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

Dietmar Eggemann
2025-02-19T13:10:02Z

On 19/02/2025 11:02, Juri Lelli wrote:
> On 19/02/25 10:29, Dietmar Eggemann wrote:

[...]

> So you don't have the one with which we ignore special tasks while
> rebuilding domains?
>
> https://lore.kernel.org/all/Z6spnwykg6YSXBX_@jlelli-thinkpadt14gen4.remote.csb/
>
> Could you please double check again against
>
> git@github.com:jlelli/linux.git experimental/dl-debug

Sorry, I forgot this one. Yes, I have it as well.

2993 void dl_add_task_root_domain(struct task_struct *p)
2994 {
2995 struct rq_flags rf;
2996 struct rq *rq;
2997 struct dl_bw *dl_b;
2998
2999 raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
3000 if (!dl_task(p) || dl_entity_is_special(&p->dl)) {
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
3001 raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags);
3002 return;
3003 }

>> The suspend issue still persists.
>>
>> My hunch is that it's rather an issue with having 0 CPUs left in DEF
>> while deactivating the last isol CPU (CPU3) so we set overflow = 1 w/o
>> calling __dl_overflow(). We want to account fair_server_bw=52428
>> against 0 CPUs.
>>
>> l B B l l l
>>
>> ^^^
>> isolcpus=[3,4]
>>
>>
>> cpumask_and(mask, rd->span, cpu_active_mask)
>>
>> mask = [3-5] & [0-3] = [3] -> dl_bw_cpus(3) = 1
>>
>> ---
>>
>> dl_bw_deactivate() called cpu=5
>>
>> dl_bw_deactivate() called cpu=4
>>
>> dl_bw_deactivate() called cpu=3
>>
>> dl_bw_cpus() cpu=6 rd->span=3-5 cpu_active_mask=0-3 cpus=1 type=DEF
>> ^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^
>> cpumask_subset(rd->span, cpu_active_mask) is false
>>
>> for_each_cpu_and(i, rd->span, cpu_active_mask)
>> cpus++ <-- cpus is 1 !!!
>>
>> dl_bw_manage: cpu=3 cap=0 fair_server_bw=52428 total_bw=104856 dl_bw_cpus=1 type=DEF span=3-5
> ^^^^^^
> This still looks wrong: with a single cpu remaining we should only have
> the corresponding dl server bandwidth present (unless there is some
> other DL task running.

That's true. '104856 - 52428 = 52428' so util of 51 ? Which is 50% of a
sugov task? Or exactly the fair_server_bw.

But the bw numbers don't matter here since we go straight into the else
path since dl_bw_cpus(3) = 1.

3587 if (dl_bw_cpus(cpu) - 1)
3588 overflow = __dl_overflow(dl_b, cap, fair_server_bw, 0);
3589 else
3590 overflow = 1;

> If you already had the patch ignoring sugovs bandwidth in your set, could
> you please share the full dmesg?

Will do later today ... busy with other stuff right now ;-(

BTW, I just saw that this issue also happens for me w/o sugov threads
(running with Performance CPUfreq governor)! So the remaining
'total_bw=104856' must be the contribution from 2 CPUs of DEF. Maybe we
just have a CPU-offset in this accounting somewhere during suspend?