[RFCv5 PATCH 02/46] sched: Make load tracking frequency scale-invariant

Morten Rasmussen
2015-07-07T18:22:28Z

From: Dietmar Eggemann <dietmar.eggemann@arm.com>

Apply frequency scale-invariance correction factor to load tracking.
Each segment of the sched_avg::runnable_avg_sum geometric series is now
scaled by the current frequency so the sched_avg::load_avg_contrib of each
entity will be invariant with frequency scaling. As a result,
cfs_rq::runnable_load_avg which is the sum of sched_avg::load_avg_contrib,
becomes invariant too. So the load level that is returned by
weighted_cpuload, stays relative to the max frequency of the cpu.

Then, we want the keep the load tracking values in a 32bits type, which
implies that the max value of sched_avg::{runnable|running}_avg_sum must
be lower than 2^32/88761=48388 (88761 is the max weight 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: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
---
kernel/sched/fair.c | 28 ++++++++++++++++------------
1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4b6e5f6..376ec75 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2551,9 +2551,9 @@ static __always_inline int __update_entity_runnable_avg(u64 now, int cpu,
int runnable,
int running)
{
- u64 delta, periods;
- u32 runnable_contrib;
- int delta_w, decayed = 0;
+ u64 delta, scaled_delta, periods;
+ u32 runnable_contrib, scaled_runnable_contrib;
+ int delta_w, scaled_delta_w, decayed = 0;
unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu);

delta = now - sa->last_runnable_update;
@@ -2587,11 +2587,12 @@ static __always_inline int __update_entity_runnable_avg(u64 now, int cpu,
* period and accrue it.
*/
delta_w = 1024 - delta_w;
+ scaled_delta_w = (delta_w * scale_freq) >> SCHED_CAPACITY_SHIFT;
+
if (runnable)
- sa->runnable_avg_sum += delta_w;
+ sa->runnable_avg_sum += scaled_delta_w;
if (running)
- sa->running_avg_sum += delta_w * scale_freq
- >> SCHED_CAPACITY_SHIFT;
+ sa->running_avg_sum += scaled_delta_w;
sa->avg_period += delta_w;

delta -= delta_w;
@@ -2609,20 +2610,23 @@ static __always_inline int __update_entity_runnable_avg(u64 now, int cpu,

/* Efficiently calculate \sum (1..n_period) 1024*y^i */
runnable_contrib = __compute_runnable_contrib(periods);
+ scaled_runnable_contrib = (runnable_contrib * scale_freq)
+ >> SCHED_CAPACITY_SHIFT;
+
if (runnable)
- sa->runnable_avg_sum += runnable_contrib;
+ sa->runnable_avg_sum += scaled_runnable_contrib;
if (running)
- sa->running_avg_sum += runnable_contrib * scale_freq
- >> SCHED_CAPACITY_SHIFT;
+ sa->running_avg_sum += scaled_runnable_contrib;
sa->avg_period += runnable_contrib;
}

/* Remainder of delta accrued against u_0` */
+ scaled_delta = (delta * scale_freq) >> SCHED_CAPACITY_SHIFT;
+
if (runnable)
- sa->runnable_avg_sum += delta;
+ sa->runnable_avg_sum += scaled_delta;
if (running)
- sa->running_avg_sum += delta * scale_freq
- >> SCHED_CAPACITY_SHIFT;
+ sa->running_avg_sum += scaled_delta;
sa->avg_period += delta;

return decayed;
--
1.9.1