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

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Date:	Mon, 08 Dec 2008 06:29:33 -0800
From:	Mike Travis <travis@....com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Avi Kivity <avi@...hat.com>, kvm-devel <kvm@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] kvm: use cpumask_var_t for cpus_hardware_enabled

Rusty Russell wrote:
> On Monday 08 December 2008 20:16:44 Avi Kivity wrote:
>> 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?
> 
> It adds up, and 4096 seems to be only the start of the insanityH^H^Hfun.

The real win though is when cpumask_size represents the actual size of the
cpumask (based on # of possible cpus) instead of the pre-configured size
of NR_CPUS.  So for 99.9% of the systems (having 64 or fewer cpus), the
savings will be 504 bytes not allocated.

> 
>>> 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.
> 
> We also need to prevent assignment, eg:
> 
> 	*foo = *bar;
> 
> Because when we allocate them, we'll cut them to size.
> 
> Cheers,
> Rusty.

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