linux-kernel - Re: [PATCH 2/2] kvm: use cpumask_var_t for cpus_hardware

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Message-ID: <493CED04.6020209@redhat.com>
Date:	Mon, 08 Dec 2008 11:46:44 +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 2/2] kvm: use cpumask_var_t for cpus_hardware_enabled

Rusty Russell wrote:
>> This isn't on stack, so it isn't buying us anything.
>>     
>
> It's the CONFIG_NR_CPUS=4096 but nr_cpu_ids=4 case which we win using
> dynamic allocation.  Gotta love distribution kernels.
>
>   

What does it buy? 4096/8 = 512 bytes statically allocated?

I understand passing things as pointers, but allocating everything 
dynamically is unCish.

>> Is the plan to drop cpumask_t?
>>     
>
> Yes.  And undefine 'struct cpumask' if CONFIG_CPUMASK_OFFSTACK.  That
> will stop assignment and on-stack declarations for all but the most
> determined.
>
>   
>> If so, we're penalizing non-stack users 
>> by forcing them to go through another pointer (and cacheline).
>>     
>
> Not quite.  If !CONFIG_CPUMASK_OFFSTACK, cpumask_var_t == cpumask_t[1].
> Blame Linus :)
>   

Hm, is there a C trick which will error out when allocating something on 
the stack, but work when allocating statically?  I can think of 
something to do the reverse, but that doesn't help.

Maybe a weak or visibility attribute?  These don't make sense on 
function locals.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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