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

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.