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Subject[PATCH v9 04/10] sched: Make sched entity usage tracking scale-invariant
Date
From: Morten Rasmussen <morten.rasmussen@arm.com>

Apply frequency scale-invariance correction factor to usage tracking.
Each segment of the running_load_avg geometric series is now scaled by the
current frequency so the utilization_avg_contrib of each entity will be
invariant with frequency scaling. As a result, utilization_load_avg which is
the sum of utilization_avg_contrib, becomes invariant too. So the usage level
that is returned by get_cpu_usage, stays relative to the max frequency as the
cpu_capacity which is is compared against.
Then, we want the keep the load tracking values in a 32bits type, which implies
that the max value of {runnable|running}_avg_sum must be lower than
2^32/88761=48388 (88761 is the max weigth of a task). As LOAD_AVG_MAX = 47742,
arch_scale_freq_capacity must return a value less than
(48388/47742) << SCHED_CAPACITY_SHIFT = 1037 (SCHED_SCALE_CAPACITY = 1024).
So we define the range to [0..SCHED_SCALE_CAPACITY] in order to avoid overflow.

cc: Paul Turner <pjt@google.com>
cc: Ben Segall <bsegall@google.com>

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
kernel/sched/fair.c | 21 ++++++++++++++-------
1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a96affd..a5039da 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2266,6 +2266,8 @@ 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
@@ -2294,7 +2296,7 @@ static u32 __compute_runnable_contrib(u64 n)
* load_avg = u_0` + y*(u_0 + u_1*y + u_2*y^2 + ... )
* = u_0 + u_1*y + u_2*y^2 + ... [re-labeling u_i --> u_{i+1}]
*/
-static __always_inline int __update_entity_runnable_avg(u64 now,
+static __always_inline int __update_entity_runnable_avg(u64 now, int cpu,
struct sched_avg *sa,
int runnable,
int running)
@@ -2302,6 +2304,7 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
u64 delta, periods;
u32 runnable_contrib;
int delta_w, decayed = 0;
+ unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu);

delta = now - sa->last_runnable_update;
/*
@@ -2337,7 +2340,8 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
if (runnable)
sa->runnable_avg_sum += delta_w;
if (running)
- sa->running_avg_sum += delta_w;
+ sa->running_avg_sum += delta_w * scale_freq
+ >> SCHED_CAPACITY_SHIFT;
sa->avg_period += delta_w;

delta -= delta_w;
@@ -2358,7 +2362,8 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
if (runnable)
sa->runnable_avg_sum += runnable_contrib;
if (running)
- sa->running_avg_sum += runnable_contrib;
+ sa->running_avg_sum += runnable_contrib * scale_freq
+ >> SCHED_CAPACITY_SHIFT;
sa->avg_period += runnable_contrib;
}

@@ -2366,7 +2371,8 @@ static __always_inline int __update_entity_runnable_avg(u64 now,
if (runnable)
sa->runnable_avg_sum += delta;
if (running)
- sa->running_avg_sum += delta;
+ sa->running_avg_sum += delta * scale_freq
+ >> SCHED_CAPACITY_SHIFT;
sa->avg_period += delta;

return decayed;
@@ -2474,8 +2480,8 @@ static inline void __update_group_entity_contrib(struct sched_entity *se)

static inline void update_rq_runnable_avg(struct rq *rq, int runnable)
{
- __update_entity_runnable_avg(rq_clock_task(rq), &rq->avg, runnable,
- runnable);
+ __update_entity_runnable_avg(rq_clock_task(rq), cpu_of(rq), &rq->avg,
+ runnable, runnable);
__update_tg_runnable_avg(&rq->avg, &rq->cfs);
}
#else /* CONFIG_FAIR_GROUP_SCHED */
@@ -2553,6 +2559,7 @@ static inline void update_entity_load_avg(struct sched_entity *se,
{
struct cfs_rq *cfs_rq = cfs_rq_of(se);
long contrib_delta, utilization_delta;
+ int cpu = cpu_of(rq_of(cfs_rq));
u64 now;

/*
@@ -2564,7 +2571,7 @@ static inline void update_entity_load_avg(struct sched_entity *se,
else
now = cfs_rq_clock_task(group_cfs_rq(se));

- if (!__update_entity_runnable_avg(now, &se->avg, se->on_rq,
+ if (!__update_entity_runnable_avg(now, cpu, &se->avg, se->on_rq,
cfs_rq->curr == se))
return;

--
1.9.1


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