Re: stable 6.6: commit "sched/cpufreq: Rework schedutil governor performance estimation' causes a regression

[Christian Loehle <christian.loehle@arm.com>]

On 11/21/25 15:37, Yu-Che Cheng wrote:
> Hi Vincent,
> 
> On Fri, Nov 21, 2025 at 10:00 PM Vincent Guittot <vincent.guittot@linaro.org>
> wrote:
>>
>> On Fri, 21 Nov 2025 at 04:55, Sergey Senozhatsky
>> <senozhatsky@chromium.org> wrote:
>>>
>>> Hi Christian,
>>>
>>> On (25/11/20 10:15), Christian Loehle wrote:
>>>> On 11/20/25 04:45, Sergey Senozhatsky wrote:
>>>>> Hi,
>>>>>
>>>>> We are observing a performance regression on one of our arm64
> boards.
>>>>> We tracked it down to the linux-6.6.y commit ada8d7fa0ad4
> ("sched/cpufreq:
>>
>> You mentioned that you tracked down to linux-6.6.y but which kernel
>> are you using ?
>>
> 
> We're using ChromeOS 6.6 kernel, which is currently on top of linux-v6.6.99.
> But we've tested that the performance regression still happens on exactly
> the same scheduler codes (`kernel/sched`) as upstream v6.6.99, compared to
> those on v6.6.88.
> 
>>>>> Rework schedutil governor performance estimation").
>>>>>
>>>>> UI speedometer benchmark:
>>>>> w/commit:   395  +/-38
>>>>> w/o commit: 439  +/-14
>>>>>
>>>>
>>>> Hi Sergey,
>>>> Would be nice to get some details. What board?
>>>
>>> It's an MT8196 chromebook.
>>>
>>>> What do the OPPs look like?
>>>
>>> How do I find that out?
>>
>> In /sys/kernel/debug/opp/cpu*/
>> or
>> /sys/devices/system/cpu/cpufreq/policy*/scaling_available_frequencies
>> with related_cpus
>>
> 
> The energy model on the device is:
> 
> CPU0-3:
> +------------+------------+
> | freq (khz) | power (uw) |
> +============+============+
> |     339000 |      34362 |
> |     400000 |      42099 |
> |     500000 |      52907 |
> |     600000 |      63795 |
> |     700000 |      74747 |
> |     800000 |      88445 |
> |     900000 |     101444 |
> |    1000000 |     120377 |
> |    1100000 |     136859 |
> |    1200000 |     154162 |
> |    1300000 |     174843 |
> |    1400000 |     196833 |
> |    1500000 |     217052 |
> |    1600000 |     247844 |
> |    1700000 |     281464 |
> |    1800000 |     321764 |
> |    1900000 |     352114 |
> |    2000000 |     383791 |
> |    2100000 |     421809 |
> |    2200000 |     461767 |
> |    2300000 |     503648 |
> |    2400000 |     540731 |
> +------------+------------+
> 
> CPU4-6:
> +------------+------------+
> | freq (khz) | power (uw) |
> +============+============+
> |     622000 |     131738 |
> |     700000 |     147102 |
> |     800000 |     172219 |
> |     900000 |     205455 |
> |    1000000 |     233632 |
> |    1100000 |     254313 |
> |    1200000 |     288843 |
> |    1300000 |     330863 |
> |    1400000 |     358947 |
> |    1500000 |     400589 |
> |    1600000 |     444247 |
> |    1700000 |     497941 |
> |    1800000 |     539959 |
> |    1900000 |     584011 |
> |    2000000 |     657172 |
> |    2100000 |     746489 |
> |    2200000 |     822854 |
> |    2300000 |     904913 |
> |    2400000 |    1006581 |
> |    2500000 |    1115458 |
> |    2600000 |    1205167 |
> |    2700000 |    1330751 |
> |    2800000 |    1450661 |
> |    2900000 |    1596740 |
> |    3000000 |    1736568 |
> |    3100000 |    1887001 |
> |    3200000 |    2048877 |
> |    3300000 |    2201141 |
> +------------+------------+
> 
> CPU7:
> 
> +------------+------------+
> | freq (khz) | power (uw) |
> +============+============+
> |     798000 |     320028 |
> |     900000 |     330714 |
> |    1000000 |     358108 |
> |    1100000 |     384730 |
> |    1200000 |     410669 |
> |    1300000 |     438355 |
> |    1400000 |     469865 |
> |    1500000 |     502740 |
> |    1600000 |     531645 |
> |    1700000 |     560380 |
> |    1800000 |     588902 |
> |    1900000 |     617278 |
> |    2000000 |     645584 |
> |    2100000 |     698653 |
> |    2200000 |     744179 |
> |    2300000 |     810471 |
> |    2400000 |     895816 |
> |    2500000 |     985234 |
> |    2600000 |    1097802 |
> |    2700000 |    1201162 |
> |    2800000 |    1332076 |
> |    2900000 |    1439847 |
> |    3000000 |    1575917 |
> |    3100000 |    1741987 |
> |    3200000 |    1877346 |
> |    3300000 |    2161512 |
> |    3400000 |    2437879 |
> |    3500000 |    2933742 |
> |    3600000 |    3322959 |
> |    3626000 |    3486345 |
> +------------+------------+
> 
>>>
>>>> Does this system use uclamp during the benchmark? How?
>>>
>>> How do I find that out?
>>
>> it can be set per cgroup
>> /sys/fs/cgroup/system.slice/<name>/cpu.uclam.min|max
>> or per task with sched_setattr()
>>
>> You most probably use it because it's the main reason for ada8d7fa0ad4
>> to remove wrong overestimate of OPP
>>
> 
> For the speedometer case, yes, we set the uclamp.min to 20 for the whole
> browser and UI (chrome).
> There's no system-wide uclamp settings though.

(From Sergey's traces)
Per-cluster time‑weighted average frequency base => revert:
little (cpu0–3, max 2.4 GHz): 0.746 GHz => 1.132 GHz (+51.6%)
mid (cpu4–6, max 3.3 GHz): 1.043 GHz => 1.303 GHz (+24.9%)
big (cpu7, max 3.626 GHz): 2.563 GHz => 3.116 GHz (+21.6%)

And in particular time spent at OPPs (base => revert):
Big core at upper 10%: 29.6% => 61.5%
little cluster at 339 MHz: 50.1% => 1.0% 

Interesting that a uclamp.min of 20 (which shouldn't really have
much affect on big CPU at all, with or without headroom AFAICS?)
makes such a big difference here?

> 
> But we also found other performance regressions in an Android guest VM,
> where there's no uclamp for the VM and vCPU processes from the host side.
> Particularly, the RAR extraction throughput reduces about 20% in the RAR
> app (from RARLAB).
> Although it's hard to tell if this is some sort of a side-effect of the UI
> regression as the UI is also running at the same time.
> 
I'd be inclined to say that is because of the vastly different DVFS from the
UI workload, yes.