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

[Tvrtko Ursulin]

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