Re: [PATCH 5/8] locking, mutex: Cancelable MCS lock for adaptive spinning

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, Feb 10, 2014 at 02:04:22PM -0800, Jason Low wrote:
> > On Mon, 2014-02-10 at 22:32 +0100, Peter Zijlstra wrote:
> > > Is adding that really much faster than the relatively straight path
> > > oqs_wait_next() would walk to bit the same exit?
> > > 
> > > The only reason I pulled out the above cmpxchg() is because its the
> > > uncontended fast path, which seems like a special enough case.
> > 
> > So it would avoid 2 extra checks (*lock == node) and (node->next) in the
> > oqs_wait_next() path, which aren't necessary when node->next != NULL.
> > 
> > And I think node->next != NULL can be considered a special enough case
> > after the cmpxchg() fails because in the contended case, we're expecting
> > the node->next to be pointing at something. The only times node->next is
> > NULL after cmpxchg() fails are during a very small race window with the
> > osq_lock(), and when the next node is unqueuing due to need_resched,
> > which is also a very small window.
> 
> True all; now if only we had a useful benchmark so we could test if 
> it makes a difference or not :-)

Having useful 'perf bench lock' sub-test(s) that mimic the AIM7 
workload (and other workloads that excercise locking) would address 
that concern to a large degree.

Thanks,

	Ingo