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

Oleg Nesterov
2023-02-16T08:06:01Z

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 ?

> 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) ?

I am just curious,

Oleg.