On Tue, Apr 30, 2019 at 12:03:18PM +0200, Peter Zijlstra wrote:
> On Sat, Apr 27, 2019 at 11:02:46AM -0700, Paul E. McKenney wrote:
>
> > This actually passes rcutorture. But, as Andrea noted, not klitmus.
> > After some investigation, it turned out that klitmus was creating kthreads
> > with PF_NO_SETAFFINITY, hence the failures. But that prompted me to
> > put checks into my code: After all, rcutorture can be fooled.
> >
> > void synchronize_rcu(void)
> > {
> > int cpu;
> >
> > for_each_online_cpu(cpu) {
> > sched_setaffinity(current->pid, cpumask_of(cpu));
> > WARN_ON_ONCE(raw_smp_processor_id() != cpu);
> > }
> > }
> >
> > This triggers fairly quickly, usually in less than a minute of rcutorture
> > testing.
> >
> > And further investigation shows that sched_setaffinity()
> > always returned 0.
>
> > Is this expected behavior? Is there some configuration or setup that I
> > might be missing?
>
> ISTR there is hotplug involved in RCU torture? In that case, it can be
> sched_setaffinity() succeeds to place us on a CPU, which CPU hotplug
> then takes away. So when we run the WARN thingy, we'll be running on a
> different CPU than expected.
There can be CPU hotplug involved in rcutorture, but it was disabled
during this run.
> If OTOH, your loop is written like (as it really should be):
>
> void synchronize_rcu(void)
> {
> int cpu;
>
> cpus_read_lock();
> for_each_online_cpu(cpu) {
> sched_setaffinity(current->pid, cpumask_of(cpu));
> WARN_ON_ONCE(raw_smp_processor_id() != cpu);
> }
> cpus_read_unlock();
> }
>
> Then I'm not entirely sure how we can return 0 and not run on the
> expected CPU. If we look at __set_cpus_allowed_ptr(), the only paths out
> to 0 are:
>
> - if the mask didn't change
> - if we already run inside the new mask
> - if we migrated ourself with the stop-task
> - if we're not in fact running
>
> That last case should never trigger in your circumstances, since @p ==
> current and current is obviously running. But for completeness, the
> wakeup of @p would do the task placement in that case.
Are there some diagnostics I could add that would help track this down,
be it my bug or yours?
Thanx, Paul
