Re: [PATCH v5 08/24] sched: Introduce per memory space current virtual cpu id

[Mathieu Desnoyers]

On 2022-11-09 04:42, Peter Zijlstra wrote:
> On Thu, Nov 03, 2022 at 04:03:43PM -0400, Mathieu Desnoyers wrote:
>
> > +void sched_vcpu_exit_signals(struct task_struct *t)
> > +{
> > +	struct mm_struct *mm = t->mm;
> > +	unsigned long flags;
> > +
> > +	if (!mm)
> > +		return;
> > +	local_irq_save(flags);
> > +	mm_vcpu_put(mm, t->mm_vcpu);
> > +	t->mm_vcpu = -1;
> > +	t->mm_vcpu_active = 0;
> > +	local_irq_restore(flags);
> > +}
> > +
> > +void sched_vcpu_before_execve(struct task_struct *t)
> > +{
> > +	struct mm_struct *mm = t->mm;
> > +	unsigned long flags;
> > +
> > +	if (!mm)
> > +		return;
> > +	local_irq_save(flags);
> > +	mm_vcpu_put(mm, t->mm_vcpu);
> > +	t->mm_vcpu = -1;
> > +	t->mm_vcpu_active = 0;
> > +	local_irq_restore(flags);
> > +}
> > +
> > +void sched_vcpu_after_execve(struct task_struct *t)
> > +{
> > +	struct mm_struct *mm = t->mm;
> > +	unsigned long flags;
> > +
> > +	WARN_ON_ONCE((t->flags & PF_KTHREAD) || !t->mm);
> > +
> > +	local_irq_save(flags);
> > +	t->mm_vcpu = mm_vcpu_get(mm);
> > +	t->mm_vcpu_active = 1;
> > +	local_irq_restore(flags);
> > +	rseq_set_notify_resume(t);
> > +}
>
> > +static inline void mm_vcpu_put(struct mm_struct *mm, int vcpu)
> > +{
> > +	lockdep_assert_irqs_disabled();
> > +	if (vcpu < 0)
> > +		return;
> > +	spin_lock(&mm->vcpu_lock);
> > +	__cpumask_clear_cpu(vcpu, mm_vcpumask(mm));
> > +	spin_unlock(&mm->vcpu_lock);
> > +}
> > +
> > +static inline int mm_vcpu_get(struct mm_struct *mm)
> > +{
> > +	int ret;
> > +
> > +	lockdep_assert_irqs_disabled();
> > +	spin_lock(&mm->vcpu_lock);
> > +	ret = __mm_vcpu_get(mm);
> > +	spin_unlock(&mm->vcpu_lock);
> > +	return ret;
> > +}
>
>
> This:
>
> 	local_irq_disable()
> 	spin_lock()
>
> thing is a PREEMPT_RT anti-pattern.
>
> At the very very least this should then be raw_spin_lock(), not in the
> least because you're calling this from under rq->lock, which itself is a
> raw_spin_lock_t.

Very good point, will fix using raw_spinlock_t.

Thanks,

Mathieu

>

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com