Date:   Thu, 2 Feb 2023 20:29:04 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     John Stultz <jstultz@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Josh Triplett <josh@joshtriplett.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>
Subject: Re: [RFC][PATCH 1/4] locktorture: Add nested_[un]lock() hooks and
 nlocks parameter
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On Thu, Feb 02, 2023 at 07:56:05PM -0800, John Stultz wrote:
> On Thu, Feb 2, 2023 at 6:05 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Thu, Feb 02, 2023 at 10:34:06PM +0000, John Stultz wrote:
> > > In order ot extend locktorture to support lock nesting, add
> > > nested_lock() and nested_unlock() hooks to the torture ops.
> > >
> > > These take a 32bit lockset mask which is generated at random,
> > > so some number of locks will be taken before the main lock is
> > > taken and released afterwards.
> > >
> > > Additionally, add nlocks module parameter to allow specifying
> > > the number of nested locks to be used.
> > >
> > > This has been helpful to uncover issues in the proxy-exec
> > > series development.
> > >
> > > This was inspired by locktorture extensions originally implemented
> > > by Connor O'Brien, for stress testing the proxy-execution series:
> > >   https://lore.kernel.org/lkml/20221003214501.2050087-12-connoro@google.com/
> > >
> > > Comments or feedback would be greatly appreciated!
> >
> > I have pulled this in for testing and further review, thank you!
> >
> > Should either of these files have a locktorture.nlocks=<whatever>
> > added?
> >
> > tools/testing/selftests/rcutorture/configs/lock/LOCK02.boot
> > tools/testing/selftests/rcutorture/configs/lock/LOCK05.boot
> >
> > The first is for mutex_lock and the second for rtmutex_lock.
> 
> Potentially? I wasn't aware of these files. Is there some
> documentation on the intent behind them?

There is a LOCK02 file that contains Kconfig options and the LOCK02.boot
file contains kernel boot parameters.  There is a CFLIST file that
contains the list of such files that the command below will test by
default.

The best documentation is probably here:

https://paulmck.livejournal.com/57769.html
https://paulmck.livejournal.com/58077.html

> While the nested locking is useful to cause different lock chains to
> test boosting or proxy-execution, I worry they may cause extra noise
> that could distract from just thrashing the *mutex lock primitive if
> that's the existing intent.

The intent is to find bugs by whatever means necessary, within reason.

> So we might want additional config files for the nested case.

That would work.

> > This did pass a quick "torture.sh --do-none --do-locktorture", which is
> > good, but this uses the existing .boot files.
> 
> Yeah, probably no effective change in that case.

At least nothing else got broken.  ;-)

							Thanx, Paul