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

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

On 2/13/25 13:33, Juri Lelli wrote:
> On 13/02/25 12:27, Christian Loehle wrote:
>> On 2/13/25 06:20, Juri Lelli wrote:
>>> On 12/02/25 19:22, Dietmar Eggemann wrote:
>>>> On 11/02/2025 11:42, Juri Lelli wrote:
>>>
>>> ...
>>>
>>>>> 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?
>>>>
>>>> It at least seems to solve the issue. And like you mentioned on irc, we
>>>> don't know the bw req of sugov anyway.
>>>>
>>>> So with this change we start with 'dl_bw->total_bw = 0' even w/ sugov tasks.
>>>>
>>>> dl_rq[0]:
>>>>   .dl_nr_running                 : 0
>>>>   .dl_bw->bw                     : 996147
>>>>   .dl_bw->total_bw               : 0       <-- !
>>>>
>>>> IMHO, people who want to run serious DL can always check whether there
>>>> are already these infrastructural DL tasks or even avoid schedutil.
>>>
>>> It definitely not ideal and admittedly gross, but not worse than what we
>>> are doing already considering we ignore sugovs at AC and the current
>>> bandwidth allocation its there only to help with PI. So, duck tape. :/
>>>
>>> A more proper way to work with this would entail coming up with sensible
>>> bandwidth allocation for sugovs, but that's most probably hardware
>>> specific, so I am not sure how we can make that general enough.
>>>
>>> Anyway, looks like Jon was still seeing the issue. I asked him to verify
>>> he is using all the proposed changes. Let's see what he reports.
>>
>> FWIW it also fixes my reproducer.
>>
>> I agree that dummy numbers for sugov bw is futile, but real bw numbers
>> also don't make a lot of sense (what if we exceed them? The system
>> won't be able to change frequency, i.e. might not be able to provide
>> bw for other DL tasks then either?).
>> I'm slightly worried about now allowing the last legal CPU for a sugov
>> cluster to offline, which would lead to a cluster still being active
>> but sugov DL unable to run anywhere. I can't reproduce this currently
>> though. Is this an issue in theory? Or am I missing something?
> 
> Not sure I get what your worry is, sorry. In my understanding when the
> last cpu of a policy/cluster gets offlined the corresponding sugov
> kthread gets stopped as well (sugov_exit)?
> 

The other way round.
We may have sugov kthread of cluster [6,7] affined to CPU1. Is it
guaranteed that we cannot offline CPU1 (while CPU6 or CPU7 are still
online)?
Or without the affinity:
cluster [6,7] with isolcpu=6 (i.e. sugov kthread of that cluster can
only run on CPU7). Is offlining of CPU6 then prevented (as long as
CPU7 is online)?
I don't see how.
Anyway we probably want to change isolcpu and affinity to merely be 
a suggestion for the sugov DL case. Fundamentally it belongs to what
is run on that CPU anyway.