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

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Message-ID: <CAM9d7cgq+jxu6FJuKhZkprn7dO4DiG5pDjmYZzneQYTfKOM85g@mail.gmail.com>
Date:   Wed, 9 Feb 2022 16:32:58 -0800
From:   Namhyung Kim <namhyung@...nel.org>
To:     Peter Zijlstra <peterz@...radead.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>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Radoslaw Burny <rburny@...gle.com>, Tejun Heo <tj@...nel.org>,
        rcu <rcu@...r.kernel.org>, cgroups <cgroups@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        intel-gfx <intel-gfx@...ts.freedesktop.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [RFC 00/12] locking: Separate lock tracepoints from
 lockdep/lock_stat (v1)

On Wed, Feb 9, 2022 at 1:09 AM Peter Zijlstra <peterz@...radead.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.

Thanks,
Namhyung