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

Paul E. McKenney
2023-03-29T23:13:12Z

On Wed, Mar 29, 2023 at 11:35:36PM +0200, Frederic Weisbecker wrote:
> 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.

All fair points!

And yes, any non-lazy callback would delazify everything, so as it
is currently constituted, it would not be testing very much of the
lazy-callback state space.

Thanx, Paul