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)

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2010-10-20 at 16:14 +0200, Peter Zijlstra wrote:

> ---
>  include/linux/sched.h    |    8 ++++++++
>  kernel/sched.c           |   28 +++++++++++++++++++++-------
>  kernel/sched_idletask.c  |    1 -
>  kernel/time/tick-sched.c |    2 ++
>  4 files changed, 31 insertions(+), 8 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..79a29e6 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 +3114,29 @@ 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_add(this_rq->calc_load_inactive, &calc_load_tasks_idle);

So that should read: that atomic_long_sub()

Trouble is, load goes down with that patch fixed, it just never goes
up :/


> +}
> +
>  static long calc_load_fold_idle(void)
>  {
>  	long delta = 0;
> @@ -3128,16 +3144,14 @@ static long calc_load_fold_idle(void)
>  	/*
>  	 * Its got a race, we don't care...
>  	 */
> -	if (atomic_long_read(&calc_load_tasks_idle))
> +	if (atomic_long_read(&calc_load_tasks_idle)) {
> +		atomic_inc(&calc_load_seq);
>  		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 */
>