Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

[Waiman Long]

On 10/13/2015 03:39 PM, Peter Zijlstra wrote:
> On Tue, Sep 22, 2015 at 04:50:43PM -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.
> Changelog does not explain the implementation, which is subtle enough to
> warrant a few words.

Will add more information into the changelog.

> > @@ -417,7 +415,8 @@ queue:
> >   	 * does not imply a full barrier.
> >   	 *
> >   	 */
> > -	pv_wait_head(lock, node);
> > +	if (pv_wait_head_and_lock(lock, node, tail))
> > +		goto release;
> That's very much: pv_wait_head_or_lock(), maybe _or_steal() is even
> better.

I am not very good at naming function. Changing it to _or_steal() is 
fine for me.

> >   	while ((val = smp_load_acquire(&lock->val.counter))&  _Q_LOCKED_PENDING_MASK)
> >   		cpu_relax();
> >
> > @@ -454,7 +453,6 @@ queue:
> >   		cpu_relax();
> >
> >   	arch_mcs_spin_unlock_contended(&next->locked);
> > -	pv_kick_node(lock, next);
> Not sure about removing that, breaks symmetry.
>
> >   /*
> > + * Allow one unfair trylock when entering the PV slowpath to reduce the
> > + * performance impact of lock waiter preemption (either explicitly via
> > + * pv_wait or implicitly via PLE). This function will be called once when
> > + * a lock waiter enter the slowpath before being queued.
> > + *
> > + * A little bit of unfairness here can improve performance without many
> > + * of the downsides of a real unfair lock.
> > + */
> > +#define queued_spin_trylock(l)	pv_queued_spin_trylock_unfair(l)
> > +static inline bool pv_queued_spin_trylock_unfair(struct qspinlock *lock)
> > +{
> > +	struct __qspinlock *l = (void *)lock;
> > +
> > +	if (READ_ONCE(l->locked))
> > +		return 0;
> > +	/*
> > +	 * Wait a bit here to ensure that an actively spinning queue head vCPU
> > +	 * has a fair chance of getting the lock.
> > +	 */
> > +	cpu_relax();
> > +
> > +	return cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0;
> > +}
> This doesn't seem to make any sense.. Its also very much distinct from
> the rest of the patch and can easily be added in a separate patch with
> separate performance numbers to show it does (or does not) make a
> difference.

If you mean I don't need an extra cpu_relax() here, I can take that out. 
It was there to make the active queue head vCPU having a higher chance 
of getting the lock, but it is not essential.

Cheers,
Longman

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