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

[Pekka Enberg]

Hi Ingo,

On Fri, Aug 22, 2008 at 9:28 AM, Ingo Molnar <mingo@xxxxxxx> wrote:
>
> * Jeremy Fitzhardinge <jeremy@xxxxxxxx> 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?

Christoph might have an idea about it.
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