Re: [PATCH v2 2/5] cpumask: Introduce for_each_cpu_andnot()

[Valentin Schneider <vschneid@redhat.com>]

On 18/08/22 15:38, Yury Norov wrote:
> On Wed, Aug 17, 2022 at 06:58:09PM +0100, Valentin Schneider wrote:
>> for_each_cpu_and() is very convenient as it saves having to allocate a
>> temporary cpumask to store the result of cpumask_and(). The same issue
>> applies to cpumask_andnot() which doesn't actually need temporary storage
>> for iteration purposes.
>>
>> Following what has been done for for_each_cpu_and(), introduce
>> for_each_cpu_andnot().
>>
>> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
>
> I'm concerned that this series doesn't give us real examples and tests
> for the new API. If we take it as-is, we'll end up with a dead code for
> a while, quite probably for long.
>

Tariq has at least two uses of for_each_numa_hop_cpu() (which uses
for_each_cpu_andnot()) in net/mlx5e and net/enic). My plan here is to make
sure the cpumask and sched/topology changes are OK, and then I'd let Tariq
carry the whole set with actual users on top.

I wouldn't want to see this merged without users, especially given the
EXPORT_SYMBOL_GPL() in 3/5.

> Can you please submit a new code with a real application for the new API?
> Alternatively, you can rework some existing code.
>
> Briefly grepping, I found good candidate in a core code: __sched_core_flip(),
> and one candidate in arch code: arch/powerpc/kernel/smp.c: update_coregroup_mask.
> I believe there are much more.
>

Some of these look fairly trivial, I'll have a look around.

> Regarding the test, I don't think it's strictly necessary to have it as soon as
> we'll have real users, but it's always good to backup with tests.
>

That sounds sensible enough, I'll have a look at that.

> Thanks,
> Yury