Re: [RFC 1/6] sched: Add nice value change notifier

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 04, 2021 at 09:12:37AM +0100, Tvrtko Ursulin wrote:
> On 01/10/2021 16:48, Peter Zijlstra wrote:

> > Hmm? That's for normalize_rt_tasks() only, right? Just don't have it
> > call the notifier in that special case (that's a magic sysrq thing
> > anyway).
> 
> You mean my talk about tasklist_lock? No, it is also on the syscall part I
> am interested in as well. Call chain looks like this:

Urgh, I alwys miss that because it lives outside of sched.. :/

> sys_setpriority()
> {
>   ...
>   rcu_read_lock();
>   read_lock(&tasklist_lock);
>   ...
>   set_one_prio()
>     set_user_nice()
>     {
>       ...
>       task_rq_lock();
>         -> my notifier from this RFC [1]
>       task_rq_unlock();
>         -> I can move the notifier here for _some_ improvement [2]
>     }
>   ...
>   read_unlock(&tasklist_lock);
>   rcu_read_unlock();
> }
> 
> So this RFC had the notifier call chain at [1], which I understood was the
> thing you initially pointed was horrible, being under a scheduler lock.
> 
> I can trivially move it to [2] but that still leaves it under the tasklist
> lock. I don't have a good feel how much better that would be. If not good
> enough then I will look for a smarter solution with less opportunity for
> global impact.

So task_list lock is pretty terrible and effectively unbound already
(just create more tasks etc..) so adding a notifier call there shouldn't
really make it much worse.