Re: [RFC] sched/eevdf: sched feature to dismiss lag on wakeup

[Vincent Guittot]

On Tue, 19 Mar 2024 at 10:08, Tobias Huschle <huschle@xxxxxxxxxxxxx> wrote:
>
> On 2024-03-18 15:45, Luis Machado wrote:
> > On 3/14/24 13:45, Tobias Huschle wrote:
> >> On Fri, Mar 08, 2024 at 03:11:38PM +0000, Luis Machado wrote:
> >>> On 2/28/24 16:10, Tobias Huschle wrote:
> >>>>
> >>>> Questions:
> >>>> 1. The kworker getting its negative lag occurs in the following
> >>>> scenario
> >>>>    - kworker and a cgroup are supposed to execute on the same CPU
> >>>>    - one task within the cgroup is executing and wakes up the
> >>>> kworker
> >>>>    - kworker with 0 lag, gets picked immediately and finishes its
> >>>>      execution within ~5000ns
> >>>>    - on dequeue, kworker gets assigned a negative lag
> >>>>    Is this expected behavior? With this short execution time, I
> >>>> would
> >>>>    expect the kworker to be fine.
> >>>
> >>> That strikes me as a bit odd as well. Have you been able to determine
> >>> how a negative lag
> >>> is assigned to the kworker after such a short runtime?
> >>>
> >>
> >> I did some more trace reading though and found something.
> >>
> >> What I observed if everything runs regularly:
> >> - vhost and kworker run alternating on the same CPU
> >> - if the kworker is done, it leaves the runqueue
> >> - vhost wakes up the kworker if it needs it
> >> --> this means:
> >>   - vhost starts alone on an otherwise empty runqueue
> >>   - it seems like it never gets dequeued
> >>     (unless another unrelated task joins or migration hits)
> >>   - if vhost wakes up the kworker, the kworker gets selected
> >>   - vhost runtime > kworker runtime
> >>     --> kworker gets positive lag and gets selected immediately next
> >> time
> >>
> >> What happens if it does go wrong:
> >> From what I gather, there seem to be occasions where the vhost either
> >> executes suprisingly quick, or the kworker surprinsingly slow. If
> >> these
> >> outliers reach critical values, it can happen, that
> >>    vhost runtime < kworker runtime
> >> which now causes the kworker to get the negative lag.
> >>
> >> In this case it seems like, that the vhost is very fast in waking up
> >> the kworker. And coincidentally, the kworker takes, more time than
> >> usual
> >> to finish. We speak of 4-digit to low 5-digit nanoseconds.
> >>
> >> So, for these outliers, the scheduler extrapolates that the kworker
> >> out-consumes the vhost and should be slowed down, although in the
> >> majority
> >> of other cases this does not happen.
> >
> > Thanks for providing the above details Tobias. It does seem like EEVDF
> > is strict
> > about the eligibility checks and making tasks wait when their lags are
> > negative, even
> > if just a little bit as in the case of the kworker.
> >
> > There was a patch to disable the eligibility checks
> > (https://lore.kernel.org/lkml/20231013030213.2472697-1-youssefesmat@xxxxxxxxxxxx/),
> > which would make EEVDF more like EVDF, though the deadline comparison
> > would
> > probably still favor the vhost task instead of the kworker with the
> > negative lag.
> >
> > I'm not sure if you tried it, but I thought I'd mention it.
>
> Haven't seen that one yet. Unfortunately, it does not help to ignore the
> eligibility.
>
> I'm inclined to rather propose propose a documentation change, which
> describes that tasks should not rely on woken up tasks being scheduled
> immediately.

Where do you see such an assumption ? Even before eevdf, there were
nothing that ensures such behavior. When using CFS (legacy or eevdf)
tasks, you can't know if the newly wakeup task will run 1st or not


>
> Changing things in the code to address for the specific scenario I'm
> seeing seems to mostly create unwanted side effects and/or would require
> the definition of some magic cut-off values.
>
>