Re: [PATCH] sched: avg_overlap decay

[Mike Galbraith <efault@gmx.de>]

On Tue, 2009-03-10 at 19:18 +0100, Peter Zijlstra wrote:
> Mike, are you good with this patch as it stands?

Yes, works for me.

	-Mike

> ---
> Subject: sched: avg_overlap decay
> From: Mike Galbraith <efault@gmx.de>
> Date: Tue Mar 10 19:08:11 CET 2009
> 
> avg_overlap is used to measure the runtime overlap of the waker and wakee.
> 
> However, when a process changes behaviour, eg a pipe becomes un-congested
> and we don't need to go to sleep after a wakeup for a while, the avg_overlap
> value grows stale.
> 
> When running we use the avg runtime between preemption as a measure for
> avg_overlap since the amount of runtime can be correlated to cache footprint.
> 
> The longer we run, the less likely we'll be wanting to be migrated to another
> CPU.
> 
> Signed-off-by: Mike Galbraith <efault@gmx.de>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  kernel/sched.c |   24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -4692,6 +4692,28 @@ static inline void schedule_debug(struct
>  #endif
>  }
>  
> +static void put_prev_task(struct rq *rq, struct task_struct *prev)
> +{
> +	if (prev->state == TASK_RUNNING) {
> +		u64 runtime = prev->se.sum_exec_runtime;
> +
> +		runtime -= prev->se.prev_sum_exec_runtime;
> +		runtime = min_t(u64, runtime, 2*sysctl_sched_migration_cost);
> +
> +		/*
> +		 * In order to avoid avg_overlap growing stale when we are
> +		 * indeed overlapping and hence not getting put to sleep, grow
> +		 * the avg_overlap on preemption.
> +		 *
> +		 * We use the average preemption runtime because that
> +		 * correlates to the amount of cache footprint a task can
> +		 * build up.
> +		 */
> +		update_avg(&prev->se.avg_overlap, runtime);
> +	}
> +	prev->sched_class->put_prev_task(rq, prev);
> +}
> +
>  /*
>   * Pick up the highest-prio task:
>   */
> @@ -4768,7 +4790,7 @@ need_resched_nonpreemptible:
>  	if (unlikely(!rq->nr_running))
>  		idle_balance(cpu, rq);
>  
> -	prev->sched_class->put_prev_task(rq, prev);
> +	put_prev_task(rq, prev);
>  	next = pick_next_task(rq);
>  
>  	if (likely(prev != next)) {
>