Re: [PATCH 4/4] rcu/nocb: Make shrinker to iterate only NOCB CPUs

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Mar 29, 2023 at 01:58:06PM -0700, Paul E. McKenney wrote:
> On Wed, Mar 29, 2023 at 06:02:03PM +0200, Frederic Weisbecker wrote:
> > Callbacks can only be queued as lazy on NOCB CPUs, therefore iterating
> > over the NOCB mask is enough for both counting and scanning. Just lock
> > the mostly uncontended barrier mutex on counting as well in order to
> > keep rcu_nocb_mask stable.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> 
> Looks plausible.  ;-)
> 
> What are you doing to test this?  For that matter, what should rcutorture
> be doing to test this?  My guess is that the current callback flooding in
> rcu_torture_fwd_prog_cr() should do the trick, but figured I should ask.

All I did was to trigger these shrinker callbacks through debugfs
(https://docs.kernel.org/admin-guide/mm/shrinker_debugfs.html)

But rcutorture isn't testing it because:

- No torture config has CONFIG_RCU_LAZY
- rcutorture doesn't do any lazy call_rcu() (always calls hurry for the
  main RCU flavour).

And I suspect rcutorture isn't ready for accepting the lazy delay, that would
require some special treatment.

Thanks.