the clarification, now I know what you mean.

-Longman
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