[RFCv5 PATCH 06/46] sched: Make usage tracking cpu scale-invariant

Morten Rasmussen
2015-07-07T18:23:06Z

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

Besides the existing frequency scale-invariance correction factor, apply
cpu scale-invariance correction factor to usage tracking.

Cpu scale-invariance takes cpu performance deviations due to
micro-architectural differences (i.e. instructions per seconds) between
cpus in HMP systems (e.g. big.LITTLE) and differences in the frequency
value of the highest OPP between cpus in SMP systems into consideration.

Each segment of the sched_avg::running_avg_sum geometric series is now
scaled by the cpu performance factor too so the
sched_avg::utilization_avg_contrib of each entity will be invariant from
the particular cpu of the HMP/SMP system it is gathered on.

So the usage level that is returned by get_cpu_usage stays relative to
the max cpu performance of the system.

In contrast to usage, load (sched_avg::runnable_avg_sum) is currently
not considered to be made cpu scale-invariant because this will have a
negative effect on the the existing load balance code based on
s[dg]_lb_stats::avg_load in overload scenarios.

example: 7 always running tasks
4 on cluster 0 (2 cpus w/ cpu_capacity=512)
3 on cluster 1 (1 cpu w/ cpu_capacity=1024)

cluster 0 cluster 1

capacity 1024 (2*512) 1024 (1*1024)
load 4096 3072
cpu-scaled load 2048 3072

Simply using cpu-scaled load in the existing lb code would declare
cluster 1 busier than cluster 0, although the compute capacity budget
for one task is higher on cluster 1 (1024/3 = 341) than on cluster 0
(2*512/4 = 256).

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
kernel/sched/fair.c | 13 +++++++++++++
kernel/sched/sched.h | 2 +-
2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5b4e8c1..39871a4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2555,6 +2555,7 @@ static __always_inline int __update_entity_runnable_avg(u64 now, int cpu,
u32 runnable_contrib, scaled_runnable_contrib;
int delta_w, scaled_delta_w, decayed = 0;
unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu);
+ unsigned long scale_cpu = arch_scale_cpu_capacity(NULL, cpu);

delta = now - sa->last_runnable_update;
/*
@@ -2591,6 +2592,10 @@ static __always_inline int __update_entity_runnable_avg(u64 now, int cpu,

if (runnable)
sa->runnable_avg_sum += scaled_delta_w;
+
+ scaled_delta_w *= scale_cpu;
+ scaled_delta_w >>= SCHED_CAPACITY_SHIFT;
+
if (running)
sa->running_avg_sum += scaled_delta_w;
sa->avg_period += delta_w;
@@ -2615,6 +2620,10 @@ static __always_inline int __update_entity_runnable_avg(u64 now, int cpu,

if (runnable)
sa->runnable_avg_sum += scaled_runnable_contrib;
+
+ scaled_runnable_contrib *= scale_cpu;
+ scaled_runnable_contrib >>= SCHED_CAPACITY_SHIFT;
+
if (running)
sa->running_avg_sum += scaled_runnable_contrib;
sa->avg_period += runnable_contrib;
@@ -2625,6 +2634,10 @@ static __always_inline int __update_entity_runnable_avg(u64 now, int cpu,

if (runnable)
sa->runnable_avg_sum += scaled_delta;
+
+ scaled_delta *= scale_cpu;
+ scaled_delta >>= SCHED_CAPACITY_SHIFT;
+
if (running)
sa->running_avg_sum += scaled_delta;
sa->avg_period += delta;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index af08a82..4e7bce7 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1403,7 +1403,7 @@ unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu)
static __always_inline
unsigned long arch_scale_cpu_capacity(struct sched_domain *sd, int cpu)
{
- if ((sd->flags & SD_SHARE_CPUCAPACITY) && (sd->span_weight > 1))
+ if (sd && (sd->flags & SD_SHARE_CPUCAPACITY) && (sd->span_weight > 1))
return sd->smt_gain / sd->span_weight;

return SCHED_CAPACITY_SCALE;
--
1.9.1