Re: [PATCH] sched/uclamp: Introduce a method to transform UCLAMP_MIN into BOOST

[Xuewen Yan <xuewen.yan94@gmail.com>]

Hi Qais

Thanks for your patient reply, and I have got that I need to do more
work in uclamp to balance the performance and power, especially in
per-task API.
And If there is any progress in the future, I hope to keep
communicating with you.

Thank you very much!

BR
xuewen

On Tue, Jul 27, 2021 at 9:45 PM Qais Yousef <qais.yousef@arm.com> wrote:
>
> Hi Xuewen
>
> On 07/27/21 20:16, Xuewen Yan wrote:
> > Hi Qais
> >
> > On Tue, Jul 27, 2021 at 1:17 AM Qais Yousef <qais.yousef@arm.com> wrote:
> > >
> > > > > >
> > > > > > The uclamp can clamp the util within uclamp_min and uclamp_max,
> > > > > > it is benifit to some tasks with small util, but for those tasks
> > > > > > with middle util, it is useless.
> > >
> > > It's not really useless, it works as it's designed ;-)
> >
> > Yes, my expression problem...
>
> No worries, I understood what you meant. But I had to highlight that this is
> the intended design behavior :-)
>
> >
> > >
> > > As Dietmar highlighted, you need to pick a higher boost value that gives you
> > > the best perf/watt for your use case.
> > >
> > > I assume that this is a patch in your own Android 5.4 kernel, right? I'm not
> >
> > Yes, the patch indeed is used in my own Android12 with kernel5.4.
> >
> > > aware of any such patch in Android Common Kernel. If it's there, do you mind
> > > pointing me to the gerrit change that introduced it?
> >
> > emmm, sorry I kind of understand what that means.  Your means is  what
> > I need to do is to send this patch to google?
>
> Oh no. I meant if you are *not* carrying this patch in your own, I'd appreciate
> getting a link to when it was merged into Google' tree. But you already said
> you carry this patch on your own kernel, so there's nothing to do :)
>
> >
> > >
> > > > Because the kernel used in Android do not have the schedtune, and the
> > > > uclamp can not
> > > > boost all the util, and this is the reason for the design of the patch.
> > >
> > > Do you have a specific workload in mind here that is failing? It would help if
> > > you can explain in detail the mode of failure you're seeing to help us
> > > understand the problem better.
> >
> > The patch has has been working with me for a while, I can redo this
> > data, but this might take a while :)
>
> But there must have been a reason you introduced it in the first place, what
> was that reason?
>
> >
> > > >
> > > > >
> > > > > > Scenario:
> > > > > > if the task_util = 200, {uclamp_min, uclamp_max} = {100, 1024}
> > > > > >
> > > > > > without patch:
> > > > > > clamp_util = 200;
> > > > > >
> > > > > > with patch:
> > > > > > clamp_util = 200 + (100 / 1024) * (1024 - 200) = 280;
> > >
> > > If a task util was 200, how long does it take for it to reach 280? Why do you
> > > need to have this immediate boost value applied and can't wait for this time to
> > > lapse? I'm not sure, but ramping up by 80 points shouldn't take *that* long,
> > > but don't quote me on this :-)
> >
> > Here is just one example to illustrate that , with this patch, It also
> > can boost the util which in {UCLAMP_MIN, UCLAMP_MAX}...
> >
> > >
> > > > >
> > > > > The same could be achieved by using {uclamp_min, uclamp_max} = {280, 1024}?
> > > >
> > > > Yes, for per-task, that is no problem, but for per-cgroup, most times,
> > > > we can not always only put the special task into the cgroup.
> > > > For example, in Android , there is a cgroup named "top-app", often all
> > > > the threads of a app would be put into it.
> > > > But, not all threads of this app need to be boosted, if we set the
> > > > uclamp_min too big, the all the small task would be clamped to
> > > > uclamp_min,
> > > > the power consumption would be increased, howerever, if setting the
> > > > uclamp_min smaller, the performance may be increased.
> > > > Such as:
> > > > a task's util is 50,  {uclamp_min, uclamp_max} = {100, 1024}
> > > > the boost_util =  50 + (100 / 1024) * (1024 - 50) = 145;
> > > > but if we set {uclamp_min, uclamp_max} = {280, 1024}, without patch:
> > > > the clamp_util = 280.
> > >
> > > I assume {uclamp_min, uclamp_max} = {145, 1024} is not good enough because you
> > > want this 200 task to be boosted to 280. One can argue that not all tasks at
> > > 200 need to be boosted to 280 too. So the question is, like above, what type
> > > of tasks that are failing here and how do you observe this failure? It seems
> > > there's a class of performance critical tasks that need this fast boosting.
> > > Can't you identify them and boost them individually?
> >
> > Yes, the best way to do that is boosting them individually, but
> > usually, it may not be so easy...
>
> Yes I appreciate that, but cgroup is a coarse grain controller. Even with your
> approach, you will still have to find the best compromise because some tasks
> will get more boosting than they really need to and waste power even with your
> approach.
>
> For best outcome with uclamp; the cgroup should be used to specify the minimum
> performance requirement of a class of tasks, then use the per-task API to fine
> tune the settings for specific tasks.
>
> I appreciate it'll take time to get there, but this is the best way forward.
>
> If you have a specific use case that's failing, it will still be good to share
> the details to think more if there's something we can do about it at the kernel
> level.
>
> Thanks
>
> --
> Qais Yousef