Re: [Xen-devel] [PATCH 1/2] IRQ: allocate CPU masks dynamically

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.11.11 at 16:43, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 03/11/11 15:17, Jan Beulich wrote:
>>>>> On 03.11.11 at 15:49, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 03/11/11 14:26, Jan Beulich wrote:
>>>> IRQ: allocate CPU masks dynamically
>>>>
>>>> This includes delaying the initialization of dynamically created IRQs
>>>> until their actual first use and some further elimination of uses of
>>>> struct irq_cfg.
>>>>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>> Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>
>>> One query which may or may not affect the patch.  Would we get better
>>> caching characteristics if all cpumasks were allocated in consecutive
>>> memory, rather than having 3 individual allocs in arch_init_one_irq_desc ?
>> That was what the first version of the patch did, rejected by Keir
>> (and not liked too much by me either).
>>
>> Jan
> 
> My understanding of the objection was hiding the variables themselves as
> an array in the code.
> 
> An alternative approach such as alloc'ing 3*sizeof(cpu mask) (cache
> aligned) and assigning the relevant pointers to the current
> cpumask_var_t's would be a suitable approach which causes the cpumasks
> to be in contiguous memory, but not changing how they are referenced in
> the code.

That would mean just open-coding what the former patch did by
abstraction. In my opinion that is even worse - either we want a
generally usable mechanism to do this, or we don't do it at all.

Jan


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