Re: [PATCH 3/9] sched: Move idle_stamp up to the core

[Peter Zijlstra]

On Tue, Jan 21, 2014 at 12:17:57PM +0100, Peter Zijlstra wrote:
> From: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> 
> The idle_balance modifies the idle_stamp field of the rq, making this
> information to be shared across core.c and fair.c. As we can know if the
> cpu is going to idle or not with the previous patch, let's encapsulate the
> idle_stamp information in core.c by moving it up to the caller. The
> idle_balance function returns true in case a balancing occured and the cpu
> won't be idle, false if no balance happened and the cpu is going idle.
> 
> Cc: alex.shi@xxxxxxxxxx
> Cc: peterz@xxxxxxxxxxxxx
> Cc: mingo@xxxxxxxxxx
> Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Link: http://lkml.kernel.org/r/1389949444-14821-3-git-send-email-daniel.lezcano@xxxxxxxxxx
> ---
>  kernel/sched/core.c  |    2 +-
>  kernel/sched/fair.c  |   14 ++++++--------
>  kernel/sched/sched.h |    2 +-
>  3 files changed, 8 insertions(+), 10 deletions(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2680,7 +2680,7 @@ static void __sched __schedule(void)
>  	pre_schedule(rq, prev);
>  
>  	if (unlikely(!rq->nr_running))
> -		idle_balance(rq);
> +		rq->idle_stamp = idle_balance(rq) ? 0 : rq_clock(rq);

OK, spotted a problem here..

So previously idle_stamp was set _before_ actually doing idle_balance(),
and that was RIGHT, because that way we include the cost of actually
doing idle_balance() into the idle time.

By not including the cost of idle_balance() you underestimate the 'idle'
time in that if idle_balance() filled the entire idle time we account 0
idle, even though we had 'plenty' of time to run the entire thing.

This leads to not running idle_balance() even though we have the time
for it.

So we very much want something like:


  if (!rq->nr_running)
  	rq->idle_stamp = rq_clock(rq);

  p = pick_next_task(rq, prev);

  if (!is_idle_task(p))
  	rq->idle_stamp = 0;


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