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

[Vincent Guittot]

On 27 May 2014 17:39, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, May 27, 2014 at 05:19:02PM +0200, Vincent Guittot wrote:
>> On 27 May 2014 14:48, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> > On Fri, May 23, 2014 at 05:52:56PM +0200, Vincent Guittot wrote:
>> >> I have tried to understand the meaning of the condition :
>> >>  (this_load <= load &&
>> >>   this_load + target_load(prev_cpu, idx) <= tl_per_task)
>> >> but i failed to find a use case that can take advantage of it and i haven't
>> >> found description of it in the previous commits' log.
>> >
>> > commit 2dd73a4f09beacadde827a032cf15fd8b1fa3d48
>> >
>> >     int try_to_wake_up():
>> >
>> >     in this function the value SCHED_LOAD_BALANCE is used to represent the load
>> >     contribution of a single task in various calculations in the code that
>> >     decides which CPU to put the waking task on.  While this would be a valid
>> >     on a system where the nice values for the runnable tasks were distributed
>> >     evenly around zero it will lead to anomalous load balancing if the
>> >     distribution is skewed in either direction.  To overcome this problem
>> >     SCHED_LOAD_SCALE has been replaced by the load_weight for the relevant task
>> >     or by the average load_weight per task for the queue in question (as
>> >     appropriate).
>> >
>> >                         if ((tl <= load &&
>> > -                               tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) ||
>> > -                               100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {
>> > +                               tl + target_load(cpu, idx) <= tl_per_task) ||
>> > +                               100*(tl + p->load_weight) <= imbalance*load) {
>>
>> The oldest patch i had found was: https://lkml.org/lkml/2005/2/24/34
>> where task_hot had been replaced by
>> + if ((tl <= load &&
>> + tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) ||
>> + 100*(tl + SCHED_LOAD_SCALE) <= imbalance*load) {
>>
>> but as explained, i haven't found a clear explanation of this condition
>
> Yeah, that's the commit I had below; but I suppose we could ask Nick if
> we really want, I've heard he still replies to email, even though he's
> locked up in a basement somewhere :-)

ok, I have added him in the list

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
>> >
>> > 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 :-)
>
>> >
>> > Now for idx==0 this is hard, because even when sync=1 you can only make
>> > it true if both cpus are completely idle, in which case you really want
>> > to move to the waking cpu I suppose.
>>
>> This use case is already taken into account by
>>
>> if (this_load > 0)
>> ..
>> else
>>  balance = true
>
> Agreed.
>
>> > One task running will have it == SCHED_LOAD_SCALE.
>> >
>> > But for idx>0 this can trigger in all kinds of situations of light load.
>>
>> target_load is the max between load for idx == 0 and load for the
>> selected idx so we have even less chance to match the condition : both
>> cpu are completely idle
>
> Ah, yes, I forgot to look at the target_load() thing and missed the max,
> yes that all makes it entirely less likely.
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