[PATCH 1/4] sched/fair: Generalize the load/util averages resolution definition

[Yuyang Du]

Metric needs certain resolution to allow detail we can look into,
which also determines the range of the metric.

For instance, increasing the resolution of [0, 1] (two levels), one
can multiply 1024 and get [0..1024] (1025 levels).

In sched/fair, a few metrics depend on the resolution: weight, load,
load_avg, util_avg, freq, and capacity. In order to reduce the risks
of making mistakes relating to the resolution and range, we generalize
the resolution by defining a basic resolution constant number, and
then formalize all metrics to base on the basic resolution.

The basic resolution is 1024 or (1 << 10). Further, one can recursively
apply the basic resolution to have higher resolution.

Pointed out by Ben Segall, weight (visible to user, e.g., NICE-0 has
1024) and load (e.g., NICE_0_LOAD) have independent resolution, but
they must be well calibrated.

Signed-off-by: Yuyang Du <yuyang.du@xxxxxxxxx>
---
 include/linux/sched.h | 15 ++++++++++++---
 kernel/sched/fair.c   |  4 ----
 kernel/sched/sched.h  | 15 ++++++++++-----
 3 files changed, 22 insertions(+), 12 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index bd38b3e..b3ba0fb 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -910,9 +910,18 @@ enum cpu_idle_type {
 };
 
 /*
+ * Integer metrics need certain resolution to allow how much detail we
+ * can look into, e.g., load, load_avg, util_avg, freq, and capacity.
+ * We define a basic resolution constant number, and then formalize
+ * all these metrics based on that basic resolution.
+ */
+# define SCHED_RESOLUTION_SHIFT	10
+# define SCHED_RESOLUTION_SCALE	(1L << SCHED_RESOLUTION_SHIFT)
+
+/*
  * Increase resolution of cpu_capacity calculations
  */
-#define SCHED_CAPACITY_SHIFT	10
+#define SCHED_CAPACITY_SHIFT	SCHED_RESOLUTION_SHIFT
 #define SCHED_CAPACITY_SCALE	(1L << SCHED_CAPACITY_SHIFT)
 
 /*
@@ -1180,8 +1189,8 @@ struct load_weight {
  * 1) load_avg factors frequency scaling into the amount of time that a
  * sched_entity is runnable on a rq into its weight. For cfs_rq, it is the
  * aggregated such weights of all runnable and blocked sched_entities.
- * 2) util_avg factors frequency and cpu scaling into the amount of time
- * that a sched_entity is running on a CPU, in the range [0..SCHED_LOAD_SCALE].
+ * 2) util_avg factors frequency and cpu capacity scaling into the amount of time
+ * that a sched_entity is running on a CPU, in the range [0..SCHED_CAPACITY_SCALE].
  * For cfs_rq, it is the aggregated such times of all runnable and
  * blocked sched_entities.
  * The 64 bit load_sum can:
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4df37a4..c61fd8e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2522,10 +2522,6 @@ static u32 __compute_runnable_contrib(u64 n)
 	return contrib + runnable_avg_yN_sum[n];
 }
 
-#if (SCHED_LOAD_SHIFT - SCHED_LOAD_RESOLUTION) != 10 || SCHED_CAPACITY_SHIFT != 10
-#error "load tracking assumes 2^10 as unit"
-#endif
-
 #define cap_scale(v, s) ((v)*(s) >> SCHED_CAPACITY_SHIFT)
 
 /*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 3845a71..31b4022 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -53,18 +53,23 @@ static inline void update_cpu_load_active(struct rq *this_rq) { }
  * increased costs.
  */
 #if 0 /* BITS_PER_LONG > 32 -- currently broken: it increases power usage under light load  */
-# define SCHED_LOAD_RESOLUTION	10
-# define scale_load(w)		((w) << SCHED_LOAD_RESOLUTION)
-# define scale_load_down(w)	((w) >> SCHED_LOAD_RESOLUTION)
+# define SCHED_LOAD_SHIFT	(SCHED_RESOLUTION_SHIFT + SCHED_RESOLUTION_SHIFT)
+# define scale_load(w)		((w) << SCHED_RESOLUTION_SHIFT)
+# define scale_load_down(w)	((w) >> SCHED_RESOLUTION_SHIFT)
 #else
-# define SCHED_LOAD_RESOLUTION	0
+# define SCHED_LOAD_SHIFT	(SCHED_RESOLUTION_SHIFT)
 # define scale_load(w)		(w)
 # define scale_load_down(w)	(w)
 #endif
 
-#define SCHED_LOAD_SHIFT	(10 + SCHED_LOAD_RESOLUTION)
 #define SCHED_LOAD_SCALE	(1L << SCHED_LOAD_SHIFT)
 
+/*
+ * NICE_0's weight (visible to user) and its load (invisible to user) have
+ * independent resolution, but they should be well calibrated. We use scale_load()
+ * and scale_load_down(w) to convert between them, the following must be true:
+ * scale_load(prio_to_weight[20]) == NICE_0_LOAD
+ */
 #define NICE_0_LOAD		SCHED_LOAD_SCALE
 #define NICE_0_SHIFT		SCHED_LOAD_SHIFT
 
-- 
2.1.4

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