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

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Message-ID: <alpine.LFD.1.10.0804221218410.2779@woody.linux-foundation.org>
Date:	Tue, 22 Apr 2008 12:22:32 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Jens Axboe <jens.axboe@...cle.com>, linux-arch@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	npiggin@...e.de, peterz@...radead.org, sam@...nborg.org
Subject: Re: [PATCH 2/11] x86: convert to generic helpers for IPI function
 calls

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
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