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Date: Wed, 12 Jul 2023 18:59:38 +0200
From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: kernel test robot <oliver.sang@intel.com>, oe-lkp@lists.linux.dev,
	lkp@intel.com, linux-kernel@vger.kernel.org, rcu@vger.kernel.org
Subject: Re: [linus:master] [rcu/nocb]  7625926086:
 WARNING:at_kernel/rcu/tree_nocb.h:#rcu_nocb_try_bypass
Message-ID: <ZK7b+vIJpOZhndbm@lothringen>
References: <202307111535.4823c255-oliver.sang@intel.com>
 <ZK1983kQCrN+zCqs@lothringen>
 <842683bc-5859-48be-8ca5-17a1e4bf3f39@paulmck-laptop>
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In-Reply-To: <842683bc-5859-48be-8ca5-17a1e4bf3f39@paulmck-laptop>

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.