Re: [RFC] Cancellable MCS spinlock rework

[Peter Zijlstra]

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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