Re: [PATCH v4] sched/core: Use zero length to reset cpumasks in sched_setaffinity()

[Waiman Long <longman@redhat.com>]

On 10/4/23 06:06, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@infradead.org> wrote:
>
>> On Wed, Oct 04, 2023 at 11:23:41AM +0200, Ingo Molnar wrote:
>>
>>>>   	if (user_mask) {
>>>> -		cpumask_copy(user_mask, in_mask);
>>>> +		/*
>>>> +		 * All-set user cpumask resets affinity and drops the explicit
>>>> +		 * user mask.
>>>> +		 */
>>>> +		cpumask_and(user_mask, in_mask, cpu_possible_mask);
>>>> +		if (cpumask_equal(user_mask, cpu_possible_mask)) {
>>>> +			kfree(user_mask);
>>>> +			user_mask = NULL;
>>>> +		}
>>> Question: is there any observable behavioral difference between current
>>> (old) all-set cpumask calls and the patched (new) one?
>> Very little I think -- the main difference is that we no longer carry
>> the ->user_cpus_ptr mask around, and that saves a little masking.
> So calling with a full mask would actually work fine on 'old' kernels too,
> as it's a 'reset' event in essence. (With a bit of allocation & masking
> overhead.)
>
> This pretty unambiguously marks the full-mask solution as the superior ABI ...

I am fine with that one too. I do have a little bit concern about that 
the difference in behavior when the full mask is passed in, but that is 
reverting to the old behavior before commit 8f9ea86fdf99 ("sched: Always 
preserve the user requested cpumask").

BTW, we can probably check the in_mask directly earlier to skip an 
unnecessary cpumask allocation and free in this particular case.

Cheers,
Longman