Re: [PATCH] tasks: Extract rcu_users out of union

[David Vernet <void@manifault.com>]

On Thu, Feb 16, 2023 at 09:04:59AM +0100, Oleg Nesterov wrote:
> I won't argue with this patch, but I can't understand the changelog...
> 
> On 02/15, David Vernet wrote:
> >
> > Similarly, in sched_ext, schedulers are using integer pids to remember
> > tasks, and then looking them up with find_task_by_pid_ns(). This is
> > slow, error prone, and adds complexity. It would be more convenient and
> > performant if BPF schedulers could instead store tasks directly in maps,
> > and then leverage RCU to ensure they can be safely accessed with low
> > overhead.
> 
> To simplify, suppose we have
> 
> 	int global_pid;
> 
> 	void func(void)
> 	{
> 		rcu_read_lock();
> 		task = find_task_by_pid(global_pid);
> 		do_something(task);
> 		rcu_read_unlock();
> 	}
> 
> Could you explain how exactly can this patch help to turn global_pid into
> "task_struct *" ? Why do you need to increment task->rcu_users ?

If you're not persisting the task in a map / data structure, then I
agree that find_task_by_pid_ns() is likely sufficient. What we want to
be able to do is something like this:

void func(void)
{
	rcu_read_lock();
	task = peek_next_task();
	if (task)
		do_something(task);
	rcu_read_unlock();
}

In such an example, we could be peeking into a statically allocated
circular queue, and want to be able to ensure that a task we look at
from the top is protected with rcu. The general mechanics would be that
a task is inserted with a refcount_inc_not_zero(), and when it's
removed, we do a put_task_struct_rcu_user().

Does that make sense?

> 
> >    a task that's successfully looked
> >    up in e.g. the pid_list with find_task_by_pid_ns(), can always have a
> >    'usage' reference acquired on them, as it's guaranteed to be >
> >    0 until after the next gp.
> 
> Yes. So it seems you need another key-to-task_struct map with rcu-safe
> lookup/get and thus the add() method needs inc_not_zero(task->rcu_users) ?

Yes, exactly.

Thanks for taking a look at the patch.

- David