Date: Wed, 20 Apr 2016 16:19:49 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Pan Xinhui <xinhui@linux.vnet.ibm.com>
Cc: Waiman Long <Waiman.Long@hpe.com>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org, Scott J Norton <scott.norton@hpe.com>,
        Douglas Hatch <doug.hatch@hpe.com>
Subject: Re: [PATCH v2] locking/pvqspinlock: Add lock holder CPU argument to
 pv_wait()
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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.