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

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Message-ID: <24fe6a08-5931-8e8d-8d77-459388c4654e@redhat.com>
Date:   Wed, 9 Feb 2022 13:19:07 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Namhyung Kim <namhyung@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        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 2/9/22 04:09, Peter Zijlstra 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.
>
> 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?
>
I second that as I don't want to see the size of a spinlock exceeds 4 
bytes in a production system.

Instead of storing additional information (e.g. lock name) directly into 
the lock itself. Maybe we can store it elsewhere and use the lock 
address as the key to locate it in a hash table. We can certainly extend 
the various lock init functions to do that. It will be trickier for 
statically initialized locks, but we can probably find a way to do that too.

Cheers,
Longman