Subject: [PATCH v2 02/11] sched: remove a wake_affine condition

From: [email-masked] (Dietmar Eggemann)
Date: Wed, 28 May 2014 16:09:44 +0100

Hi Vincent & Peter,

On 28/05/14 07:49, Vincent Guittot wrote:
[...]
> 
> Nick,
> 
> While doing some rework on the wake affine part of the scheduler, i
> failed to catch the use case that takes advantage of a condition that
> you added some while ago with the commit
> a3f21bce1fefdf92a4d1705e888d390b10f3ac6f
> 
> Could you help us to clarify the 2 first lines of the test that you added ?
> +                       if ((tl <= load &&
> +                               tl + target_load(cpu, idx) <=
> SCHED_LOAD_SCALE) ||
> +                               100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {
> 
> Regards,
> Vincent
>>
>>>
>>>>> commit a3f21bce1fefdf92a4d1705e888d390b10f3ac6f
>>>>>
>>>>>
>>>>> +                       if ((tl <= load &&
>>>>> +                               tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) ||
>>>>> +                               100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {
>>>>>
>>>>>
>>>>> So back when the code got introduced, it read:
>>>>>
>>>>>         target_load(prev_cpu, idx) - sync*SCHED_LOAD_SCALE < source_load(this_cpu, idx) &&
>>>>>         target_load(prev_cpu, idx) - sync*SCHED_LOAD_SCALE + target_load(this_cpu, idx) < SCHED_LOAD_SCALE
>>>>>

Shouldn't this be 

target_load(this_cpu, idx) - sync*SCHED_LOAD_SCALE <= source_load(prev_cpu, idx) &&
target_load(this_cpu, idx) - sync*SCHED_LOAD_SCALE + target_load(prev_cpu, idx) <= SCHED_LOAD_SCALE

"[PATCH] sched: implement smpnice" (2dd73a4f09beacadde827a032cf15fd8b1fa3d48)
mentions that SCHED_LOAD_BALANCE (IMHO, should be SCHED_LOAD_SCALE) represents
the load contribution of a single task. So I read the second part as if
the sum of the load of this_cpu and prev_cpu is smaller or equal to the
(maximal) load contribution (maximal possible effect) of a single task.

There is even a comment in "[PATCH] sched: tweak affine wakeups"
(a3f21bce1fefdf92a4d1705e888d390b10f3ac6f) in try_to_wake_up() when
SCHED_LOAD_SCALE gets subtracted from tl = this_load =
target_load(this_cpu, idx):

+ * If sync wakeup then subtract the (maximum possible)
+ * effect of the currently running task from the load
+ * of the current CPU:

"[PATCH] sched: implement smpnice" then replaces SCHED_LOAD_SCALE w/ 

+static inline unsigned long cpu_avg_load_per_task(int cpu)
+{
+       runqueue_t *rq = cpu_rq(cpu);
+       unsigned long n = rq->nr_running;
+
+       return n ?  rq->raw_weighted_load / n : SCHED_LOAD_SCALE;

-- Dietmar

>>>>> So while the first line makes some sense, the second line is still
>>>>> somewhat challenging.
>>>>>
>>>>> I read the second line something like: if there's less than one full
>>>>> task running on the combined cpus.
>>>>
>>>> ok. your explanation makes sense
>>>
>>> Maybe, its still slightly weird :-)
>>>
>>>>>
[...]