Re: [PATCH v2] locking/pvqspinlock: Add lock holder CPU argument to pv_wait()

[Waiman Long]

On 04/20/2016 10:19 AM, Peter Zijlstra wrote:
> On Wed, Apr 20, 2016 at 10:15:09PM +0800, Pan Xinhui wrote:
> > So there is such case that we search the whole hashtable and the lock is not found. :(
> > Waiman assume that if l = null, the lock is not stored. however the lock might be there actually.
> > But to avoid the worst case I just mentioned above, it can quickly finish the lookup.
>
> > > > +
> > > > +			/*
> > > > +			 * We try to locate the queue head pv_node by looking
> > > > +			 * up the hash table. If it is not found, use the
> > > > +			 * CPU in the previous node instead.
> > > > +			 */
> > > > +			hn = pv_lookup_hash(lock);
> > > > +			if (!hn)
> > > > +				hn = pn;
> > > This is potentially expensive... it does not explain why this lookup can
> > > fail etc.. nor mentioned that lock stealing caveat.
> > Yes, it's expensive. Normally, PPC phyp don't always need the correct
> > holder. That means current vcpu can just give up its slice.  There is
> > one lpar hvcall H_CONFER. I paste some spec below.
> Ok, so if we can indeed scan the _entire_ hashtable, then we really
> should not have that in common code. That's seriously expensive.

No, it should not scan the entire hashtable and there is no point in 
doing so.

Cheers,
Longman