On Mon, Nov 03, 2014 at 04:54:42PM +0000, 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
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 ?
It better be fast :) It is used in critical paths. However, if you really care about latency you probably don't want frequency scaling to mess around. If the architecture provides a fast-path for arch_scale_freq_capacity() returning SCHED_CAPACITY_SCALE when frequency scaling is disabled, the overhead should be minimal. If the architecture doesn't provide arch_scale_freq_capacity() it becomes a constant multiplication and should hopefully go away completely.
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);
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?
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
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;
} 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);
I'm not sure if it makes any difference, but shouldn't it be __weak instead of extern?
unsigned long __weak arch_scale_freq_capacity(...)
Also, now that the function prototype definition is in the header file we can kill the local prototype in fair.c introduced in patch 4:
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 6fd5ac6..921b174 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2277,8 +2277,6 @@ static u32 __compute_runnable_contrib(u64 n) return contrib + runnable_avg_yN_sum[n]; }
-unsigned long __weak arch_scale_freq_capacity(struct sched_domain *sd, int cpu); - /* * We can represent the historical contribution to runnable average as * the * coefficients of a geometric series. To do this we sub-divide our * runnable