Re: [PATCH 2/2] kvm: use cpumask_var_t for cpus_hardware_enabled

Mike Travis
2008-12-08T14:29:37Z

> 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.