On Tue, Jan 21, 2014 at 12:17:57PM +0100, Peter Zijlstra wrote:
> From: Daniel Lezcano <[email protected]>
>
> 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: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Signed-off-by: Daniel Lezcano <[email protected]>
> Signed-off-by: Peter Zijlstra <[email protected]>
> Link:
> http://lkml.kernel.org/r/[email protected]
> ---
> 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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