Messages in this thread | | | Date | Wed, 2 Jul 2014 18:27:49 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC] Cancellable MCS spinlock rework |
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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.
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