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SubjectRe: cpusets not cpu hotplug aware
Paul Jackson wrote:
> Nathan wrote:
> > I think it would be more sensible for the default (i.e. user hasn't
> > explicitly configured any cpusets) behavior on a CONFIG_CPUSETS=y
> > kernel to match the behavior with a CONFIG_CPUSETS=n kernel.
>
> Basically, in this situation, that would mean that the code that
> masks a requested sched_setaffinity cpumask with the tasks
> cpuset_cpus_allowed():
>
> cpus_allowed = cpuset_cpus_allowed(p);
> cpus_and(new_mask, new_mask, cpus_allowed);
>
> would not be executed in the case of a kernel configured for cpusets,
> when running on a system that wasn't using cpusets.
>
> That's quite doable, and for systems not actually using cpusets, makes
> good sense.
>
> But it makes a bit of discontinuity in the system behaviour when
> someone starts using cpusets. If someone makes a cpuset, then suddenly
> tasks in the top cpuset start seeing failed sched_setaffinity calls
> for any CPU that was brought online after system boot.
>
> If there is some decent way I can get the cpus_allowed of the top
> cpuset to track the cpu_online_map, then we avoid this discontinuity
> in system behaviour.


How about this? I've verified it fixes the issue but I'm nervous
about the locking.


--- cpuhp-sched_setaffinity.orig/kernel/cpuset.c
+++ cpuhp-sched_setaffinity/kernel/cpuset.c
@@ -2033,6 +2033,31 @@ out:
return err;
}

+static int cpuset_handle_cpuhp(struct notifier_block *nb,
+ unsigned long phase, void *_cpu)
+{
+ unsigned long cpu = (unsigned long)_cpu;
+
+ mutex_lock(&manage_mutex);
+ lock_cpu_hotplug();
+ mutex_lock(&callback_mutex);
+
+ switch (phase) {
+ case CPU_ONLINE:
+ cpu_set(cpu, top_cpuset.cpus_allowed);
+ break;
+ case CPU_DEAD:
+ cpu_clear(cpu, top_cpuset.cpus_allowed);
+ break;
+ }
+
+ mutex_unlock(&callback_mutex);
+ unlock_cpu_hotplug();
+ mutex_unlock(&manage_mutex);
+
+ return 0;
+}
+
/**
* cpuset_init_smp - initialize cpus_allowed
*
@@ -2043,6 +2068,8 @@ void __init cpuset_init_smp(void)
{
top_cpuset.cpus_allowed = cpu_online_map;
top_cpuset.mems_allowed = node_online_map;
+
+ hotcpu_notifier(cpuset_handle_cpuhp, 0);
}

/**
-
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