Re: [patch] sched: beef up wake_wide()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 14, 2015 at 04:17:46PM +0200, Mike Galbraith wrote:
> There's a buglet, 

> We'll not look for a idle cpu when wake_wide() naks want_affine.

*sigh* indeed.. fixing that'll bring us very close to what we started
out wiht..

The one XXX there raises the question on whether we should always so
select_idle_sibling() if we do not have a suitable balance flag, or only
on wakeups.



---
Subject: sched: Beef up wake_wide()
From: Mike Galbraith <umgwanakikbuti@gmail.com>
Date: Fri, 10 Jul 2015 07:19:26 +0200

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 64bit counter.

Cc: morten.rasmussen@arm.com
Cc: riel@redhat.com
Cc: mingo@redhat.com
Signed-off-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Tested-by: Josef Bacik <jbacik@fb.com>
[peterz: frobbings]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1436505566.5715.50.camel@gmail.com
---
 include/linux/sched.h |    4 +-
 kernel/sched/fair.c   |   69 ++++++++++++++++++++++++--------------------------
 2 files changed, 36 insertions(+), 37 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -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
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4726,26 +4726,29 @@ static long effective_load(struct task_g
 
 #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;
-	}
-
-	return 0;
+	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_doma
 	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);
@@ -5015,19 +5011,19 @@ static int get_cpu_usage(int cpu)
 static int
 select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags)
 {
-	struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;
+	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 *
 
 		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_flags & 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 *
 		}
 		/* while loop will break here if sd == NULL */
 	}
-unlock:
 	rcu_read_unlock();
 
 	return new_cpu;