Re: [RFC] Cancellable MCS spinlock rework

[Peter Zijlstra]

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?

Yeah..

> 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.

Right, there's actually a 4 byte hole in there aside from the full
cacheline alignment, so yeah, tons of space.

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