On 21 November 2014 at 13:35, Morten Rasmussen [off-list ref] wrote:

On Mon, Nov 03, 2014 at 04:54:42PM +0000, Vincent Guittot wrote:

[snip]

quoted

The average running time of RT tasks is used to estimate the remaining compute

@@ -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);

I haven't looked through all the details of the rt avg tracking, but if
'used' is in the range [0..SCHED_CAPACITY_SCALE], I believe it should
work. Is it guaranteed that total > 0 so we don't get division by zero?

static inline u64 sched_avg_period(void)
{
return (u64)sysctl_sched_time_avg * NSEC_PER_MSEC / 2;
}

It does get a slightly more complicated if we want to figure out the
available capacity at the current frequency (current < max) later. Say,
rt eats 25% of the compute capacity, but the current frequency is only
50%. In that case get:

curr_avail_capacity = (arch_scale_cpu_capacity() *
  (arch_scale_freq_capacity() - (SCHED_SCALE_CAPACITY - scale_rt_capacity())))
  >> SCHED_CAPACITY_SHIFT

You don't have to be so complicated but simply need to do:
curr_avail_capacity for CFS = (capacity_of(CPU) *
arch_scale_freq_capacity())  >> SCHED_CAPACITY_SHIFT

capacity_of(CPU) = 600 is the max available capacity for CFS tasks
once we have removed the 25% of capacity that is used by RT tasks
arch_scale_freq_capacity = 512 because we currently run at 50% of max freq

so curr_avail_capacity for CFS = 300

Vincent

With numbers assuming arch_scale_cpu_capacity() = 800:

curr_avail_capacity = 800 * (512 - (1024 - 758)) >> 10 = 200

Which isn't actually that bad. Anyway, it isn't needed until we start
invovling energy models.

quoted

-     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;
 }