Re: [PATCH v10 1/3] smpboot: allow excluding cpus from the smpboot threads

Frederic Weisbecker
2015-05-01T08:54:07Z

On Thu, Apr 30, 2015 at 03:39:24PM -0400, Chris Metcalf wrote:
> This change allows some cores to be excluded from running the
> smp_hotplug_thread tasks. The following commit to update
> kernel/watchdog.c to use this functionality is the motivating
> example, and more information on the motivation is provided there.
>
> A new smp_hotplug_thread field is introduced, "cpumask", which
> is cpumask field managed by the smpboot subsystem that indicates whether
> or not the given smp_hotplug_thread should run on that core; the
> cpumask is checked when deciding whether to unpark the thread.
>
> To limit the cpumask to less than cpu_possible, you must call
> smpboot_update_cpumask_percpu_thread() after registering.
>
> Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
> ---
> include/linux/smpboot.h | 5 +++++
> kernel/smpboot.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 59 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/smpboot.h b/include/linux/smpboot.h
> index d600afb21926..7c42153edfac 100644
> --- a/include/linux/smpboot.h
> +++ b/include/linux/smpboot.h
> @@ -27,6 +27,8 @@ struct smpboot_thread_data;
> * @pre_unpark: Optional unpark function, called before the thread is
> * unparked (cpu online). This is not guaranteed to be
> * called on the target cpu of the thread. Careful!
> + * @cpumask: Internal state. To update which threads are unparked,
> + * call smpboot_update_cpumask_percpu_thread().
> * @selfparking: Thread is not parked by the park function.
> * @thread_comm: The base name of the thread
> */
> @@ -41,11 +43,14 @@ struct smp_hotplug_thread {
> void (*park)(unsigned int cpu);
> void (*unpark)(unsigned int cpu);
> void (*pre_unpark)(unsigned int cpu);
> + struct cpumask cpumask;

I believe it should be allocated dynamically, otherwise it gets the size of NR_CPUS
instead of nr_cpus_bits. It's not _that_ much space spared but think there should be
several struct smp_hotplug_thread registered.

> bool selfparking;
> const char *thread_comm;
> };
>
> int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread);
> void smpboot_unregister_percpu_thread(struct smp_hotplug_thread *plug_thread);
> +int smpboot_update_cpumask_percpu_thread(struct smp_hotplug_thread *plug_thread,
> + const struct cpumask *);
>
> #endif
> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
> index c697f73d82d6..209750ab7031 100644
> --- a/kernel/smpboot.c
> +++ b/kernel/smpboot.c
> @@ -232,7 +232,8 @@ void smpboot_unpark_threads(unsigned int cpu)
>
> mutex_lock(&smpboot_threads_lock);
> list_for_each_entry(cur, &hotplug_threads, list)
> - smpboot_unpark_thread(cur, cpu);
> + if (cpumask_test_cpu(cpu, &cur->cpumask))
> + smpboot_unpark_thread(cur, cpu);
> mutex_unlock(&smpboot_threads_lock);
> }
>
> @@ -258,6 +259,15 @@ static void smpboot_destroy_threads(struct smp_hotplug_thread *ht)
> {
> unsigned int cpu;
>
> + /* Unpark any threads that were voluntarily parked. */
> + for_each_cpu_not(cpu, &ht->cpumask) {
> + if (cpu_online(cpu)) {
> + struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu);
> + if (tsk)
> + kthread_unpark(tsk);

I'm still not clear why we are doing that. kthread_stop() should be able
to handle parked kthreads, otherwise it needs to be fixed.

> + }
> + }
> +
> /* We need to destroy also the parked threads of offline cpus */
> for_each_possible_cpu(cpu) {
> struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu);
> @@ -281,6 +291,7 @@ int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread)
> unsigned int cpu;
> int ret = 0;
>
> + cpumask_copy(&plug_thread->cpumask, cpu_possible_mask);
> get_online_cpus();
> mutex_lock(&smpboot_threads_lock);
> for_each_online_cpu(cpu) {
> @@ -316,6 +327,48 @@ void smpboot_unregister_percpu_thread(struct smp_hotplug_thread *plug_thread)
> }
> EXPORT_SYMBOL_GPL(smpboot_unregister_percpu_thread);
>
> +/**
> + * smpboot_update_cpumask_percpu_thread - Adjust which per_cpu hotplug threads stay parked
> + * @plug_thread: Hotplug thread descriptor
> + * @new: Revised mask to use
> + *
> + * The cpumask field in the smp_hotplug_thread must not be updated directly
> + * by the client, but only by calling this function.
> + */
> +int smpboot_update_cpumask_percpu_thread(struct smp_hotplug_thread *plug_thread,
> + const struct cpumask *new)
> +{
> + struct cpumask *old = &plug_thread->cpumask;
> + cpumask_var_t tmp;
> + unsigned int cpu;
> +
> + if (!alloc_cpumask_var(&tmp, GFP_KERNEL))
> + return -ENOMEM;
> +
> + get_online_cpus();
> + mutex_lock(&smpboot_threads_lock);
> +
> + /* Park threads that were exclusively enabled on the old mask. */
> + cpumask_andnot(tmp, old, new);
> + for_each_cpu_and(cpu, tmp, cpu_online_mask)
> + smpboot_park_thread(plug_thread, cpu);
> +
> + /* Unpark threads that are exclusively enabled on the new mask. */
> + cpumask_andnot(tmp, new, old);
> + for_each_cpu_and(cpu, tmp, cpu_online_mask)
> + smpboot_unpark_thread(plug_thread, cpu);
> +
> + cpumask_copy(old, new);
> +
> + mutex_unlock(&smpboot_threads_lock);
> + put_online_cpus();
> +
> + free_cpumask_var(tmp);
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(smpboot_update_cpumask_percpu_thread);
> +
> static DEFINE_PER_CPU(atomic_t, cpu_hotplug_state) = ATOMIC_INIT(CPU_POST_DEAD);
>
> /*
> --
> 2.1.2
>