On 2016年04月20日 22:19, 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. > Okay, I will try to add the holder lookup code in arch/...
But I just come up with one idea, in __pv_queued_spin_unlock_slowpath() we will kick the node->cpu, who will become the holder soon. I think we can somehow record the node->cpu and use it in pv_wait_node :) thanks xinhui
