Re: Question about a8ea6fc9b089 ("sched: Stop PF_NO_SETAFFINITY from being inherited by various init system threads")

Paul E. McKenney
2021-06-11T17:10:11Z

On Fri, Jun 11, 2021 at 03:19:29PM +0100, Valentin Schneider wrote:
> On 11/06/21 06:52, Paul E. McKenney wrote:
> > On Fri, Jun 11, 2021 at 11:12:29AM +0100, Valentin Schneider wrote:
> >>
> >> The way I see 570a752b7a9b is that, if a task is pinned to a single CPU but
> >> doesn't have PF_NO_SETAFFINITY, then userspace can unpin it. This means it
> >> ought to have entered check_preemption_disabled() with preemption disabled
> >> - right now it may be pinned, but that can change at any minute, and
> >> whatever code it is running needs to cope with that.
> >
> > Thank you for catching me up on this topic!
> >
> >> Could you share some details on which tasks you are hitting this with?
> >
> > Let's start with ref_scale_reader() in kernel/rcu/refscale.c. This
> > is for fine-grained in-kernel benchmarking, so it really wants kthreads
> > running this function to be pinned.
> >
> > I took a look at kthread_bind(), but it is not intended to be called by
> > the kthread itself. Looking elsewhere in the kernel, it looks like I
> > just do this right after invoking set_cpus_allowed_ptr():
> >
> > current->flags != PF_NO_SETAFFINITY;
> >
> > Or am I missing a better way to handle this?
>
> Looking at ref_scale_reader(), ISTM the initial configuration (affinity,
> niceness) should be done by its parent thread, not by itself. i.e.:
>
> p = kthread_create(ref_scale_reader);
> kthread_bind(p, cpu); // Does the pinning + sets PF_NO_SETAFFINITY
> set_user_nice(p, MAX_NICE);
> wake_up_process(p);
>
> (kthread_create_on_cpu() is also an option)
>
> >From what I can see, torture_create_kthread() immediately wakes the
> newly-created kthread, we'd need a version that calls kthread_create()
> instead of kthread_run() for the above. Would that be an issue?

It sounds much simpler for me to just continue using raw_smp_processor_id()
instead of trying to switch back to smp_processor_id(). ;-)

Thanx, Paul