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

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Message-ID: <4c045707-6f5a-44fd-b2d1-3ad13c2b11ba@arm.com>
Date: Wed, 19 Feb 2025 14:09:48 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Juri Lelli <juri.lelli@...hat.com>
Cc: Jon Hunter <jonathanh@...dia.com>,
 Christian Loehle <christian.loehle@....com>,
 Thierry Reding <treding@...dia.com>, Waiman Long <longman@...hat.com>,
 Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
 Michal Koutny <mkoutny@...e.com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
 Phil Auld <pauld@...hat.com>, Qais Yousef <qyousef@...alina.io>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 "Joel Fernandes (Google)" <joel@...lfernandes.org>,
 Suleiman Souhlal <suleiman@...gle.com>, Aashish Sharma <shraash@...gle.com>,
 Shin Kawamura <kawasin@...gle.com>,
 Vineeth Remanan Pillai <vineeth@...byteword.org>,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v2 3/2] sched/deadline: Check bandwidth overflow earlier
 for hotplug

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@...hub.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?