linux-kernel - Re: High CPU load when machine is idle (related to PROBLEM: Unusually high load average when idle in 2.6.35, 2.6.35.1 and later)

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Date:	Wed, 20 Oct 2010 19:26:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Damien Wyart <damien.wyart@...e.fr>
Cc:	Chase Douglas <chase.douglas@...onical.com>,
	Ingo Molnar <mingo@...e.hu>, tmhikaru@...il.com,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Venkatesh Pallipadi <venki@...gle.com>
Subject: Re: High CPU load when machine is idle (related to PROBLEM:
 Unusually high load average when idle in 2.6.35, 2.6.35.1 and later)



OK, how does this work for people? I find my idle load is still a tad
high, but maybe I'm not patient enough.

---
 include/linux/sched.h    |    8 ++++++++
 kernel/sched.c           |   32 ++++++++++++++++++++++++++------
 kernel/sched_idletask.c  |    1 -
 kernel/time/tick-sched.c |    2 ++
 4 files changed, 36 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 0383601..5311ef4 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -145,6 +145,14 @@ extern unsigned long this_cpu_load(void);
 
 extern void calc_global_load(void);
 
+#ifdef CONFIG_NO_HZ
+extern void calc_load_account_idle(void);
+extern void calc_load_account_nonidle(void);
+#else
+static inline void calc_load_account_idle(void) { }
+static inline void calc_load_account_nonidle(void) { }
+#endif
+
 extern unsigned long get_parent_ip(unsigned long addr);
 
 struct seq_file;
diff --git a/kernel/sched.c b/kernel/sched.c
index abf8440..f214222 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -526,6 +526,10 @@ struct rq {
 	/* calc_load related fields */
 	unsigned long calc_load_update;
 	long calc_load_active;
+#ifdef CONFIG_NO_HZ
+	long calc_load_inactive;
+	int calc_load_seq;
+#endif
 
 #ifdef CONFIG_SCHED_HRTICK
 #ifdef CONFIG_SMP
@@ -1833,7 +1837,6 @@ static void cfs_rq_set_shares(struct cfs_rq *cfs_rq, unsigned long shares)
 }
 #endif
 
-static void calc_load_account_idle(struct rq *this_rq);
 static void update_sysctl(void);
 static int get_update_sysctl_factor(void);
 static void update_cpu_load(struct rq *this_rq);
@@ -3111,16 +3114,36 @@ static long calc_load_fold_active(struct rq *this_rq)
  * When making the ILB scale, we should try to pull this in as well.
  */
 static atomic_long_t calc_load_tasks_idle;
+static atomic_t calc_load_seq;
 
-static void calc_load_account_idle(struct rq *this_rq)
+void calc_load_account_idle(void)
 {
+	struct rq *this_rq = this_rq();
 	long delta;
 
 	delta = calc_load_fold_active(this_rq);
+	this_rq->calc_load_inactive = delta;
+	this_rq->calc_load_seq = atomic_read(&calc_load_seq);
+
 	if (delta)
 		atomic_long_add(delta, &calc_load_tasks_idle);
 }
 
+void calc_load_account_nonidle(void)
+{
+	struct rq *this_rq = this_rq();
+
+	if (atomic_read(&calc_load_seq) == this_rq->calc_load_seq) {
+		atomic_long_sub(this_rq->calc_load_inactive, &calc_load_tasks_idle);
+		/*
+		 * Undo the _fold_active() from _account_idle(). This
+		 * avoids us loosing active tasks and creating a negative
+		 * bias
+		 */
+		this_rq->calc_load_active -= this_rq->calc_load_inactive;
+	}
+}
+
 static long calc_load_fold_idle(void)
 {
 	long delta = 0;
@@ -3128,16 +3151,13 @@ static long calc_load_fold_idle(void)
 	/*
 	 * Its got a race, we don't care...
 	 */
+	atomic_inc(&calc_load_seq);
 	if (atomic_long_read(&calc_load_tasks_idle))
 		delta = atomic_long_xchg(&calc_load_tasks_idle, 0);
 
 	return delta;
 }
 #else
-static void calc_load_account_idle(struct rq *this_rq)
-{
-}
-
 static inline long calc_load_fold_idle(void)
 {
 	return 0;
diff --git a/kernel/sched_idletask.c b/kernel/sched_idletask.c
index 9fa0f402..6ca191f 100644
--- a/kernel/sched_idletask.c
+++ b/kernel/sched_idletask.c
@@ -23,7 +23,6 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
 static struct task_struct *pick_next_task_idle(struct rq *rq)
 {
 	schedstat_inc(rq, sched_goidle);
-	calc_load_account_idle(rq);
 	return rq->idle;
 }
 
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 3e216e0..808abd7 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -411,6 +411,7 @@ void tick_nohz_stop_sched_tick(int inidle)
 			ts->tick_stopped = 1;
 			ts->idle_jiffies = last_jiffies;
 			rcu_enter_nohz();
+			calc_load_account_idle();
 		}
 
 		ts->idle_sleeps++;
@@ -520,6 +521,7 @@ void tick_nohz_restart_sched_tick(void)
 
 	ts->inidle = 0;
 
+	calc_load_account_nonidle();
 	rcu_exit_nohz();
 
 	/* Update jiffies first */

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