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.
