linux-kernel - Re: [PATCH 05/31] cpumask: Provide new cpumask API

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Date:	Wed, 1 Oct 2008 11:08:09 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Travis <travis@....com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/31] cpumask: Provide new cpumask API

* Mike Travis <travis@....com> wrote:

> Mike Travis wrote:
> > Ingo Molnar wrote:
> ...
> > 
> >> In what way will Rusty's changes differ? Since you incorporate some of 
> >> Rusty's changes already, could you please iterate towards a single 
> >> patchset which we could then start testing?
> > 
> > Our timezones are not very conducive to a lot of email exchanges 
> > (and he's moving.) From what I've seen I believe he's leaning 
> > towards using struct cpumask * and less trickery than I have.

actually, that's quite sane to do. const_cpumask_t looked a bit weird to 
me.

the extra indirection to a cpumask_t is not a big issue IMO, so in that 
sense whether we pass by value or pass by reference is not a _big_ 
performance item.

The complications (both present and expected ones) all come from the 
allocations.

> Oh yeah, I forgot the other major point of Rusty's approach.  He wants 
> the patchset to be completely bisectable.  That's far from true in my 
> version.

well, it should be a smooth transition and completely bisectable, 
there's hundreds of usages of cpumask_t and quite many in the pipeline. 
It's far easier to _you_ to get this stuff to work if it's all gradual 
and is expected to work all across. Have a default-off debug mode that 
turns off compatible cpumask_t perhaps - we can remove that later on.

with 'struct cpumask' we could keep cpumask_t as the compatible API, and 
could see the impact of these changes in a very finegrained and gradual 
way. Seems like a fundamentally sane approach to me ...

	Ingo
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