Re: [PATCH] sched: Take thermal pressure into account when determine rt fits capacity

[Qais Yousef <qais.yousef@arm.com>]

On 04/26/22 10:09, Vincent Guittot wrote:
> On Tue, 26 Apr 2022 at 04:07, Xuewen Yan <xuewen.yan94@gmail.com> wrote:
> >
> > On Tue, Apr 26, 2022 at 12:12 AM Qais Yousef <qais.yousef@arm.com> wrote:
> > >
> > > On 04/25/22 09:31, Xuewen Yan wrote:
> > > > On Fri, Apr 22, 2022 at 12:15 AM Qais Yousef <qais.yousef@arm.com> wrote:
> > > > > Is it okay to share what the capacities of the littles, mediums and bigs on
> > > > > your system? And how they change under worst case scenario thermal pressure?
> > > > > Only IF you have these numbers handy :-)
> > > >
> > > > Okay, the little/mid/big cpu scale capacity is 350/930/1024, but the
> > > > cpu frequency point is discrete, the big core's high freq point may is
> > > > just a few more than the mid core's highest.
> > > > In this case, once the thermal decrease the scaling_max_freq, the
> > > > maximum frequency of the large core is easily lower than that of the
> > > > medium core.
> > > > Of course, the corner case is due to the frequency design of the soc
> > > > and  our thermal algorithm.
> > >
> > > Okay, thanks for the info!
> > >
> > > >
> > > > >
> > > > > Is it actually an indication of a potential other problem if you swing into
> > > > > capacity inversion in the bigs that often? I've seen a lot of systems where the
> > > > > difference between the meds and bigs is small. But frequent inversion could be
> > > > > suspicious still.
> > > > >
> > > > > Do the littles and the mediums experience any significant thermal pressure too?
> > > >
> > > > In our platform, it's not.
> > >
> > > Good.
> > >
> > > > > It doesn't seem it'll cause a significant error, but still it seems to me this
> > > > > function wants the original capacity passed to it.
> > > > >
> > > > > There are similar questions to be asked since you modify sg_cpu->max. Every
> > > > > user needs to be audited if they're fine with this change or not.
> > > > >
> > > > > I'm not sure still what we are achieving here. You want to force schedutil not
> > > > > to request higher frequencies if thermal pressure is high? Should schedutil
> > > > > actually care? Shouldn't the cpufreq driver reject this request and pick the
> > > > > next best thing if it can't satisfy it? I could be missing something, I haven't
> > > > > looked that hard tbh :-)
> > > >
> > > > I changed this just want to make it more responsive to the real
> > > > capacity of the cpu, if it will cause other problems, maybe it would
> > > > be better not to change it.:)
> > >
> > > There are others who can give you a better opinion. But AFAICS we're not fixing
> > > anything but risking breaking other things. So I vote for not to change it :)
> > >
> > > > > It depends on the severity of the problem. The simplest thing I can suggest is
> > > > > to check if the cpu is in capacity inversion state, and if it is, then make
> > > > > rt_task_fits_capacity() return false always.
> > > > >
> > > > > If we need a generic solution to handle thermal pressure omitting OPPs, then
> > > > > the search needs to become more complex. The proposal in this patch is not
> > > > > adequate because tasks that want to run at capacity_orig_of(cpu) will wrongly
> > > > > omit some cpus because of any tiny thermal pressure. For example if the
> > > > > capacity_orig_of(medium_cpu) = 700, and uclamp_min for RT is set to 700, then
> > > > > any small thermal pressure on mediums will cause these tasks to run on big cpus
> > > > > only, which is not what we want. Especially if these big cpus can end up in
> > > > > capacity inversion later ;-)
> > > > >
> > > > > So if we want to handle this case, then we need to ensure the search returns
> > > > > false only if
> > > > >
> > > > >         1. Thermal pressure results in real OPP to be omitted.
> > > > >         2. Another CPU that can provide this performance level is available.
> > > > >
> > > > > Otherwise we should still fit it on this CPU because it'll give us the closest
> > > > > thing to what was requested.
> > > > >
> > > > > I can think of 2 ways to implement this, but none of them seem particularly
> > > > > pretty :-/
> > > >
> > > > Maybe as Lukasz Luba said:
> > > >
> > > > https://lore.kernel.org/all/ae98a861-8945-e630-8d4c-8112723d1007@arm.com/
> > > >
> > > > > Let's meet in the middle:
> > > > > 1) use the thermal PELT signal in RT:
> > > > > capacity = capacity_orig_of(cpu) - thermal_load_avg(cpu_rq(cpu))
> > > > > 2) introduce a more configurable thermal_pressure shifter instead
> > > > > 'sched_thermal_decay_shift', which would allow not only to make the
> > > > > decaying longer, but also shorter when the platform already might do
> > > > > that, to not cause too much traffic.
> > > >
> > > > But even if this is changed, there will still be the same problem, I
> > > > look forward to Lukasz's patch:)
> > >
> > > This will not address my concern unless I missed something.
> > >
> > > The best (simplest) way forward IMHO is to introduce a new function
> > >
> > >         bool cpu_in_capacity_inversion(int cpu);
> > >
> > > (feel free to pick another name) which will detect the scenario you're in. You
> > > can use this function then in rt_task_fits_capacity()
> > >
> > >         diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > >         index a32c46889af8..d48811a7e956 100644
> > >         --- a/kernel/sched/rt.c
> > >         +++ b/kernel/sched/rt.c
> > >         @@ -462,6 +462,9 @@ static inline bool rt_task_fits_capacity(struct task_struct *p, int cpu)
> > >                 if (!static_branch_unlikely(&sched_asym_cpucapacity))
> > >                         return true;
> > >
> > >         +       if (cpu_in_capacity_inversion(cpu))
> > >         +               return false;
> > >         +
> > >                 min_cap = uclamp_eff_value(p, UCLAMP_MIN);
> > >                 max_cap = uclamp_eff_value(p, UCLAMP_MAX);
> > >
> > > You'll probably need to do something similar in dl_task_fits_capacity().
> > >
> > > This might be a bit aggressive though as we'll steer away all RT tasks from
> > > this CPU (as long as there's another CPU that can fit it). I need to think more
> > > about it. But we could do something like this too
> > >
> > >         diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > >         index a32c46889af8..f2a34946a7ab 100644
> > >         --- a/kernel/sched/rt.c
> > >         +++ b/kernel/sched/rt.c
> > >         @@ -462,11 +462,14 @@ static inline bool rt_task_fits_capacity(struct task_struct *p, int cpu)
> > >                 if (!static_branch_unlikely(&sched_asym_cpucapacity))
> > >                         return true;
> > >
> > >         +       cpu_cap = capacity_orig_of(cpu);
> > >         +
> > >         +       if (cpu_in_capacity_inversion(cpu))
> >
> > It's  a good idea, but as you said, in mainline, the
> > sysctl_sched_uclamp_util_min_rt_default is always 1024,
> > Maybe it's better to add it to the judgment?
> >
> >  +       if (sysctl_sched_uclamp_util_min_rt_default !=
> > SCHED_CAPACITY_SCALE && cpu_in_capacity_inversion(cpu))
> >
> > >         +               cpu_cap -= thermal_load_avg(cpu_rq(cpu));
> >
> > Why use thermal_load_avg? If thermal is always in effect，the
> > thermal_load_avg would get bigger and bigger, as a result, the cpu_cap
> > maybe smaller than (capacity_orig - thermal_pressure).
> 
> For a fixed thermal_pressure(), thermal_load_avg() will not be higher
> than thermal_pressure() but will increase to reach thermal_pressure()
> 
> In the current implementation for sched_asym_cpucapacity topology, we
> do a 1st iteration trying to find a cpu that fits a task's capacity
> but if it fails, we run a normal cpupri_find that doesn't care about
> capacity.
> 
> Do I understand correctly that in your case you would like to run
> a 1st iteration that takes into account capacity_orig_of(cpu) -
> thermal_load_avg(cpu_rq(cpu))
> If it fails run another iteration only with capacity_orig_of(cpu)
> and finally tries without capacity constraint

Wouldn't this be expensive to have 3 loops? That was my other suggestion but
wasn't sure the complexity was worth it. So I suggested handling the capacity
inversion case only.


Thanks

--
Qais Yousef