[Lucid] SRU: request pull EC2 Revert "sched: update load count only once per cpu in 10 tick update window"

[Steve Conklin]

On Fri, 2010-08-20 at 11:01 -0700, John Johansen wrote:
> The following changes since commit 65316c3b7e1fccaae5f2c0cf5f98cd3220a5be64:
>   Stefan Bader (1):
>         UBUNTU: Ubuntu-2.6.32-308.15
> 
> are available in the git repository at:
> 
>   git://kernel.ubuntu.com/jj/ubuntu-lucid ec2
> 
> John Johansen (1):
>       UBUNTU: SAUCE: Revert "sched: update load count only once per cpu in 10 tick update window"
> 
>  kernel/sched.c |   24 ++----------------------
>  1 files changed, 2 insertions(+), 22 deletions(-)
> 
> ------
> 
> UBUNTU: SAUCE: Revert "sched: update load count only once per cpu in 10 tick update window"
> 
> BugLink: http://bugs.launchpad.net/bugs/574910
> 
> SRU Justification:
> 
> Impact:
> Fixes loadavg reporting on EC2.
> 
> Fix:
> This reverts commit 0d843425672f4d2dc99b9004409aae503ef4d39f which fixes a bug in load
> accounting when a tickless (no idle HZ) kernel is used.  However the Xen patchset used
> on EC2 is not tickless but the accounting modifications are still being done, resulting
> in phantom load.
> 
> Testcase:
> Start any Ubuntu Lucid based instance on EC2, let it idle while logging the load average.
>      while true ; do cat /proc/loadavg >>load.log ; sleep 5 ; done
> Alternately simply run top or htop and monitor the load average.
> 
> Without the revert the reported load will vary from 0 up to about .5 for a clean image
> with no extra tasks launched.
> 
> With the revert the load stays steady around 0 with only occasional small bump when
> a background task is run.
> 
> Signed-off-by: John Johansen <john.johansen at canonical.com>
Acked-by: Steve Conklin <sconklin at canonical.com>
> ---
>  kernel/sched.c |   24 ++----------------------
>  1 files changed, 2 insertions(+), 22 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 31a25d0..cbb111e 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2964,7 +2964,6 @@ unsigned long this_cpu_load(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);
> @@ -3019,7 +3018,7 @@ void calc_global_load(void)
>   */
>  static void calc_load_account_active(struct rq *this_rq)
>  {
> -	long nr_active, delta, deferred;
> +	long nr_active, delta;
>  
>  	nr_active = this_rq->nr_running;
>  	nr_active += (long) this_rq->nr_uninterruptible;
> @@ -3027,25 +3026,6 @@ 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);
>  	}
>  }
> @@ -3089,8 +3069,8 @@ static void update_cpu_load(struct rq *this_rq)
>  	}
>  
>  	if (time_after_eq(jiffies, this_rq->calc_load_update)) {
> -		calc_load_account_active(this_rq);
>  		this_rq->calc_load_update += LOAD_FREQ;
> +		calc_load_account_active(this_rq);
>  	}
>  }
>  
> -- 
> 1.7.0.4
> 
> 

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