linux-kernel - Re: [PATCHv5] sched/deadline: Walk up cpuset hierarchy to decide root domain when hot-unplug

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Message-ID: <d42cd14d-46da-41b4-b580-f112baf0dfdb@redhat.com>
Date: Mon, 10 Nov 2025 17:08:56 -0500
From: Waiman Long <llong@...hat.com>
To: Juri Lelli <juri.lelli@...hat.com>, Pingfan Liu <piliu@...hat.com>
Cc: linux-kernel@...r.kernel.org, Chen Ridong <chenridong@...weicloud.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Pierre Gondois <pierre.gondois@....com>, Ingo Molnar <mingo@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCHv5] sched/deadline: Walk up cpuset hierarchy to decide root
 domain when hot-unplug

On 11/10/25 4:07 PM, Waiman Long wrote:
> On 11/10/25 6:14 AM, Juri Lelli wrote:
>> Hi,
>>
>> Looks like this has two issues.
>>
>> On 10/11/25 09:47, Pingfan Liu wrote:
>>
>> ...
>>
>>> +/*
>>> + * This function always returns a non-empty bitmap in @cpus. This 
>>> is because
>>> + * if a root domain has reserved bandwidth for DL tasks, the DL 
>>> bandwidth
>>> + * check will prevent CPU hotplug from deactivating all CPUs in 
>>> that domain.
>>> + */
>>> +static void dl_get_task_effective_cpus(struct task_struct *p, 
>>> struct cpumask *cpus)
>>> +{
>>> +    const struct cpumask *hk_msk;
>>> +
>>> +    hk_msk = housekeeping_cpumask(HK_TYPE_DOMAIN);
>>> +    if (housekeeping_enabled(HK_TYPE_DOMAIN)) {
>>> +        if (!cpumask_intersects(p->cpus_ptr, hk_msk)) {
>>> +            /*
>>> +             * CPUs isolated by isolcpu="domain" always belong to
>>> +             * def_root_domain.
>>> +             */
>>> +            cpumask_andnot(cpus, cpu_active_mask, hk_msk);
>>> +            return;
>>> +        }
>>> +    }
>>> +
>>> +    /*
>>> +     * If a root domain holds a DL task, it must have active CPUs. So
>>> +     * active CPUs can always be found by walking up the task's cpuset
>>> +     * hierarchy up to the partition root.
>>> +     */
>>> +    cpuset_cpus_allowed(p, cpus);
>> Grabs callbak_lock spin_lock (sleepable on RT) under pi_lock
>> raw_spin_lock.
> I have been thinking about changing callback_lock to a raw_spinlock_t, 
> but need to find a good use case for this change. So it is a solvable 
> problem. 

Actually, we don't need to acquire the callback_lock if cpuset_mutex is 
held. So another possibility is to create a cpuset_cpus_allowed() 
variant that doesn't acquire the callback_mutex but assert that 
cpuset_mutex is held.

Cheers,
Longman