Messages in this thread | | | Date | Fri, 10 Jun 2022 16:15:23 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v7] sched/fair: Consider cpu affinity when allowing NUMA imbalance in find_idlest_group |
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On Fri, Jun 10, 2022 at 11:55:37AM +0100, Valentin Schneider wrote: > 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)
At that point we might just as well do the whole cpumask_and() thing, no? There's not much cost difference between these two operations.
> 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).
I'm confused, NR_CPUS is the right value for CPU_MASK_ALL.
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