[PATCH linux-next][RFC]torture: avoid offline tick_do_timer_cpu

[Paul E. McKenney]

On Sun, Nov 27, 2022 at 01:40:28PM +0100, Thomas Gleixner wrote:

[ . . . ]

> >> No. We are not exporting this just to make a bogus test case happy.
> >>
> >> Fix the torture code to handle -EBUSY correctly.
> > I am going to do a study on this, for now, I do a grep in the kernel tree:
> > find . -name "*.c"|xargs grep cpuhp_setup_state|wc -l
> > The result of the grep command shows that there are 268
> > cpuhp_setup_state* cases.
> > which may make our task more complicated.
> 
> Why? The whole point of this torture thing is to stress the
> infrastructure.

Indeed.

> There are quite some reasons why a CPU-hotplug or a hot-unplug operation
> can fail, which is not a fatal problem, really.
> 
> So if a CPU hotplug operation fails, then why can't the torture test
> just move on and validate that the system still behaves correctly?
> 
> That gives us more coverage than just testing the good case and giving
> up when something unexpected happens.

Agreed, with access to a function like the tick_nohz_full_timekeeper()
suggested earlier in this email thread, then yes, it would make sense to
try to offline the CPU anyway, then forgive the failure in cases where
the CPU matches that indicated by tick_nohz_full_timekeeper().

> I even argue that the torture test should inject random failures into
> the hotplug state machine to achieve extended code coverage.

I could imagine torture_onoff() telling various CPU-hotplug notifiers
to refuse the transition using some TBD interface.  That would better
test the CPU-hotplug common code's ability to deal with failures.

Or did you have something else/additional in mind?

							Thanx, Paul