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

[Peter Zijlstra]

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).