linux-kernel - [patch] sched: beef up wake

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Date:	Wed, 08 Jul 2015 08:13:46 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Josef Bacik <jbacik@...com>, riel@...hat.com, mingo@...hat.com,
	linux-kernel@...r.kernel.org, morten.rasmussen@....com,
	kernel-team <Kernel-team@...com>
Subject: [patch] sched: 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 wakeup of the 1:N waker,
returning 1 only when the 1:N waker is waking one of its minions.

Correct that, and don't bother doing domain traversal when we know
that all we need to do is check for an idle cpu.

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

Signed-off-by: Mike Galbraith <umgwanakikbuti@...il.com>
Tested-by: Josef Bacik <jbacik@...com>
---
 include/linux/sched.h |    4 +--
 kernel/sched/fair.c   |   56 ++++++++++++++++++++++++--------------------------
 2 files changed, 29 insertions(+), 31 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1351,9 +1351,9 @@ 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
@@ -4730,26 +4730,27 @@ static long effective_load(struct task_g
 
 #endif
 
+/*
+ * Detect 1:N waker/wakee relationship 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 we are seeing a 1:N
+ * relationship, and that load size exceeds socket size.
+ */
 static int wake_wide(struct task_struct *p)
 {
+	unsigned int waker_flips = current->wakee_flips;
+	unsigned int wakee_flips = 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 (waker_flips < wakee_flips)
+		swap(waker_flips, wakee_flips);
+	if (wakee_flips < factor || waker_flips < wakee_flips * factor)
+		return 0;
+	return 1;
 }
 
 static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
@@ -4761,13 +4762,6 @@ 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);
@@ -5021,14 +5015,17 @@ select_task_rq_fair(struct task_struct *
 {
 	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));
-
 	rcu_read_lock();
+	if (sd_flag & SD_BALANCE_WAKE) {
+		want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
+		if (!want_affine)
+			goto select_idle;
+	}
+
 	for_each_domain(cpu, tmp) {
 		if (!(tmp->flags & SD_LOAD_BALANCE))
 			continue;
@@ -5048,10 +5045,11 @@ select_task_rq_fair(struct task_struct *
 	}
 
 	if (affine_sd && cpu != prev_cpu && wake_affine(affine_sd, p, sync))
-		prev_cpu = cpu;
+		new_cpu = cpu;
 
 	if (sd_flag & SD_BALANCE_WAKE) {
-		new_cpu = select_idle_sibling(p, prev_cpu);
+select_idle:
+		new_cpu = select_idle_sibling(p, new_cpu);
 		goto unlock;
 	}
 


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