linux-kernel - Re: [RFC 09/13] genapic: reduce stack pressuge in io_apic.c step 1 temp cpumask

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Date:	Mon, 08 Sep 2008 09:03:02 -0700
From:	Mike Travis <travis@....com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	davej@...emonkey.org.uk, David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jack Steiner <steiner@....com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Jes Sorensen <jes@....com>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 09/13] genapic: reduce stack pressuge in io_apic.c step
 1 temp cpumask_ts

Andi Kleen wrote:
> Mike Travis <travis@....com> writes:
> 
>>   * Step 1 of cleaning up io_apic.c removes local cpumask_t variables
>>     from the stack.
> 
> Sorry that patch seems incredibly messy. Global variables
> and a tricky ordering and while it's at least commented it's still a mess
> and maintenance unfriendly.
> 
> Also I think set_affinity is the only case where a truly arbitary cpu
> mask can be passed in anyways. And it's passed in from elsewhere. 
> 
> The other cases generally just want to handle a subset of CPUs which
> are nearby. How about you define a new cpumask like type that 
> consists of a start/stop CPU and a mask for that range only 
> and is not larger than a few words?
> 
> I think with that the nearby assignments could be handled 
> reasonably cleanly with arguments and local variables.
> 
> And I suspect with some restructuring set_affinity could
> be also made to support such a model.
> 
> -Andi

Thanks for the comments.  I did mull over something like this early on
in researching this "cpumask" problem, but the problem of having different
cpumask operators didn't seem worthwhile.  But perhaps for a very limited
use (with very few ops), it would be worthwhile.

But how big to make these?  Variable sized?  Config option?  Should I
introduce some kind of MAX_CPUS_PER_NODE constant?  (NR_CPUS/MAX_NUMNODES
I don't think is the right answer.)

Thanks,
Mike
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