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

There is of course an example ioprio which solves the runtime
adjustments via a dedicated system call. But I don't currently feel that
a third one would be a good solution. At least I don't see a case for
being able to decouple the priority of CPU and GPU and computations.

Have I opened a large can of worms? :)

Regards,

Tvrtko

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