linux-kernel - Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

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Date:	Mon, 28 Apr 2008 09:38:17 +0200
From:	Jes Sorensen <jes@....com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Jens Axboe <jens.axboe@...cle.com>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, npiggin@...e.de,
	torvalds@...ux-foundation.org, peterz@...radead.org,
	sam@...nborg.org
Subject: Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

Jeremy Fitzhardinge wrote:
> Jens Axboe wrote:
>> On Thu, Apr 24 2008, Russell King wrote:
>>> On Tue, Apr 22, 2008 at 08:50:17PM +0200, Jens Axboe wrote:
>>>> +        data->csd.func(data->csd.info);
>>>> +
>>>> +        spin_lock(&data->lock);
>>>> +        cpu_clear(cpu, data->cpumask);
>>>> +        WARN_ON(data->refs == 0);
>>>> +        data->refs--;
>>>> +        refs = data->refs;
>>>>       
>>> Probably a silly question, but what does data->refs do that
>>> cpus_empty(data->cpumask) wouldn't do?  (as indeed ARM presently does.)
>>
>> I guess it can be marginally slower for NR_CPUS > BITS_PER_LONG,
>> otherwise there's absolutely no reason to have a seperate ref counter.
> 
> Jes was concerned about scanning bitmasks on a 4096 CPU Altix.  I'm not 
> sure its all that important, but a refcount check would definitely be 
> quicker.

I just felt it was silly to do a bigger test if it wasn't necessary.
Even on a 4096 CPU box it's probably barely noticeable, but if it adds
cost then I'd be in favor of keeping the slightly faster version. Maybe
it would be worth doing a branched version, one for
NR_CPUS <= BITS_PER_LONG and one for the other case?

Cheers,
Jes

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