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Subject[RFC PATCH 6/7] sched/cpufreq: Improve sugov_cpu_is_busy accuracy
Date
From: Douglas RAILLARD <douglas.raillard@arm.com>

Avoid assuming a CPU is busy when it has begun being idle before
get_next_freq() is called. This is achieved by making sure the CPU will
not be detected as busy by other CPUs whenever its utilization is
decreasing.

Signed-off-by: Douglas RAILLARD <douglas.raillard@arm.com>
---
kernel/sched/cpufreq_schedutil.c | 42 ++++++++++++++++++++++++++++----
1 file changed, 37 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index a12b7e5bc028..ce4b90cafbb5 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -62,6 +62,7 @@ struct sugov_cpu {
/* The field below is for single-CPU policies only: */
#ifdef CONFIG_NO_HZ_COMMON
unsigned long saved_idle_calls;
+ unsigned long previous_util;
#endif
};

@@ -181,14 +182,35 @@ static bool sugov_cpu_is_busy(struct sugov_cpu *sg_cpu)
return ret;
}

-static void sugov_cpu_is_busy_update(struct sugov_cpu *sg_cpu)
+static void sugov_cpu_is_busy_update(struct sugov_cpu *sg_cpu,
+ unsigned long util)
{
unsigned long idle_calls = tick_nohz_get_idle_calls_cpu(sg_cpu->cpu);
sg_cpu->saved_idle_calls = idle_calls;
+
+ /*
+ * Make sure that this CPU will not be immediately considered as busy in
+ * cases where the CPU has already entered an idle state. In that case,
+ * the number of idle_calls will not vary anymore until it exits idle,
+ * which would lead sugov_cpu_is_busy() to say that this CPU is busy,
+ * because it has not (re)entered idle since the last time we looked at
+ * it.
+ * Assuming cpu0 and cpu1 are in the same policy, that will make sure
+ * this sequence of events leads to right cpu1 business status from
+ * get_next_freq(cpu=1)
+ * cpu0: [enter idle] -> [get_next_freq] -> [doing nothing] -> [wakeup]
+ * cpu1: ... -> [get_next_freq] -> ...
+ */
+ if (util <= sg_cpu->previous_util)
+ sg_cpu->saved_idle_calls--;
+
+ sg_cpu->previous_util = util;
}
#else
static inline bool sugov_cpu_is_busy(struct sugov_cpu *sg_cpu) { return false; }
-static void sugov_cpu_is_busy_update(struct sugov_cpu *sg_cpu) {}
+static void sugov_cpu_is_busy_update(struct sugov_cpu *sg_cpu
+ unsigned long util)
+{}
#endif /* CONFIG_NO_HZ_COMMON */

/**
@@ -507,10 +529,9 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
if (!sugov_should_update_freq(sg_policy, time))
return;

- busy = sugov_cpu_is_busy(sg_cpu);
- sugov_cpu_is_busy_update(sg_cpu);
-
util = sugov_get_util(sg_cpu);
+ busy = sugov_cpu_is_busy(sg_cpu);
+ 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);
@@ -545,12 +566,15 @@ static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
struct cpufreq_policy *policy = sg_policy->policy;
unsigned long util = 0, max = 1;
unsigned int j;
+ unsigned long sg_cpu_util = 0;

for_each_cpu(j, policy->cpus) {
struct sugov_cpu *j_sg_cpu = &per_cpu(sugov_cpu, j);
unsigned long j_util, j_max;

j_util = sugov_get_util(j_sg_cpu);
+ if (j_sg_cpu == sg_cpu)
+ sg_cpu_util = j_util;
j_max = j_sg_cpu->max;
j_util = sugov_iowait_apply(j_sg_cpu, time, j_util, j_max);

@@ -560,6 +584,14 @@ static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time)
}
}

+ /*
+ * Only update the business status if we are looking at the CPU for
+ * which a utilization change triggered a call to get_next_freq(). This
+ * way, we don't affect the "busy" status of CPUs that don't have any
+ * change in utilization.
+ */
+ sugov_cpu_is_busy_update(sg_cpu, sg_cpu_util);
+
return get_next_freq(sg_policy, util, max);
}

--
2.21.0
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