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.
