linux-kernel - Re: [PATCH 1/2] kvm: use modern cpumask primitives, no cpumask

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Message-ID: <493BF648.6060504@redhat.com>
Date:	Sun, 07 Dec 2008 18:14:00 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	kvm-devel <kvm@...r.kernel.org>, linux-kernel@...r.kernel.org,
	Mike Travis <travis@....com>
Subject: Re: [PATCH 1/2] kvm: use modern cpumask primitives, no cpumask_t
 on stack

Avi Kivity wrote:
> Rusty Russell wrote:
>> We're getting rid on on-stack cpumasks for large NR_CPUS.
>>
>> 1) Use cpumask_var_t and alloc_cpumask_var (a noop normally).  Fallback
>>    code is inefficient but never happens in practice.
>
> Wow, code duplication from Rusty. Things must be bad.
>
> Since we're in a get_cpu() here, how about a per_cpu static cpumask 
> instead? I don't mind the inefficient fallback, just the duplication.
>

Btw, for the general case, instead of forcing everyone to duplicate, how 
about:

cpumask_var_t cpus;

with_cpumask(cpus) {
   ... code to populate cpus
   smp_call_function_some(...);
} end_with_cpumask(cpus);

Where with_cpumask() allocates cpus, and uses a mutex + static fallback 
on failure.

May need a couple of variants (spinlock + GFP_NOWAIT, mutex with 
sleeping allocation).

-- 
error compiling committee.c: too many arguments to function

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