On Wed, 2014-07-02 at 18:27 +0200, Peter Zijlstra wrote: > On Wed, Jul 02, 2014 at 09:21:10AM -0700, Jason Low wrote: > > The cancellable MCS spinlock is currently used to queue threads that are > > doing optimistic spinning. It uses per-cpu nodes, where a thread obtaining > > the lock would access and queue the local node corresponding to the CPU that > > it's running on. Currently, the cancellable MCS lock is implemented by using > > pointers to these nodes. > > > > In this RFC patch, instead of operating on pointers to the per-cpu nodes, we > > store the CPU numbers in which the per-cpu nodes correspond to in atomic_t. > > A similar concept is used with the qspinlock. > > > > We add 1 to the CPU number to retrive an "encoded value" representing the > > node > > of that CPU. By doing this, 0 can represent "no CPU", which allows us to > > keep the simple "if (CPU)" and "if (!CPU)" checks. In this patch, the next > > and > > prev pointers in each node were also modified to store encoded CPU values. > > > > By operating on the CPU # of the nodes using atomic_t instead of pointers > > to those nodes, this can reduce the overhead of the cancellable MCS spinlock > > by 32 bits (on 64 bit systems). > > Still struggling to figure out why you did this.
Why I converted pointers to atomic_t? This would avoid the potentially racy ACCESS_ONCE stores + cmpxchg while also using less overhead, since atomic_t is often only 32 bits while pointers could be 64 bits. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
