On 26 November 2014 at 06:18, Wanpeng Li kernellwp@gmail.com wrote:
Hi Vincent,
On 11/25/14, 9:52 PM, Vincent Guittot wrote:
On 25 November 2014 at 03:24, Wanpeng Li kernellwp@gmail.com wrote:
Hi Vincent, On 11/4/14, 12:54 AM, 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)
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 .
Do you mean 'sched_avg_period() + delta' should be replaced by 'delta' since sched_avg_period() is "long"?
No, Both are useful. sched_avg_period is useful to keep the value more stable across the time. Without sched_avg_period, it will be too much reactive ( especially at the beg of a period ).
Regards, Wanpeng Li
Regards, Wanpeng Li
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;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