Re: [PATCH] smp, ipi: Speed up IPI handling by invoking the callbacks in reverse order

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 06/05/2014 12:56 PM, Peter Zijlstra wrote:
> On Thu, Jun 05, 2014 at 01:37:25AM +0530, Srivatsa S. Bhat wrote:
>> On 06/05/2014 01:17 AM, Peter Zijlstra wrote:
>>> On Thu, Jun 05, 2014 at 01:09:35AM +0530, Srivatsa S. Bhat wrote:
>>>> The current implementation of lockless list (llist) has a drawback: if we
>>>> want to traverse the list in FIFO order (oldest to newest), we need to
>>>> reverse the list first (and this can be expensive if the list is large,
>>>> since this is an O(n) operation).
>>>
>>> Have you actually looked at the queue depth of this thing? Large queues
>>> are a problem for interrupt latency.
>>>
>>
>> Actually, I wrote this patch just by looking at the code and realizing
>> that we don't need to reverse the list. In practice, I haven't actually
>> seen any noticeable interrupt latencies or large queues so far. So I think
>> this patch is just a very tiny optimization, that's all.
> 
> So conceptually it makes sense to service in FIFO because the first
> entry is waiting longest, by servicing them in LIFO order you get far
> more variance in latency.
> 
> And if the list is small, the cost isn't high.
> 
> Then again, we don't have any good numbers one way or the other.
> 

Hmm, right. I thought hard to see if there is a clever way to maintain
the llist in the FIFO order itself, while still preserving the atomicity
guarantees, but I couldn't think of anything sane :-(

Regards,
Srivatsa S. Bhat