[PATCH 8/8] sched: prefer cpufreq_scale_freq_capacity

[Michael Turquette]

arch_scale_freq_capacity is weird. It specifies an arch hook for an
implementation that could easily vary within an architecture or even a
chip family.

This patch helps to mitigate this weirdness by defaulting to the
cpufreq-provided implementation, which should work for all cases where
CONFIG_CPU_FREQ is set.

If CONFIG_CPU_FREQ is not set, then try to use an implementation
provided by the architecture. Failing that, fall back to
SCHED_CAPACITY_SCALE.

It may be desirable for cpufreq drivers to specify their own
implementation of arch_scale_freq_capacity in the future. The same is
true for platform code within an architecture. In both cases an
efficient implementation selector will need to be created and this patch
adds a comment to that effect.

Signed-off-by: Michael Turquette <mturquette+renesas@xxxxxxxxxxxx>
---
 kernel/sched/sched.h | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 469d11d..37502ea 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1368,7 +1368,21 @@ static inline int hrtick_enabled(struct rq *rq)
 #ifdef CONFIG_SMP
 extern void sched_avg_update(struct rq *rq);
 
-#ifndef arch_scale_freq_capacity
+/*
+ * arch_scale_freq_capacity can be implemented by cpufreq, platform code or
+ * arch code. We select the cpufreq-provided implementation first. If it
+ * doesn't exist then we default to any other implementation provided from
+ * platform/arch code. If those do not exist then we use the default
+ * SCHED_CAPACITY_SCALE value below.
+ *
+ * Note that if cpufreq drivers or platform/arch code have competing
+ * implementations it is up to those subsystems to select one at runtime with
+ * an efficient solution, as we cannot tolerate the overhead of indirect
+ * functions (e.g. function pointers) in the scheduler fast path
+ */
+#ifdef CONFIG_CPU_FREQ
+#define arch_scale_freq_capacity cpufreq_scale_freq_capacity
+#elif !defined(arch_scale_freq_capacity)
 static __always_inline
 unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu)
 {
-- 
2.1.4