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

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Message-ID: <84fd8b61-b6db-4b8e-ac02-89e00b267e77@paulmck-laptop>
Date:   Wed, 29 Mar 2023 16:12:30 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, rcu <rcu@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 4/4] rcu/nocb: Make shrinker to iterate only NOCB CPUs

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@...nel.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