Re: [PATCH RESEND v9 05/10] sched: make scale_rt invariant with frequency

Peter Zijlstra
2015-02-19T16:52:55Z

On Thu, Jan 15, 2015 at 11:09:25AM +0100, Vincent Guittot wrote:
> The average running time of RT tasks is used to estimate the remaining compute
> capacity for CFS tasks. This remaining capacity is the original capacity scaled
> down by a factor (aka scale_rt_capacity). This estimation of available capacity
> must also be invariant with frequency scaling.
>
> A frequency scaling factor is applied on the running time of the RT tasks for
> computing scale_rt_capacity.
>
> In sched_rt_avg_update, we scale the RT execution time like below:
> rq->rt_avg += rt_delta * arch_scale_freq_capacity() >> SCHED_CAPACITY_SHIFT
>
> Then, scale_rt_capacity can be summarized by:
> scale_rt_capacity = SCHED_CAPACITY_SCALE -
> ((rq->rt_avg << SCHED_CAPACITY_SHIFT) / period)
>
> We can optimize by removing right and left shift in the computation of rq->rt_avg
> and scale_rt_capacity

So far so good..

> The call to arch_scale_frequency_capacity in the rt scheduling path might be
> a concern for RT folks because I'm not sure whether we can rely on
> arch_scale_freq_capacity to be short and efficient ?

No, that is, arch_scale_frequency_capacity() _must_ be short and
efficient, event for the fair class, its called in very hot paths.

I think we've talked about this before; this function should basically
only return a cached value, which is periodically updated through some
means.

But lets see, I've yet to see an actual implementation of it; and its
got that sd argument, curious what you're going to do with that.

> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> kernel/sched/fair.c | 17 +++++------------
> kernel/sched/sched.h | 4 +++-
> 2 files changed, 8 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a5039da..b37c27b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c

> @@ -5801,19 +5801,12 @@ static unsigned long scale_rt_capacity(int cpu)
>
> total = sched_avg_period() + delta;
>
> - if (unlikely(total < avg)) {
> - /* Ensures that capacity won't end up being negative */
> - available = 0;
> - } else {
> - available = total - avg;
> - }
> + used = div_u64(avg, total);
>
> - if (unlikely((s64)total < SCHED_CAPACITY_SCALE))
> - total = SCHED_CAPACITY_SCALE;
> + if (likely(used < SCHED_CAPACITY_SCALE))
> + return SCHED_CAPACITY_SCALE - used;
>
> - total >>= SCHED_CAPACITY_SHIFT;
> -
> - return div_u64(available, total);
> + return 1;
> }

This makes a lot of changes not commented on in the Changelog.

I think it makes sense, but this has always been horrible.