linux-kernel - Re: [PATCH v3 4/8] sched/deadline: Rebuild root domain accounting after every update

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Message-ID: <be2c47b8-a5e4-4591-ac4d-3cbc92e2ce5d@redhat.com>
Date: Tue, 11 Mar 2025 08:34:52 -0400
From: Waiman Long <llong@...hat.com>
To: Juri Lelli <juri.lelli@...hat.com>, Waiman Long <llong@...hat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 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>,
 Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
 Michal Koutný <mkoutny@...e.com>,
 Qais Yousef <qyousef@...alina.io>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Swapnil Sapkal <swapnil.sapkal@....com>,
 Shrikanth Hegde <sshegde@...ux.ibm.com>, Phil Auld <pauld@...hat.com>,
 luca.abeni@...tannapisa.it, tommaso.cucinotta@...tannapisa.it,
 Jon Hunter <jonathanh@...dia.com>
Subject: Re: [PATCH v3 4/8] sched/deadline: Rebuild root domain accounting
 after every update

On 3/11/25 7:59 AM, Juri Lelli wrote:
> On 10/03/25 20:16, Waiman Long wrote:
>> On 3/10/25 3:18 PM, Waiman Long wrote:
>>> On 3/10/25 2:54 PM, Dietmar Eggemann wrote:
>>>> On 10/03/2025 10:37, Juri Lelli wrote:
>>>>> Rebuilding of root domains accounting information (total_bw) is
>>>>> currently broken on some cases, e.g. suspend/resume on aarch64. Problem
>>>> Nit: Couldn't spot any arch dependency here. I guess it was just tested
>>>> on Arm64 platforms so far.
>>>>
>>>> [...]
>>>>
>>>>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>>>>> index 44093339761c..363ad268a25b 100644
>>>>> --- a/kernel/sched/topology.c
>>>>> +++ b/kernel/sched/topology.c
>>>>> @@ -2791,6 +2791,7 @@ void partition_sched_domains_locked(int
>>>>> ndoms_new, cpumask_var_t doms_new[],
>>>>>        ndoms_cur = ndoms_new;
>>>>>          update_sched_domain_debugfs();
>>>>> +    dl_rebuild_rd_accounting();
>>>> Won't dl_rebuild_rd_accounting()'s lockdep_assert_held(&cpuset_mutex)
>>>> barf when called via cpuhp's:
>>>>
>>>> sched_cpu_deactivate()
>>>>
>>>>     cpuset_cpu_inactive()
>>>>
>>>>       partition_sched_domains()
>>>>
>>>>         partition_sched_domains_locked()
>>>>
>>>>           dl_rebuild_rd_accounting()
>>>>
>>>> ?
> Good catch. Guess I didn't notice while testing with LOCKDEP as I was
> never able to hit this call path on my systems.
>
>>> Right. If cpuhp_tasks_frozen is true, partition_sched_domains() will be
>>> called without holding cpuset mutex.
>>>
>>> Well, I think we will need an additional wrapper in cpuset.c that
>>> acquires the cpuset_mutex first before calling partition_sched_domains()
>>> and use the new wrapper in these cases.
>> Actually, partition_sched_domains() is called with the special arguments (1,
>> NULL, NULL) to reset the domain to a single one. So perhaps something like
>> the following will be enough to avoid this problem.
> I think this would work, as we will still rebuild the accounting after
> last CPU comes back from suspend. The thing I am still not sure about is
> what we want to do in case we have DEADLINE tasks around, since with
> this I belive we would be ignoring them and let suspend proceed.

That is the current behavior. You can certainly create a test case to 
trigger such condition and see what to do about it. Alternatively, you 
can document that and come up with a follow-up patch later on.

Cheers,
Longman