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

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On 01/10/2021 16:48, Peter Zijlstra wrote:
On Fri, Oct 01, 2021 at 11:32:16AM +0100, Tvrtko Ursulin wrote:

On 01/10/2021 10:04, Tvrtko Ursulin wrote:

Hi Peter,

On 30/09/2021 19:33, Peter Zijlstra wrote:
On Thu, Sep 30, 2021 at 06:15:47PM +0100, Tvrtko Ursulin wrote:
   void set_user_nice(struct task_struct *p, long nice)
   {
       bool queued, running;
-    int old_prio;
+    int old_prio, ret;
       struct rq_flags rf;
       struct rq *rq;
@@ -6913,6 +6945,9 @@ void set_user_nice(struct task_struct *p,
long nice)
        */
       p->sched_class->prio_changed(rq, p, old_prio);
+    ret = atomic_notifier_call_chain(&user_nice_notifier_list,
nice, p);
+    WARN_ON_ONCE(ret != NOTIFY_DONE);
+
   out_unlock:
       task_rq_unlock(rq, p, &rf);
   }

No, we're not going to call out to exported, and potentially unbounded,
functions under scheduler locks.

Agreed, that's another good point why it is even more hairy, as I have
generally alluded in the cover letter.

Do you have any immediate thoughts on possible alternatives?

Like for instance if I did a queue_work from set_user_nice and then ran
a notifier chain async from a worker? I haven't looked at yet what
repercussion would that have in terms of having to cancel the pending
workers when tasks exit. I can try and prototype that and see how it
would look.

Hm or I simply move calling the notifier chain to after task_rq_unlock? That
would leave it run under the tasklist lock so probably still quite bad.

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:

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.

Regards,

Tvrtko



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