[RFC PATCH 3/7] sched/fair: Remove magic margin in fits_capacity()

[Qais Yousef <qyousef@layalina.io>]

80% margin is a magic value that has served its purpose for now, but it
no longer fits the variety of systems exist today. If a system is over
powered specifically, this 80% will mean we leave a lot of capacity
unused before we decide to upmigrate on HMP system.

The upmigration behavior should rely on the fact that a bad decision
made will need load balance to kick in to perform misfit migration. And
I think this is an adequate definition for what to consider as enough
headroom to consider whether a util fits capacity or not.

Use the new approximate_util_avg() function to predict the util if the
task continues to run for TICK_US. If the value is not strictly less
than the capacity, then it must not be placed there, ie considered
misfit.

Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
---
 kernel/sched/fair.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0b7445cd5af9..facbf3eb7141 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -109,16 +109,31 @@ int __weak arch_asym_cpu_priority(int cpu)
 }
 
 /*
- * The margin used when comparing utilization with CPU capacity.
+ * The util will fit the capacity if it has enough headroom to grow within the
+ * next tick - which is when any load balancing activity happens to do the
+ * correction.
  *
- * (default: ~20%)
+ * If util stays within the capacity before tick has elapsed, then it should be
+ * fine. If not, then a correction action must happen shortly after it starts
+ * running, hence we treat it as !fit.
+ *
+ * TODO: TICK is not actually accurate enough. balance_interval is the correct
+ * one to use as the next load balance doesn't not happen religiously at tick.
+ * Accessing balance_interval might be tricky and will require some refactoring
+ * first.
  */
-#define fits_capacity(cap, max)	((cap) * 1280 < (max) * 1024)
+static inline bool fits_capacity(unsigned long util, unsigned long capacity)
+{
+	return approximate_util_avg(util, TICK_USEC) < capacity;
+}
 
 /*
  * The margin used when comparing CPU capacities.
  * is 'cap1' noticeably greater than 'cap2'
  *
+ * TODO: use approximate_util_avg() to give something more quantifiable based
+ * on time? Like 1ms?
+ *
  * (default: ~5%)
  */
 #define capacity_greater(cap1, cap2) ((cap1) * 1024 > (cap2) * 1078)
-- 
2.34.1