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Date:   Thu, 10 Feb 2022 12:10:58 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Byungchul Park <byungchul.park@lge.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Radoslaw Burny <rburny@google.com>, Tejun Heo <tj@kernel.org>,
        rcu <rcu@vger.kernel.org>, cgroups <cgroups@vger.kernel.org>,
        linux-btrfs <linux-btrfs@vger.kernel.org>,
        intel-gfx <intel-gfx@lists.freedesktop.org>
Subject: Re: [RFC 00/12] locking: Separate lock tracepoints from
 lockdep/lock_stat (v1)
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On Thu, Feb 10, 2022 at 02:27:11PM -0500, Waiman Long wrote:
> On 2/10/22 14:14, Paul E. McKenney wrote:
> > On Thu, Feb 10, 2022 at 10:13:53AM +0100, Peter Zijlstra wrote:
> > > On Wed, Feb 09, 2022 at 04:32:58PM -0800, Namhyung Kim wrote:
> > > > On Wed, Feb 9, 2022 at 1:09 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > > > On Tue, Feb 08, 2022 at 10:41:56AM -0800, Namhyung Kim wrote:
> > > > > 
> > > > > > Eventually I'm mostly interested in the contended locks only and I
> > > > > > want to reduce the overhead in the fast path.  By moving that, it'd be
> > > > > > easy to track contended locks with timing by using two tracepoints.
> > > > > So why not put in two new tracepoints and call it a day?
> > > > > 
> > > > > Why muck about with all that lockdep stuff just to preserve the name
> > > > > (and in the process continue to blow up data structures etc..). This
> > > > > leaves distros in a bind, will they enable this config and provide
> > > > > tracepoints while bloating the data structures and destroying things
> > > > > like lockref (which relies on sizeof(spinlock_t)), or not provide this
> > > > > at all.
> > > > If it's only lockref, is it possible to change it to use arch_spinlock_t
> > > > so that it can remain in 4 bytes?  It'd be really nice if we can keep
> > > > spin lock size, but it'd be easier to carry the name with it for
> > > > analysis IMHO.
> > > It's just vile and disgusting to blow up the lock size for convenience
> > > like this.
> > > 
> > > And no, there's more of that around. A lot of effort has been spend to
> > > make sure spinlocks are 32bit and we're not going to give that up for
> > > something as daft as this.
> > > 
> > > Just think harder on the analysis side. Like said; I'm thinking the
> > > caller IP should be good enough most of the time.
> > 
> > Another option is to keep any additional storage in a separate data
> > structure keyed off of lock address, lockdep class, or whatever.
> > 
> > Whether or not this is a -good- option, well, who knows?  ;-)
> 
> I have suggested that too. Unfortunately, I was replying to an email with
> your wrong email address. So you might not have received it.

Plus I was too lazy to go look at lore.  ;-)

For whatever it is worth, we did something similar in DYNIX/ptx, whose
spinlocks were limited to a single byte.  But it does have its drawbacks.

							Thanx, Paul