linux-kernel - Re: [PATCH 06/13] cpuset: cleanup cpuset[_can]

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Message-ID: <871ue35bzk.fsf@rustcorp.com.au>
Date:	Thu, 03 Jan 2013 11:17:11 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Tejun Heo <tj@...nel.org>
Cc:	Li Zefan <lizefan@...wei.com>, paul@...lmenage.org,
	glommer@...allels.com, containers@...ts.linux-foundation.org,
	cgroups@...r.kernel.org, peterz@...radead.org, mhocko@...e.cz,
	bsingharora@...il.com, hannes@...xchg.org,
	kamezawa.hiroyu@...fujitsu.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/13] cpuset: cleanup cpuset[_can]_attach()

Tejun Heo <tj@...nel.org> writes:
> Hello, Rusty.
>
> On Wed, Jan 02, 2013 at 03:12:15PM +1030, Rusty Russell wrote:
>> > Hmmm?  cpumask_t can't be used for stack but other than that I don't
>> > see how it would be deprecated completely.  Rusty, can you please
>> > chime in?
>> 
>> The long-never-quite-complete-plan was for struct cpumask to be
>> undefined when CONFIG_CPUMASK_OFFSTACK=y.  That means noone can declare
>> them, or pass them on the stack, since they'll get a compiler error.
>> 
>> Now, there are some cases where it really is a reason to use a static
>> bitmap, and 1/2 a K of wasted space be damned.  There's a
>> deliberately-ugly way of doing that: declare a bitmap and use
>> to_cpumask().  Of course, if we ever really want to remove NR_CPUS and
>> make it completely generic, we have to kill all these too, but noone is
>> serious about that.
>
> So, I guess this currently is caught in a place which isn't here or
> there.  I'm pretty skeptical whether it makes sense to bother about
> static usages tho.  Can I keep them for static ones?

I didn't realize that cpuset_attach was a fastpath.  If it is, put a
static there and I'll fix turn it into a bitmap when I need to.
Otherwise, please don't change the code in the first place.

Cheers,
Rusty.
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