[PATCH v3?] sched: update load count only once per cpu in 10 tick update window

[Colin Ian King]

On Thu, 2010-04-08 at 12:02 -0400, Chase Douglas wrote:
> There's a period of 10 ticks where calc_load_tasks is updated by all the
> cpus for the load avg. Usually all the cpus do this during the first
> tick. If any cpus go idle, calc_load_tasks is decremented accordingly.
> However, if they wake up calc_load_tasks is not incremented. Thus, if
> cpus go idle during the 10 tick period, calc_load_tasks may be
> decremented to a non-representative value. This issue can lead to
> systems having a load avg of exactly 0, even though the real load avg
> could theoretically be up to NR_CPUS.
> 
> This change defers calc_load_tasks accounting after each cpu updates the
> count until after the 10 tick update window.
> 
> A few points:
> 
> * A global atomic deferral counter, and not per-cpu vars, is needed
>   because a cpu may go NOHZ idle and not be able to update the global
>   calc_load_tasks variable for subsequent load calculations.
> * It is not enough to add calls to account for the load when a cpu is
>   awakened:
>   - Load avg calculation must be independent of cpu load.
>   - If a cpu is awakend by one tasks, but then has more scheduled before
>     the end of the update window, only the first task will be accounted.
> 
> BugLink: http://bugs.launchpad.net/bugs/513848
> 
> Signed-off-by: Chase Douglas <chase.douglas at canonical.com>
> ---
>  kernel/sched.c |   24 ++++++++++++++++++++++--
>  1 files changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 81ede13..c372249 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2967,6 +2967,7 @@ unsigned long nr_iowait(void)
>  
>  /* Variables and functions for calc_load */
>  static atomic_long_t calc_load_tasks;
> +static atomic_long_t calc_load_tasks_deferred;
>  static unsigned long calc_load_update;
>  unsigned long avenrun[3];
>  EXPORT_SYMBOL(avenrun);
> @@ -3021,7 +3022,7 @@ void calc_global_load(void)
>   */
>  static void calc_load_account_active(struct rq *this_rq)
>  {
> -	long nr_active, delta;
> +	long nr_active, delta, deferred;
>  
>  	nr_active = this_rq->nr_running;
>  	nr_active += (long) this_rq->nr_uninterruptible;
> @@ -3029,6 +3030,25 @@ static void calc_load_account_active(struct rq *this_rq)
>  	if (nr_active != this_rq->calc_load_active) {
>  		delta = nr_active - this_rq->calc_load_active;
>  		this_rq->calc_load_active = nr_active;
> +
> +		/*
> +		 * Update calc_load_tasks only once per cpu in 10 tick update
> +		 * window.
> +		 */
> +		if (unlikely(time_before(jiffies, this_rq->calc_load_update) &&
> +			     time_after_eq(jiffies, calc_load_update))) {
> +			if (delta)
> +				atomic_long_add(delta,
> +						&calc_load_tasks_deferred);
> +			return;
> +		}
> +
> +		if (atomic_long_read(&calc_load_tasks_deferred)) {
> +			deferred = atomic_long_xchg(&calc_load_tasks_deferred,
> +						    0);
> +			delta += deferred;
> +		}
> +
>  		atomic_long_add(delta, &calc_load_tasks);
>  	}
>  }
> @@ -3072,8 +3092,8 @@ static void update_cpu_load(struct rq *this_rq)
>  	}
>  
>  	if (time_after_eq(jiffies, this_rq->calc_load_update)) {
> -		this_rq->calc_load_update += LOAD_FREQ;
>  		calc_load_account_active(this_rq);
> +		this_rq->calc_load_update += LOAD_FREQ;
>  	}
>  }


OK - this does have some subtly to it that's for sure, especially with
the this_rg->calc_load_update dependency and hence the need to
re-ordering this after calc_load_account_active. One needs to tread
carefully here.

The description of the change and change needs some hard thought, but
I believe it does the trick as you claim. 

Acked-by: Colin King <colin.king at canonical.com>