[PATCH v2 1/6] sched/fair: Generalize the load/util averages resolution definition

[Yuyang Du]

Integer metric needs fixed point arithmetic. In sched/fair, a few
metrics, e.g., weight, load, load_avg, util_avg, freq, and capacity,
may have different fixed point ranges, which makes their update and
usage error-prone.

In order to avoid the errors relating to the fixed point range, we
definie a basic fixed point range, and then formalize all metrics to
base on the basic range.

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

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

Signed-off-by: Yuyang Du <yuyang.du@xxxxxxxxx>
---
 include/linux/sched.h | 16 +++++++++++++---
 kernel/sched/fair.c   |  4 ----
 kernel/sched/sched.h  | 15 ++++++++++-----
 3 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index bd38b3e..ab59390 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -910,9 +910,19 @@ enum cpu_idle_type {
 };
 
 /*
+ * Integer metrics need fixed point arithmetic, e.g., sched/fair
+ * has a few: load, load_avg, util_avg, freq, and capacity.
+ *
+ * We define a basic fixed point arithmetic range, and then formalize
+ * all these metrics based on that basic range.
+ */
+# define SCHED_FIXEDPOINT_SHIFT	10
+# define SCHED_FIXEDPOINT_SCALE	(1L << SCHED_FIXEDPOINT_SHIFT)
+
+/*
  * Increase resolution of cpu_capacity calculations
  */
-#define SCHED_CAPACITY_SHIFT	10
+#define SCHED_CAPACITY_SHIFT	SCHED_FIXEDPOINT_SHIFT
 #define SCHED_CAPACITY_SCALE	(1L << SCHED_CAPACITY_SHIFT)
 
 /*
@@ -1180,8 +1190,8 @@ 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..61a5858 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_FIXEDPOINT_SHIFT + SCHED_FIXEDPOINT_SHIFT)
+# define scale_load(w)		((w) << SCHED_FIXEDPOINT_SHIFT)
+# define scale_load_down(w)	((w) >> SCHED_FIXEDPOINT_SHIFT)
 #else
-# define SCHED_LOAD_RESOLUTION	0
+# define SCHED_LOAD_SHIFT	(SCHED_FIXEDPOINT_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 ranges, 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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