On 21 November 2014 at 13:35, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Mon, Nov 03, 2014 at 04:54:42PM +0000, Vincent Guittot wrote:
[snip]
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.
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;}