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Subject[PATCH v3 2/2] cpufreq: schedutil: consolidate capacity margin calculation
Date
Scheduler CFS class has variable 'capacity_margin' to represent the
capacity margin, currently in the kernel 'capacity_margin' is 1280;
on the other hand schedutil governor also needs to compensate the
margin for frequency tipping point. Below are formulas which are used
in CFS class and schedutil governor separately, from the formulas we
can get to know essentially both of them uses the same margin:

CFS: U` = U * capacity_margin / 1024 = U * 1.25
Schedutil: U` = U + U >> 2 = U + U * 0.25 = U * 1.25

This patch consolidates the usage of the capacity margin value and
lets schedutil use the same formula as the CFS class. Thus we can
avoid the mismatch between schedutil and CFS class if
'capacity_margin' is changed to other values in the future.

Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>
Cc: Chris Redpath <Chris.Redpath@arm.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
---
kernel/sched/cpufreq_schedutil.c | 7 +++++--
kernel/sched/sched.h | 1 +
2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 9209d83..79abbaa 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -155,7 +155,9 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
*
* next_freq = C * curr_freq * util_raw / max
*
- * Take C = 1.25 for the frequency tipping point at (util / max) = 0.8.
+ * Take C = capacity_margin >> SCHED_CAPACITY_SHIFT; if capacity_margin is
+ * 1280 and SCHED_CAPACITY_SHIFT is 10, this results in C = 1.25 and the
+ * frequency tipping point at (util / max) = 0.8.
*
* The lowest driver-supported frequency which is equal or greater than the raw
* next_freq (as calculated above) is returned, subject to policy min/max and
@@ -168,7 +170,8 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
unsigned int freq = arch_scale_freq_invariant() ?
policy->cpuinfo.max_freq : policy->cur;

- freq = (freq + (freq >> 2)) * util / max;
+ freq = freq * capacity_margin >> SCHED_CAPACITY_SHIFT;
+ freq = freq * util / max;

if (freq == sg_policy->cached_raw_freq && sg_policy->next_freq != UINT_MAX)
return sg_policy->next_freq;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 14db76c..cf75bdc 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -52,6 +52,7 @@ struct cpuidle_state;
#define TASK_ON_RQ_MIGRATING 2

extern __read_mostly int scheduler_running;
+extern unsigned int capacity_margin __read_mostly;

extern unsigned long calc_load_update;
extern atomic_long_t calc_load_tasks;
--
2.7.4
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