Messages in this thread | | | Subject | Re: [RFC] Cancellable MCS spinlock rework | From | Jason Low <> | Date | Mon, 07 Jul 2014 10:22:28 -0700 |
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On Fri, 2014-07-04 at 09:51 +0200, Peter Zijlstra wrote: > 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;
Okay, I will be sending out the patchset I had so that there's something more concrete.
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