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SubjectRe: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu

* Jeremy Fitzhardinge <jeremy@goop.org> wrote:

> RCU can only control the lifetime of allocated memory blocks, which
> forces all the call structures to be allocated. This is expensive
> compared to allocating them on the stack, which is the common case for
> synchronous calls.
>
> This patch takes a different approach. Rather than using RCU, the
> queues are managed under rwlocks. Adding or removing from the queue
> requires holding the lock for writing, but multiple CPUs can walk the
> queues to process function calls under read locks. In the common
> case, where the structures are stack allocated, the calling CPU need
> only wait for its call to be done, take the lock for writing and
> remove the call structure.
>
> Lock contention - particularly write vs read - is reduced by using
> multiple queues.

hm, is there any authorative data on what is cheaper on a big box, a
full-blown MESI cache miss that occurs for every reader in this new
fastpath, or a local SLAB/SLUB allocation+free that occurs with the
current RCU approach?

Ingo


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