Messages in this thread Patch in this message | | | Date | Mon, 25 Apr 2022 17:12:09 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH] sched: Take thermal pressure into account when determine rt fits capacity |
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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)) + cpu_cap -= thermal_load_avg(cpu_rq(cpu)); + min_cap = uclamp_eff_value(p, UCLAMP_MIN); max_cap = uclamp_eff_value(p, UCLAMP_MAX); - cpu_cap = capacity_orig_of(cpu); - return cpu_cap >= min(min_cap, max_cap); } #else
Thoughts?
Thanks!
-- Qais Yousef
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