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    SubjectRe: [PATCH v3] cpu/hotplug: wait for cpuset_hotplug_work to finish on cpu onlining
    Date
    Alexey Klimov <aklimov@redhat.com> writes:
    > When a CPU offlined and onlined via device_offline() and device_online()
    > the userspace gets uevent notification. If, after receiving "online" uevent,
    > userspace executes sched_setaffinity() on some task trying to move it
    > to a recently onlined CPU, then it sometimes fails with -EINVAL. Userspace
    > needs to wait around 5..30 ms before sched_setaffinity() will succeed for
    > recently onlined CPU after receiving uevent.
    >
    > If in_mask argument for sched_setaffinity() has only recently onlined CPU,
    > it could fail with such flow:
    >
    > sched_setaffinity()
    > cpuset_cpus_allowed()
    > guarantee_online_cpus() <-- cs->effective_cpus mask does not
    > contain recently onlined cpu
    > cpumask_and() <-- final new_mask is empty
    > __set_cpus_allowed_ptr()
    > cpumask_any_and_distribute() <-- returns dest_cpu equal to nr_cpu_ids
    > returns -EINVAL
    >
    > Cpusets used in guarantee_online_cpus() are updated using workqueue from
    > cpuset_update_active_cpus() which in its turn is called from cpu hotplug callback
    > sched_cpu_activate() hence it may not be observable by sched_setaffinity() if
    > it is called immediately after uevent.
    >
    > Out of line uevent can be avoided if we will ensure that cpuset_hotplug_work
    > has run to completion using cpuset_wait_for_hotplug() after onlining the
    > cpu in cpu_device_up() and in cpuhp_smt_enable().
    >
    > Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
    > Reviewed-by: Qais Yousef <qais.yousef@arm.com>
    > Co-analyzed-by: Joshua Baker <jobaker@redhat.com>
    > Signed-off-by: Alexey Klimov <aklimov@redhat.com>

    Looks good to me.

    Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>

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