Re: [PATCH 2/11] x86: convert to generic helpers for IPI function calls

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, 22 Apr 2008, Ingo Molnar wrote:
> 
> > Or is it just a performance optimization? [...]
> 
> yes and i gave in - Nick and Jens wants to do some crazy stuff and they 
> had the numbers. Here's the previous discussion:
> 
>   http://lkml.org/lkml/2008/2/27/125

No, the previous discussion was about single *queues* vs single *vectors*.

I agree unconditionally with the decision to use a separate per-cpu queue 
from the shared queue (in fact, I would argue that the "mask" code might 
want to notice when the mask is just a single CPU, and turn a mask request 
into a targeted request). 

But I wonder why we want to then have two IPI target vectors, when it 
would appear to be perfectly fine and cheap to have just a single vector 
that can handle both the per-cpu case and the shared queue case (since the 
thing would tend to be one or the other, not both).

A single vector is still pefectly fine, if 99% of all usage cases are the 
targeted-to-a-single-cpu thing, because the shared queue will basically be 
empty (and you can test that without even taking any locks).

		Linus