[PATCH v9 05/10] sched: make scale_rt invariant with frequency
Morten Rasmussen
morten.rasmussen at arm.com
Fri Nov 21 04:35:59 PST 2014
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 fastpath 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.
>
> Signedoffby: Vincent Guittot <vincent.guittot at 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 subdivide our
* runnable
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