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

[Peter Zijlstra]

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     SCHED_LOAD_SCALE has been replaced by the load_weight for the relevant task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 where task_hot had been replaced by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 :-)

> > 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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