sched/fair: Align rq->avg_idle and rq->avg_scan_cost

sched/core.c uses update_avg() for rq->avg_idle and sched/fair.c uses an
open-coded version (with the exact same decay factor) for
rq->avg_scan_cost. On top of that, select_idle_cpu() expects to be able to
compare these two fields.

The only difference between the two is that rq->avg_scan_cost is computed
using a pure division rather than a shift. Turns out it actually matters,
first of all because the shifted value can be negative, and the standard
has this to say about it:

  """
  The result of E1 >> E2 is E1 right-shifted E2 bit positions. [...] If E1
  has a signed type and a negative value, the resulting value is
  implementation-defined.
  """

Not only this, but (arithmetic) right shifting a negative value (using 2's
complement) is *not* equivalent to dividing it by the corresponding power
of 2. Let's look at a few examples:

  -4      -> 0xF..FC
  -4 >> 3 -> 0xF..FF == -1 != -4 / 8

  -8      -> 0xF..F8
  -8 >> 3 -> 0xF..FF == -1 == -8 / 8

  -9      -> 0xF..F7
  -9 >> 3 -> 0xF..FE == -2 != -9 / 8

Make update_avg() use a division, and export it to the private scheduler
header to reuse it where relevant. Note that this still lets compilers use
a shift here, but should prevent any unwanted surprise. The disassembly of
select_idle_cpu() remains unchanged on arm64, and ttwu_do_wakeup() gains 2
instructions; the diff sort of looks like this:

  - sub x1, x1, x0
  + subs x1, x1, x0 // set condition codes
  + add x0, x1, #0x7
  + csel x0, x0, x1, mi // x0 = x1 < 0 ? x0 : x1
    add x0, x3, x0, asr #3

which does the right thing (i.e. gives us the expected result while still
using an arithmetic shift)

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20200330090127.16294-1-valentin.schneider@arm.com

kernel/sched/core.c

@@ -2119,12 +2119,6 @@ int select_task_rq(struct task_struct *p, int cpu, int sd_flags, int wake_flags)
 	return cpu;
 }
 
-static void update_avg(u64 *avg, u64 sample)
-{
-	s64 diff = sample - *avg;
-	*avg += diff >> 3;
-}
-
 void sched_set_stop_task(int cpu, struct task_struct *stop)
 {
 	struct sched_param param = { .sched_priority = MAX_RT_PRIO - 1 };

kernel/sched/fair.c

@@ -6080,8 +6080,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
 	struct sched_domain *this_sd;
 	u64 avg_cost, avg_idle;
-	u64 time, cost;
-	s64 delta;
+	u64 time;
 	int this = smp_processor_id();
 	int cpu, nr = INT_MAX;
 
@@ -6119,9 +6118,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 	}
 
 	time = cpu_clock(this) - time;
-	cost = this_sd->avg_scan_cost;
-	delta = (s64)(time - cost) / 8;
-	this_sd->avg_scan_cost += delta;
+	update_avg(&this_sd->avg_scan_cost, time);
 
 	return cpu;
 }

kernel/sched/sched.h

@@ -195,6 +195,12 @@ static inline int task_has_dl_policy(struct task_struct *p)
 
 #define cap_scale(v, s) ((v)*(s) >> SCHED_CAPACITY_SHIFT)
 
+static inline void update_avg(u64 *avg, u64 sample)
+{
+	s64 diff = sample - *avg;
+	*avg += diff / 8;
+}
+
 /*
  * !! For sched_setattr_nocheck() (kernel) only !!
  *