linux-kernel - Re: [linus:master] [rcu/nocb] 7625926086: WARNING:at_kernel/rcu/tree_nocb.h:#rcu_nocb_try

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Message-ID: <ZK7b+vIJpOZhndbm@lothringen>
Date:   Wed, 12 Jul 2023 18:59:38 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
        lkp@...el.com, linux-kernel@...r.kernel.org, rcu@...r.kernel.org
Subject: Re: [linus:master] [rcu/nocb]  7625926086:
 WARNING:at_kernel/rcu/tree_nocb.h:#rcu_nocb_try_bypass

On Tue, Jul 11, 2023 at 09:41:58AM -0700, Paul E. McKenney wrote:
> On Tue, Jul 11, 2023 at 06:06:11PM +0200, Frederic Weisbecker wrote:
> Heh!
> 
> The purpose was to see if this lock was ever contended.  I guess we now
> have an answer, which is "Yes, but rarely."
> 
> It looks like the victim commit increased the size of the ->nocb_lock
> critical section, just enough to make this happen sometimes.
> 
> Removing the WARN_ON_ONCE() seems appropriate, especially given that
> this only happens when shrinking.

Ok, I'll check that.

> Should we add something that monitors that lock's contention?  It is
> often the case that lock contention rises over time as new features and
> optimizations are added.

I'm not sure. Should we keep the current ->nocb_lock_contended based engine
to report contention somehow somewhere? Also does it behave better than our
current spinlock slow path implementations?

Thanks.