Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

[Waiman Long]

On 09/14/2015 10:04 AM, Peter Zijlstra wrote:
> On Fri, Sep 11, 2015 at 02:37:37PM -0400, Waiman Long wrote:
> > This patch allows one attempt for the lock waiter to steal the lock
> > when entering the PV slowpath.  This helps to reduce the performance
> > penalty caused by lock waiter preemption while not having much of
> > the downsides of a real unfair lock.
>
> > @@ -416,7 +414,8 @@ queue:
> >   	 * does not imply a full barrier.
> >   	 *
> >   	 */
> If it really were once, like the Changelog says it is, then you could
> have simply added:
>
> 	if (pv_try_steal_lock(...))
> 		goto release;

My previous mail has clarified where the lock stealing happen. Will add 
the necessary comment to the patch.

> here, and not wrecked pv_wait_head() like you did. Note that if you do
> it like this, you also do not need to play games with the hash, because
> you'll never get into that situation.
>
> > -	pv_wait_head(lock, node);
> > +	if (pv_wait_head_and_lock(lock, node, tail))
> > +		goto release;
> >   	while ((val = smp_load_acquire(&lock->val.counter))&  _Q_LOCKED_PENDING_MASK)
> >   		cpu_relax();

Because we need to use atomic op to get the lock, we can't use the 
native logic to do the acquire. I know it is kind of hacky, but I don't 
have a good alternative here.

Cheers,
Longman
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