On 25 November 2014 at 03:24, Wanpeng Li [off-list ref] wrote:

Hi Vincent,
On 11/4/14, 12:54 AM, Vincent Guittot wrote:

quoted

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)

The 'period' aka 'total' in the scale_rt_capacity(), why it is
sched_avg_period() + delta instead of sched_avg_period()?

The default value of sched_avg_period is 1sec which is "long" so we
take into account the time consumed by RT tasks in the ongoing period
.

Regards,
Wanpeng Li

quoted

We can optimize by removing right and left shift in the computation of
rq->rt_avg
and scale_rt_capacity

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 ?

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

@@ -5785,7 +5785,7 @@ unsigned long __weak arch_scale_cpu_capacity(struct

sched_domain *sd, int cpu)
  static unsigned long scale_rt_capacity(int cpu)
  {
        struct rq *rq = cpu_rq(cpu);
-       u64 total, available, age_stamp, avg;
+       u64 total, used, age_stamp, avg;
        s64 delta;
        /*

@@ -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;
  }
    static void update_cpu_capacity(struct sched_domain *sd, int cpu)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c34bd11..fc5b152 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h

@@ -1312,9 +1312,11 @@ static inline int hrtick_enabled(struct rq *rq)
    #ifdef CONFIG_SMP
  extern void sched_avg_update(struct rq *rq);
+extern unsigned long arch_scale_freq_capacity(struct sched_domain *sd,

int cpu);
+
  static inline void sched_rt_avg_update(struct rq *rq, u64 rt_delta)
  {
-       rq->rt_avg += rt_delta;
+       rq->rt_avg += rt_delta * arch_scale_freq_capacity(NULL,
cpu_of(rq));
        sched_avg_update(rq);
  }
  #else