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Commit 63b0e9ed authored by Mike Galbraith's avatar Mike Galbraith Committed by Ingo Molnar
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sched/fair: Beef up wake_wide() 



Josef Bacik reported that Facebook sees better performance with their
1:N load (1 dispatch/node, N workers/node) when carrying an old patch
to try very hard to wake to an idle CPU.  While looking at wake_wide(),
I noticed that it doesn't pay attention to the wakeup of a many partner
waker, returning 1 only when waking one of its many partners.

Correct that, letting explicit domain flags override the heuristic.

While at it, adjust task_struct bits, we don't need a 64-bit counter.

Tested-by: default avatarJosef Bacik <jbacik@fb.com>
Signed-off-by: default avatarMike Galbraith <umgwanakikbuti@gmail.com>
[ Tidy things up. ]
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-team<Kernel-team@fb.com>
Cc: morten.rasmussen@arm.com
Cc: riel@redhat.com
Link: http://lkml.kernel.org/r/1436888390.7983.49.camel@gmail.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent fbd705a0

include/linux/sched.h

+2 −2
Original line number Diff line number Diff line
@@ -1359,9 +1359,9 @@ struct task_struct { 
#ifdef CONFIG_SMP

	struct llist_node wake_entry;

	int on_cpu;

	struct task_struct *last_wakee;

	unsigned long wakee_flips;

	unsigned int wakee_flips;

	unsigned long wakee_flip_decay_ts;

	struct task_struct *last_wakee;



	int wake_cpu;

#endif

kernel/sched/fair.c

+33 −34
Original line number Diff line number Diff line
@@ -4726,26 +4726,29 @@ static long effective_load(struct task_group *tg, int cpu, long wl, long wg) 


#endif



/*

 * Detect M:N waker/wakee relationships via a switching-frequency heuristic.

 * A waker of many should wake a different task than the one last awakened

 * at a frequency roughly N times higher than one of its wakees.  In order

 * to determine whether we should let the load spread vs consolodating to

 * shared cache, we look for a minimum 'flip' frequency of llc_size in one

 * partner, and a factor of lls_size higher frequency in the other.  With

 * both conditions met, we can be relatively sure that the relationship is

 * non-monogamous, with partner count exceeding socket size.  Waker/wakee

 * being client/server, worker/dispatcher, interrupt source or whatever is

 * irrelevant, spread criteria is apparent partner count exceeds socket size.

 */

static int wake_wide(struct task_struct *p)

{

	unsigned int master = current->wakee_flips;

	unsigned int slave = p->wakee_flips;

	int factor = this_cpu_read(sd_llc_size);



	/*

	 * Yeah, it's the switching-frequency, could means many wakee or

	 * rapidly switch, use factor here will just help to automatically

	 * adjust the loose-degree, so bigger node will lead to more pull.

	 */

	if (p->wakee_flips > factor) {

		/*

		 * wakee is somewhat hot, it needs certain amount of cpu

		 * resource, so if waker is far more hot, prefer to leave

		 * it alone.

		 */

		if (current->wakee_flips > (factor * p->wakee_flips))

			return 1;

	}



	if (master < slave)

		swap(master, slave);

	if (slave < factor || master < slave * factor)

		return 0;

	return 1;

}



static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
@@ -4757,13 +4760,6 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync) 
	unsigned long weight;

	int balanced;



	/*

	 * If we wake multiple tasks be careful to not bounce

	 * ourselves around too much.

	 */

	if (wake_wide(p))

		return 0;



	idx	  = sd->wake_idx;

	this_cpu  = smp_processor_id();

	prev_cpu  = task_cpu(p);
@@ -5017,17 +5013,17 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f 
{

	struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;

	int cpu = smp_processor_id();

	int new_cpu = cpu;

	int new_cpu = prev_cpu;

	int want_affine = 0;

	int sync = wake_flags & WF_SYNC;



	if (sd_flag & SD_BALANCE_WAKE)

		want_affine = cpumask_test_cpu(cpu, tsk_cpus_allowed(p));

		want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));



	rcu_read_lock();

	for_each_domain(cpu, tmp) {

		if (!(tmp->flags & SD_LOAD_BALANCE))

			continue;

			break;



		/*

		 * If both cpu and prev_cpu are part of this domain,
@@ -5041,17 +5037,21 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f 


		if (tmp->flags & sd_flag)

			sd = tmp;

		else if (!want_affine)

			break;

	}



	if (affine_sd && cpu != prev_cpu && wake_affine(affine_sd, p, sync))

		prev_cpu = cpu;



	if (sd_flag & SD_BALANCE_WAKE) {

		new_cpu = select_idle_sibling(p, prev_cpu);

		goto unlock;

	if (affine_sd) {

		sd = NULL; /* Prefer wake_affine over balance flags */

		if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))

			new_cpu = cpu;

	}



	while (sd) {

	if (!sd) {

		if (sd_flag & SD_BALANCE_WAKE) /* XXX always ? */

			new_cpu = select_idle_sibling(p, new_cpu);



	} else while (sd) {

		struct sched_group *group;

		int weight;
@@ -5085,7 +5085,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f 
		}

		/* while loop will break here if sd == NULL */

	}

unlock:

	rcu_read_unlock();



	return new_cpu;