Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

[Oleg Nesterov]

On 10/11, Peter Zijlstra wrote:
>
> As a penance I'll start by removing all get_online_cpus() usage from the
> scheduler.

I only looked at the change in setaffinity,

> @@ -3706,7 +3707,6 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
>  	struct task_struct *p;
>  	int retval;
>
> -	get_online_cpus();
>  	rcu_read_lock();

Hmm. In theory task_rq_lock() doesn't imply rcu-lock, so
set_cpus_allowed_ptr() can miss the change in cpu_active_mask. But this
is probably fine, CPU_DYING does __migrate_task().

However. This means that sched_setaffinity() can fail if it races with
the failing cpu_down() (say, __cpu_notify(CPU_DOWN_PREPARE) fails).
Probably we do not really care, just this looks a bit confusing.

> @@ -3814,7 +3813,6 @@ long sched_getaffinity(pid_t pid, struct cpumask *mask)
>  	unsigned long flags;
>  	int retval;
>
> -	get_online_cpus();

This change is probably fine in any case?

>  	rcu_read_lock();
>
>  	retval = -ESRCH;
> @@ -3827,12 +3825,11 @@ long sched_getaffinity(pid_t pid, struct cpumask *mask)
>  		goto out_unlock;
>
>  	raw_spin_lock_irqsave(&p->pi_lock, flags);
> -	cpumask_and(mask, &p->cpus_allowed, cpu_online_mask);
> +	cpumask_and(mask, &p->cpus_allowed, cpu_active_mask);

But I am just curious, is this change is strictly needed?

Afaics we do not care if we race with set_cpu_online(true/false).

Oleg.

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