Re: [PATCH v2 5/7] sched/fair: Take into account latency nice at wakeup

[Josh Don <joshdon@google.com>]

On Thu, May 19, 2022 at 6:57 AM Vincent Guittot
<vincent.guittot@linaro.org> wrote:
>
> On Tue, 17 May 2022 at 02:54, Josh Don <joshdon@google.com> wrote:
> >
> > Hi Vincent,
> >
> > On Thu, May 12, 2022 at 9:36 AM Vincent Guittot
> > <vincent.guittot@linaro.org> wrote:
> > >
> > > Take into account the nice latency priority of a thread when deciding to
> > > preempt the current running thread. We don't want to provide more CPU
> > > bandwidth to a thread but reorder the scheduling to run latency sensitive
> > > task first whenever possible.
> > >
> > > As long as a thread didn't use its bandwidth, it will be able to preempt
> > > the current thread.
> > >
> > > At the opposite, a thread with a low latency priority will preempt current
> > > thread at wakeup only to keep fair CPU bandwidth sharing. Otherwise it will
> > > wait for the tick to get its sched slice.
> >
> > Following up from the discussion on the prior series, I'm still not
> > sure why this approach is exclusive of extending the entity placement
> > code; I think both changes together would be useful.
> >
> > By only changing the wakeup preemption decision, you're only
> > guaranteeing that the latency-sensitive thing on cpu won't be
> > preempted until the next sched tick, which can occur at any time
> > offset from the wakeup, from 0ns to the length of one tick. If a
>
> In fact, you are ensured to run a minimum time of 3ms at least on >=8
> cores system before tick can preempt you. I considered updating this
> part as well to increase the value for negative weight but didn't do
> it for now. I can have a look

If the latency sensitive thing on cpu has already been running close
to or greater than that min granularity, that doesn't apply; can still
get preempted pretty quickly from the tick by the newly woken task.

A possible change would be to reduce the sleeper credit when the
waking entity has lower latency priority than the current entity (ie.
similar to the se_is_idle() check already being made in
place_entity()).

> > latency-tolerant task wakes up with a lot of sleeper credit, it would
> > pretty quickly preempt a latency-sensitive task on-cpu, even if it
> > doesn't initially do so due to the above changes to wakeup preemption.
> >
> > Adjusting place_entity wouldn't impact cpu bandwidth in steady-state
> > competition between threads of different latency prio, it would only
> > impact slightly at wakeup, in a similar but more consistent manner to
> > the changes above to wakeup_preempt_entity (ie. a task that is not
> > latency sensitive might have to wait a few ticks to preempt a latency
> > sensitive task, even if it was recently sleeping for a while).