Messages in this thread | | | Date | Fri, 4 Jul 2014 09:51:22 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC] Cancellable MCS spinlock rework |
| |
On Thu, Jul 03, 2014 at 06:07:23PM -0700, Jason Low wrote: > On Thu, 2014-07-03 at 16:35 -0400, Waiman Long wrote: > > > I do see a point in reducing the size of the rwsem structure. However, I > > don't quite understand the point of converting pointers in the > > optimistic_spin_queue structure to atomic_t. The structure is cacheline > > aligned and there is no saving in size. Converting them to atomic_t does > > have a bit of additional overhead of converting the encoded cpu number > > back to the actual pointer. > > > > So my suggestion is to just change what is stored in the mutex and rwsem > > structure to atomic_t, but keep the pointers in the > > optimistic_spin_queue structure. > > Peter, would you prefer going with the above? > > If we were to keep the pointers to the next and prev nodes in the struct > optimistic_spin_queue instead of converting them to atomic_t to store > their cpu #, we'd still need to keep track of the cpu #. In the unqueue > phase of osq_lock, we might have to reload prev = node->prev which we > then may cmpxchg() it with the lock tail. > > The method we can think of so far would be to add a regular int variable > to optimistic_spin_queue and initialize it to the CPU #, during the time > we also initialize node->locked and node->next at the beginning of > osq_lock. The cost wouldn't be much of an issue since > optimistic_spin_queue is cache aligned.
Let me try and have an actual look at the patch; I'm in the tail end of a flu and my head isn't quite set for details, but I'll buckle up and go look, gimme a few :-) [unhandled content-type:application/pgp-signature] | |