Re: [PATCH v7] sched/fair: Consider cpu affinity when allowing NUMA imbalance in find_idlest_group

[Valentin Schneider <vschneid@redhat.com>]

On 09/06/22 13:54, Peter Zijlstra wrote:
> Now, I can (and have) fixed up the conflict, but it did make me look at
> this in a little more detail; and the thing I noticed is that your:
>
>   'p->nr_cpus_allowed != num_online_cpus()'
>
> test makes no sense. That's basically 'true'. The thing is,
> nr_cpus_allowed is initialized to NR_CPUS, and unless someone somewhere
> did set_cpus_allowed() on it, it'll still be NR_CPUS.
>
> Also, CPU hotplug doesn't change nr_cpus_allowed, so num_online_cpus()
> is just plain wrong.
>
> Now, something that might work is:
>
>   'p->nr_cpus_allowed < num_online_cpus()'
>
> And even that is no guarantee. You can construct a situation where this
> is still false even though you actually have a constrained set.
> Consider a machine with 8 CPUs. Then set the mask to: 0x55, which has 4
> CPUs set. Then offline the last 4 so that the online mask becomes 0x0f.
>
> Then the effective mask is 0x05, and the number we're looking for above
> is 2, but the suggested test would still be false, because
> nr_cpus_allowed would be 4, as would num_online_cpus().
>

IIUC we want to pay special attention when the task isn't allowed to run on
all online CPUs, wouldn't the below do that?

  !cpumask_subset(cpu_online_mask, p->cpus_ptr)

The task affinity can be a superset of the online mask, obvious case is
init_task's CPU_MASK_ALL, and the above test is still false if both masks
are equal.

(Additionnaly we could add a step in sched_init() to "properly" initialize
the init_task mask and remove the NR_CPUS faff).