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Subject[RFC PATCH 7/7] sched/cpufreq: Boost schedutil frequency ramp up
Date
From: Douglas RAILLARD <douglas.raillard@arm.com>

In some situations, it can be interesting to spend temporarily more
power if that can give a useful frequency boost.

The sugov_cpu_is_busy() heuristic is reused to check if there has been
some idle time on all CPUs in the considered perf domain since last call
to schedutil's get_next_freq(). If not, it is assumed that at least one
CPU is in a frequency ramp up phase and the domain will be allowed to
spend extra power to reach a stable OPP in a shorter amount of time.

Since the extra power expenditure is bounded, it cannot skyrocket even
on platforms with a large number of cores in the same frequency domain
and/or very high ratio between lowest and highest OPP cost.

Signed-off-by: Douglas RAILLARD <douglas.raillard@arm.com>
---
kernel/sched/cpufreq_schedutil.c | 22 +++++++++++++++++-----
1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index ce4b90cafbb5..513b32bf14c5 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -218,6 +218,8 @@ static void sugov_cpu_is_busy_update(struct sugov_cpu *sg_cpu
* @sg_policy: schedutil policy object to compute the new frequency for.
* @util: Current CPU utilization.
* @max: CPU capacity.
+ * @busy: true if at least one CPU in the policy is busy, which means it had no
+ * idle time since its last frequency change.
*
* If the utilization is frequency-invariant, choose the new frequency to be
* proportional to it, that is
@@ -231,20 +233,28 @@ static void sugov_cpu_is_busy_update(struct sugov_cpu *sg_cpu
*
* Take C = 1.25 for the frequency tipping point at (util / max) = 0.8.
*
+ * An energy-aware boost is then applied if busy is true. The boost will allow
+ * selecting frequencies at most twice as costly in term of energy.
+ *
* 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
* cpufreq driver limitations.
*/
static unsigned int get_next_freq(struct sugov_policy *sg_policy,
- unsigned long util, unsigned long max)
+ unsigned long util, unsigned long max,
+ bool busy)
{
struct cpufreq_policy *policy = sg_policy->policy;
unsigned int freq = arch_scale_freq_invariant() ?
policy->cpuinfo.max_freq : policy->cur;
struct em_perf_domain *pd = sugov_policy_get_pd(sg_policy);

- /* Maximum power we are ready to spend. */
- unsigned int cost_margin = 0;
+ /*
+ * Maximum power we are ready to spend.
+ * When one CPU is busy in the policy, we apply a boost to help it reach
+ * the needed frequency faster.
+ */
+ unsigned int cost_margin = busy ? 1024/2 : 0;

freq = map_util_freq(util, freq, max);

@@ -534,7 +544,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
sugov_cpu_is_busy_update(sg_cpu, util);
max = sg_cpu->max;
util = sugov_iowait_apply(sg_cpu, time, util, max);
- next_f = get_next_freq(sg_policy, util, max);
+ next_f = get_next_freq(sg_policy, util, max, busy);
/*
* Do not reduce the frequency if the CPU has not been idle
* recently, as the reduction is likely to be premature then.
@@ -567,6 +577,7 @@ static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
unsigned long util = 0, max = 1;
unsigned int j;
unsigned long sg_cpu_util = 0;
+ bool busy = false;

for_each_cpu(j, policy->cpus) {
struct sugov_cpu *j_sg_cpu = &per_cpu(sugov_cpu, j);
@@ -577,6 +588,7 @@ static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
sg_cpu_util = j_util;
j_max = j_sg_cpu->max;
j_util = sugov_iowait_apply(j_sg_cpu, time, j_util, j_max);
+ busy |= sugov_cpu_is_busy(j_sg_cpu);

if (j_util * max > j_max * util) {
util = j_util;
@@ -592,7 +604,7 @@ static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
*/
sugov_cpu_is_busy_update(sg_cpu, sg_cpu_util);

- return get_next_freq(sg_policy, util, max);
+ return get_next_freq(sg_policy, util, max, busy);
}

static void
--
2.21.0
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