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

[Juri Lelli <juri.lelli@redhat.com>]

On 11/02/25 09:36, Dietmar Eggemann wrote:
> On 10/02/2025 18:09, 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)
> >>
> 
> Could reproduce on Juno-r0:
> 
> 0 1 2 3 4 5
> 
> L b b L L L
> 
>       ^^^
>       isol = [3-4] so both L
> 
> echo 0 > /sys/devices/system/cpu/cpu1/online
> echo 0 > /sys/devices/system/cpu/cpu4/online
> echo 0 > /sys/devices/system/cpu/cpu5/online
> echo 0 > /sys/devices/system/cpu/cpu2/online - isol
> echo 0 > /sys/devices/system/cpu/cpu3/online - isol
> 
> >> The issue only triggers with sugov DL threads (I guess that's obvious, but
> >> just to mention it).
> 
> IMHO, it doesn't have to be a sugov DL task. Any DL task will do.

OK, but in this case we actually want to fail. If we have allocated
bandwidth for an actual DL task (not a dl server or a 'fake' sugov), we
don't want to inadvertently leave it w/o bandwidth by turning CPUs off.

> // on a 2. shell:
> # chrt -d -T 5000000 -D 10000000 -P 16666666 -p 0 $$
> 
> # ps -eTo comm,pid,class | grep DLN
> bash             1243 DLN
> 
> 5000000/16666666 = 0.3, 0.3 << 10 = 307 (task util, bandwidth requirement)
> 
> > 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.
> 
> # echo 0 > /sys/devices/system/cpu/cpu1/online
> [   87.402722] __dl_bw_capacity() mask=0-2,5 cap=2940
> [   87.407551] dl_bw_cpus() cpu=1 rd->span=0-2,5 cpu_active_mask=0-5 cpumask_weight(rd->span)=4
> [   87.416019] dl_bw_manage: cpu=1 cap=1916 fair_server_bw=52428 total_bw=524284 dl_bw_cpus=4 type=DYN span=0-2,5
> 
> # echo 0 > /sys/devices/system/cpu/cpu2/online
> [   95.562270] __dl_bw_capacity() mask=0,2,5 cap=1916
> [   95.567091] dl_bw_cpus() cpu=2 rd->span=0,2,5 cpu_active_mask=0,2-5 cpumask_weight(rd->span)=3
> [   95.575735] dl_bw_manage: cpu=2 cap=892 fair_server_bw=52428 total_bw=157284 dl_bw_cpus=3 type=DYN span=0,2,5
> 
> # echo 0 > /sys/devices/system/cpu/cpu5/online
> [  100.573131] __dl_bw_capacity() mask=0,5 cap=892
> [  100.577713] dl_bw_cpus() cpu=5 rd->span=0,5 cpu_active_mask=0,3-5 cpumask_weight(rd->span)=2
> [  100.586186] dl_bw_manage: cpu=5 cap=446 fair_server_bw=52428 total_bw=104856 dl_bw_cpus=2 type=DYN span=0,5
> 
> # echo 0 > /sys/devices/system/cpu/cpu3/online
> [  110.232755] __dl_bw_capacity() mask=1-5 cap=892
> [  110.237333] dl_bw_cpus() cpu=6 rd->span=1-5 cpu_active_mask=0,3-4 cpus=2
> [  110.244064] dl_bw_manage: cpu=3 cap=446 fair_server_bw=52428 total_bw=419428 dl_bw_cpus=2 type=DEF span=1-5
> 
> 
> # echo 0 > /sys/devices/system/cpu/cpu4/online
> [  175.870273] __dl_bw_capacity() mask=1-5 cap=446
> [  175.874850] dl_bw_cpus() cpu=6 rd->span=1-5 cpu_active_mask=0,4 cpus=1
> [  175.881407] dl_bw_manage: cpu=4 cap=0 fair_server_bw=52428 total_bw=367000 dl_bw_cpus=1 type=DEF span=1-5
>                                    ^^^^^                                                            ^^^^^^^^
>                                    w/o/ cpu4 cap is 0!                                              cpu0 is not part of it                                                                                                     
> ...
> [  175.897600] dl_bw_manage() cpu=4 cap=0 overflow=1 return=-16
>                                           ^^^^^^^^^^ -EBUSY
>                                           
> -bash: echo: write error: Device or resource busy
> 
> sched_cpu_deactivate()
> 
>   dl_bw_deactivate(cpu)
> 
>     dl_bw_manage(dl_bw_req_deactivate, cpu, 0);
> 
>       return overflow ? -EBUSY : 0;
> 
> Looks like in DEF there is no CPU capacity left but we still have 1 DLN
> task with a bandwidth requirement of 307.
>