Re: [RFC PATCH 1/9] sched,cgroup: Add interface for latency-nice

[Joel Fernandes <joel@joelfernandes.org>]

Hi Dietmar,

On Thu, Apr 16, 2020 at 1:23 PM Dietmar Eggemann
<dietmar.eggemann@arm.com> wrote:
>
> Hi Joel,
>
> On 16.04.20 02:02, Joel Fernandes wrote:
> > On Thu, Sep 05, 2019 at 12:47:26PM +0100, Qais Yousef wrote:
> >> On 09/05/19 13:30, Peter Zijlstra wrote:
> >>> On Thu, Sep 05, 2019 at 12:13:47PM +0100, Qais Yousef wrote:
> >>>> On 09/05/19 12:46, Peter Zijlstra wrote:
> >>>
> >>>>> This is important because we want to be able to bias towards less
> >>>>> importance to (tail) latency as well as more importantance to (tail)
> >>>>> latency.
> >>>>>
> >>>>> Specifically, Oracle wants to sacrifice (some) latency for throughput.
> >>>>> Facebook OTOH seems to want to sacrifice (some) throughput for latency.
> >>>>
> >>>> Another use case I'm considering is using latency-nice to prefer an idle CPU if
> >>>> latency-nice is set otherwise go for the most energy efficient CPU.
> >>>>
> >>>> Ie: sacrifice (some) energy for latency.
> >>>>
> >>>> The way I see interpreting latency-nice here as a binary switch. But
> >>>> maybe we can use the range to select what (some) energy to sacrifice
> >>>> mean here. Hmmm.
> >>>
> >>> It cannot be binary, per definition is must be ternary, that is, <0, ==0
> >>> and >0 (or middle value if you're of that persuasion).
> >>
> >> I meant I want to use it as a binary.
> >>
> >>>
> >>> In your case, I'm thinking you mean >0, we want to lower the latency.
> >>
> >> Yes. As long as there's an easy way to say: does this task care about latency
> >> or not I'm good.
> >
> > Qais, Peter, all,
> >
> > For ChromeOS (my team), we are planning to use the upstream uclamp mechanism
> > instead of the out-of-tree schedtune mechanism to provide EAS with the
> > latency-sensitivity (binary/ternary) hint. ChromeOS is thankfully quite a bit
> > upstream focussed :)
> >
> > However, uclamp is missing an attribute to provide this biasing to EAS as we
> > know.
> >
> > What was the consensus on adding a per-task attribute to uclamp for providing
> > this? Happy to collaborate on this front.
>
> We're planning to have a session about this topic (latency-nice
> attribute per task group) during the virtual Pisa OSPM summit
> retis.sssup.it/ospm-summit in May this year.

Cool, I registered as well.

>
> There are two presentations/discussions planned:
>
> "Introducing Latency Nice for Scheduler Hints and Optimizing Scheduler
> Task Wakeup" and "The latency nice use case for Energy-Aware-Scheduling
> (EAS) in Android Common Kernel (ACK)"
>
> We'll probably merge those two into one presentation/discussion.
>
> So far we have Parth's per-task implementation
>
> https://lore.kernel.org/lkml/20200228090755.22829-1-parth@linux.ibm.com

Cool, I see it has some Reviewed-by tags so that's a good sign. Will
look more into that.

> What's missing is the per-taskgroup implementation, at least from the
> standpoint of ACK.
>
> The (mainline) EAS use-case for latency nice is already in ACK
> (android-5.4):
>
> https://android.googlesource.com/kernel/common/+/760b82c9b88d2c8125abfc5f732cc3cd460b2a54

Yes, I was aware of this. But if we use task groups, then the
transition from schedtune -> uclamp means now the tasks that use
uclamp would also be subjected to cpu.shares. That's why we were
looking into the per-task interface and glad there's some work on this
already done.

Thanks!

 - Joel