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

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Date:	Tue, 22 Apr 2008 21:26:01 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > 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).

ok. In which case the reschedule vector could be consolidated into that 
as well (it's just a special single-CPU call). Then there would be no 
new vector allocations needed at all, just the renaming of 
RESCHEDULE_VECTOR to something more generic.

	Ingo
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