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

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Message-ID: <87zk0s5h7c.fsf@rustcorp.com.au>
Date:	Wed, 02 Jan 2013 15:12:15 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>
Cc:	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:

> (cc'ing Rusty, hi!)
>
> Hello, Li.
>
> On Wed, Dec 26, 2012 at 06:20:11PM +0800, Li Zefan wrote:
>> On 2012/11/29 5:34, Tejun Heo wrote:
>> > cpuset_can_attach() prepare global variables cpus_attach and
>> > cpuset_attach_nodemask_{to|from} which are used by cpuset_attach().
>> > There is no reason to prepare in cpuset_can_attach().  The same
>> > information can be accessed from cpuset_attach().
>> > 
>> > Move the prepartion logic from cpuset_can_attach() to cpuset_attach()
>> > and make the global variables static ones inside cpuset_attach().
>> > 
>> > While at it, convert cpus_attach to cpumask_t from cpumask_var_t.
>> > There's no reason to mess with dynamic allocation on a static buffer.
>> > 
>> 
>> But Rusty had been deprecating the use of cpumask_t. I don't know why
>> the final deprecation hasn't been completed yet.
>
> 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.

Cheers,
Rusty.
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