linux-kernel - Re: commit e9e9250b: sync wakeup bustage when waker is an RT task

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Date:	Mon, 31 May 2010 13:56:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: commit e9e9250b: sync wakeup bustage when waker is an RT task

On Sun, 2010-05-16 at 09:21 +0200, Mike Galbraith wrote:
> sched: RT waker sync wakeup bugfix
> 
> An RT waker's weight is not on the runqueue, but we try to subrtact it anyway
> in the sync wakeup case,  sending this_load negative.  This leads to affine
> wakeup failure in cases where it should succeed.  This was found while testing
> an PREEMPT_RT kernel with lmbench's lat_udp.  In a PREEMPT_RT kernel, softirq
> threads act as a ~proxy for the !RT buddy.  Approximate !PREEMPT_RT sync wakeup
> behavior by looking at the buddy instead, and subtracting the maximum task weight
> that will not send this_load negative. 


Does the below work?

Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
---
 kernel/sched.c      |    4 ++--
 kernel/sched_fair.c |   36 ++++++++++++++++++++++++++++++------
 2 files changed, 32 insertions(+), 8 deletions(-)

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -1877,8 +1877,8 @@ static void dec_nr_running(struct rq *rq
 static void set_load_weight(struct task_struct *p)
 {
 	if (task_has_rt_policy(p)) {
-		p->se.load.weight = prio_to_weight[0] * 2;
-		p->se.load.inv_weight = prio_to_wmult[0] >> 1;
+		p->se.load.weight = 0;
+		p->se.load.inv_weight = WMULT_CONST;
 		return;
 	}
 
Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c
+++ linux-2.6/kernel/sched_fair.c
@@ -1220,12 +1220,26 @@ static inline unsigned long effective_lo
 
 #endif
 
+static unsigned long cpu_power(int cpu)
+{
+	struct sched_domain *sd;
+	struct sched_group *sg;
+
+	sd = rcu_dereference_check_sched_domain(cpu_rq(cpu)->sd);
+	if (!sd)
+		return 1024;
+	sg = sd->groups;
+	if (!sg)
+		return 1024;
+
+	return sg->cpu_power;
+}
+
 static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
 {
 	unsigned long this_load, load;
 	int idx, this_cpu, prev_cpu;
 	unsigned long tl_per_task;
-	unsigned int imbalance;
 	struct task_group *tg;
 	unsigned long weight;
 	int balanced;
@@ -1252,8 +1266,6 @@ static int wake_affine(struct sched_doma
 	tg = task_group(p);
 	weight = p->se.load.weight;
 
-	imbalance = 100 + (sd->imbalance_pct - 100) / 2;
-
 	/*
 	 * In low-load situations, where prev_cpu is idle and this_cpu is idle
 	 * due to the sync cause above having dropped this_load to 0, we'll
@@ -1263,9 +1275,21 @@ static int wake_affine(struct sched_doma
 	 * Otherwise check if either cpus are near enough in load to allow this
 	 * task to be woken on this_cpu.
 	 */
-	balanced = !this_load ||
-		100*(this_load + effective_load(tg, this_cpu, weight, weight)) <=
-		imbalance*(load + effective_load(tg, prev_cpu, 0, weight));
+	if (this_load) {
+		unsigned long this_eff_load, prev_eff_load;
+
+		this_eff_load = 100;
+		this_eff_load *= cpu_power(prev_cpu);
+		this_eff_load *= this_load +
+			effective_load(tg, this_cpu, weight, weight);
+
+		prev_eff_load = 100 + (sd->imbalance_pct - 100) / 2;
+		prev_eff_load *= cpu_power(this_cpu);
+		prev_eff_load *= load + effective_load(tg, prev_cpu, 0, weight);
+
+		balanced = this_eff_load <= prev_eff_load;
+	} else
+		balanced = true;
 
 	/*
 	 * If the currently running task will sleep within

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