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

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Message-ID: <20080423011153.GB17572@wotan.suse.de>
Date:	Wed, 23 Apr 2008 03:11:54 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Jens Axboe <jens.axboe@...cle.com>,
	linux-arch@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	peterz@...radead.org, sam@...nborg.org
Subject: Re: [PATCH 2/11] x86: convert to generic helpers for IPI function calls

On Tue, Apr 22, 2008 at 12:50:30PM -0700, Linus Torvalds wrote:
> 
> 
> On Tue, 22 Apr 2008, Ingo Molnar wrote:
> > 
> > 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.
> 
> Yes.
> 
> Btw, don't get me wrong - I'm not against multiple vectors per se. I just 
> wonder if there is any real reason for the code duplication. 
> 
> And there certainly *can* be tons of valid reasons for it. For example, 
> some of the LAPIC can only have something like two pending interrupts per 
> vector, and after that IPI's would get lost.
> 
> However, since the queuing is actually done with the data structures, I 
> don't think it matters for the IPI's - they don't need any hardware 
> queuing at all, afaik, since even if two IPI's would be merged into one 
> (due to lack of hw queueing) the IPI handling code still has its list of 
> events, so it doesn't matter.
> 
> And performance can be a valid reason ("too expensive to check the shared 
> queue if we only have per-cpu events"), although I$ issues can cause that 
> argument to go both ways.
> 
> I was also wondering whether there are deadlock issues (ie one type of IPI 
> has to complete even if a lock is held for the other type). 
> 
> So I don't dislike the patch per se, I just wanted to understand _why_ the 
> IPI's wanted separate vectors.

The "too expensive to check the shared queue" is one aspect of it. The
shared queue need not have events *for us* (at least, unless Jens has
changed the implementation a bit) but it can still have events that we
would need to check through.

I don't think deadlock is a problem (any more than with multiple vectors).


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