Commit b5242e98 authored by Chris Metcalf's avatar Chris Metcalf Committed by Linus Torvalds
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smpboot: allow excluding cpus from the smpboot threads 



This patch series allows the watchdog to run by default only on the
housekeeping cores when nohz_full is in effect; this seems to be a good
compromise short of turning it off completely (since the nohz_full cores
can't tolerate a watchdog).

To provide customizability, we add /proc/sys/kernel/watchdog_cpumask so
that the set of cores running the watchdog can be tuned to different
values after bootup.

To implement this customizability, we add a new
smpboot_update_cpumask_percpu_thread() API to the smpboot_thread
subsystem that lets us park or unpark "unwanted" threads.

And now that threads can be parked for long periods of time, we tweak the
/proc/<pid>/stat and /proc/<pid>/status code so parked threads aren't
reported as running, which is otherwise confusing.

This patch (of 3):

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: default avatarChris Metcalf <cmetcalf@ezchip.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 8c07a308

include/linux/smpboot.h

+5 −0
Original line number Diff line number Diff line
@@ -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);

	cpumask_var_t			cpumask;

	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

kernel/smpboot.c

+58 −1
Original line number Diff line number Diff line
@@ -232,6 +232,7 @@ void smpboot_unpark_threads(unsigned int cpu) 


	mutex_lock(&smpboot_threads_lock);

	list_for_each_entry(cur, &hotplug_threads, list)

		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);

		}

	}



	/* 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 +291,10 @@ int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread) 
	unsigned int cpu;

	int ret = 0;



	if (!alloc_cpumask_var(&plug_thread->cpumask, GFP_KERNEL))

		return -ENOMEM;

	cpumask_copy(plug_thread->cpumask, cpu_possible_mask);



	get_online_cpus();

	mutex_lock(&smpboot_threads_lock);

	for_each_online_cpu(cpu) {
@@ -313,9 +327,52 @@ void smpboot_unregister_percpu_thread(struct smp_hotplug_thread *plug_thread) 
	smpboot_destroy_threads(plug_thread);

	mutex_unlock(&smpboot_threads_lock);

	put_online_cpus();

	free_cpumask_var(plug_thread->cpumask);

}

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);



/*

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