Re: [PATCH v2 3/2] sched/deadline: Check bandwidth overflow earlier for hotplug

Jon Hunter
2025-02-13T09:53:27Z

On 13/02/2025 06:16, Juri Lelli wrote:
> On 12/02/25 23:01, Jon Hunter wrote:
>>
>> On 11/02/2025 10:42, Juri Lelli wrote:
>>> On 11/02/25 10:15, Christian Loehle wrote:
>>>> On 2/10/25 17:09, Juri Lelli wrote:
>>>>> Hi Christian,
>>>>>
>>>>> Thanks for taking a look as well.
>>>>>
>>>>> On 07/02/25 15:55, Christian Loehle wrote:
>>>>>> On 2/7/25 14:04, Jon Hunter wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 07/02/2025 13:38, Dietmar Eggemann wrote:
>>>>>>>> On 07/02/2025 11:38, Jon Hunter wrote:
>>>>>>>>>
>>>>>>>>> On 06/02/2025 09:29, Juri Lelli wrote:
>>>>>>>>>> On 05/02/25 16:56, Jon Hunter wrote:
>>>>>>>>>>
>>>>>>>>>> ...
>>>>>>>>>>
>>>>>>>>>>> Thanks! That did make it easier :-)
>>>>>>>>>>>
>>>>>>>>>>> Here is what I see ...
>>>>>>>>>>
>>>>>>>>>> Thanks!
>>>>>>>>>>
>>>>>>>>>> Still different from what I can repro over here, so, unfortunately, I
>>>>>>>>>> had to add additional debug printks. Pushed to the same branch/repo.
>>>>>>>>>>
>>>>>>>>>> Could I ask for another run with it? Please also share the complete
>>>>>>>>>> dmesg from boot, as I would need to check debug output when CPUs are
>>>>>>>>>> first onlined.
>>>>>>>>
>>>>>>>> So you have a system with 2 big and 4 LITTLE CPUs (Denver0 Denver1 A57_0
>>>>>>>> A57_1 A57_2 A57_3) in one MC sched domain and (Denver1 and A57_0) are
>>>>>>>> isol CPUs?
>>>>>>>
>>>>>>> I believe that 1-2 are the denvers (even thought they are listed as 0-1 in device-tree).
>>>>>>
>>>>>> Interesting, I have yet to reproduce this with equal capacities in isolcpus.
>>>>>> Maybe I didn't try hard enough yet.
>>>>>>
>>>>>>>
>>>>>>>> This should be easy to set up for me on my Juno-r0 [A53 A57 A57 A53 A53 A53]
>>>>>>>
>>>>>>> Yes I think it is similar to this.
>>>>>>>
>>>>>>> Thanks!
>>>>>>> Jon
>>>>>>>
>>>>>>
>>>>>> I could reproduce that on a different LLLLbb with isolcpus=3,4 (Lb) and
>>>>>> the offlining order:
>>>>>> echo 0 > /sys/devices/system/cpu/cpu5/online
>>>>>> echo 0 > /sys/devices/system/cpu/cpu1/online
>>>>>> echo 0 > /sys/devices/system/cpu/cpu3/online
>>>>>> echo 0 > /sys/devices/system/cpu/cpu2/online
>>>>>> echo 0 > /sys/devices/system/cpu/cpu4/online
>>>>>>
>>>>>> while the following offlining order succeeds:
>>>>>> echo 0 > /sys/devices/system/cpu/cpu5/online
>>>>>> echo 0 > /sys/devices/system/cpu/cpu4/online
>>>>>> echo 0 > /sys/devices/system/cpu/cpu1/online
>>>>>> echo 0 > /sys/devices/system/cpu/cpu2/online
>>>>>> echo 0 > /sys/devices/system/cpu/cpu3/online
>>>>>> (Both offline an isolcpus last, both have CPU0 online)
>>>>>>
>>>>>> The issue only triggers with sugov DL threads (I guess that's obvious, but
>>>>>> just to mention it).
>>>>>
>>>>> It wasn't obvious to me at first :). So thanks for confirming.
>>>>>
>>>>>> I'll investigate some more later but wanted to share for now.
>>>>>
>>>>> So, problem actually is that I am not yet sure what we should do with
>>>>> sugovs' bandwidth wrt root domain accounting. W/o isolation it's all
>>>>> good, as it gets accounted for correctly on the dynamic domains sugov
>>>>> tasks can run on. But with isolation and sugov affected_cpus that cross
>>>>> isolation domains (e.g., one BIG one little), we can get into troubles
>>>>> not knowing if sugov contribution should fall on the DEF or DYN domain.
>>>>>
>>>>> Hummm, need to think more about it.
>>>>
>>>> That is indeed tricky.
>>>> I would've found it super appealing to always just have sugov DL tasks activate
>>>> on this_cpu and not have to worry about all this, but then you have contention
>>>> amongst CPUs of a cluster and there are energy improvements from always
>>>> having little cores handle all sugov DL tasks, even for the big CPUs,
>>>> that's why I introduced
>>>> commit 93940fbdc468 ("cpufreq/schedutil: Only bind threads if needed")
>>>> but that really doesn't make this any easier.
>>>
>>> What about we actually ignore them consistently? We already do that for
>>> admission control, so maybe we can do that when rebuilding domains as
>>> well (until we find maybe a better way to deal with them).
>>>
>>> Does the following make any difference?
>>>
>>> ---
>>> kernel/sched/deadline.c | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>>> index b254d878789d..8f7420e0c9d6 100644
>>> --- a/kernel/sched/deadline.c
>>> +++ b/kernel/sched/deadline.c
>>> @@ -2995,7 +2995,7 @@ void dl_add_task_root_domain(struct task_struct *p)
>>> struct dl_bw *dl_b;
>>> raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
>>> - if (!dl_task(p)) {
>>> + if (!dl_task(p) || dl_entity_is_special(&p->dl)) {
>>> raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags);
>>> return;
>>> }
>>>
>>
>> I have tested this on top of v6.14-rc2, but this is still not resolving the
>> issue for me :-(
>
> Thanks for testing.
>
> Was the testing using the full stack of changes I proposed so far? I
> believe we still have to fix the accounting of dl_servers for def
> root domain (there is a patch that should do that).
>
> I updated the branch with the full set. In case it still fails, could
> you please collect dmesg and tracing output as I suggested and share?

Ah no it was not! OK, let me test the latest branch now.

Thanks
Jon

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