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

[Waiman Long]

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