linux-kernel - Re: [RFC 00/12] locking: Separate lock tracepoints from lockdep/lock

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Message-ID: <20220209090908.GK23216@worktop.programming.kicks-ass.net>
Date:   Wed, 9 Feb 2022 10:09:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Byungchul Park <byungchul.park@....com>,
        "Paul E. McKenney" <paul.mckenney@...aro.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Radoslaw Burny <rburny@...gle.com>, Tejun Heo <tj@...nel.org>,
        rcu@...r.kernel.org, cgroups@...r.kernel.org,
        linux-btrfs@...r.kernel.org, intel-gfx@...ts.freedesktop.org
Subject: Re: [RFC 00/12] locking: Separate lock tracepoints from
 lockdep/lock_stat (v1)

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.

Yes, the name is convenient, but it's just not worth it IMO. It makes
the whole proposition too much of a trade-off.

Would it not be possible to reconstruct enough useful information from
the lock callsite?