Date:   Wed, 9 Feb 2022 14:02:34 -0500
From:   Waiman Long <longman@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        rostedt <rostedt@...dmis.org>,
        Byungchul Park <byungchul.park@....com>,
        "Paul E. McKenney" <paul.mckenney@...aro.org>,
        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@...ts.freedesktop.org
Subject: Re: [RFC 00/12] locking: Separate lock tracepoints from
 lockdep/lock_stat (v1)

On 2/9/22 13:29, Mathieu Desnoyers wrote:
> ----- On Feb 9, 2022, at 1:19 PM, Waiman Long longman@...hat.com wrote:
>
>> 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.
> If we go down that route, it would be nice if we can support a few different
> use-cases for various tracers out there.
>
> One use-case (a) requires the ability to query the lock name based on its address as key.
> For this a hash table is a good fit. This would allow tracers like ftrace to
> output lock names in its human-readable output which is formatted within the kernel.
>
> Another use-case (b) is to be able to "dump" the lock { name, address } tuples
> into the trace stream (we call this statedump events in lttng), and do the
> translation from address to name at post-processing. This simply requires
> that this information is available for iteration for both the core kernel
> and module locks, so the tracer can dump this information on trace start
> and module load.
>
> Use-case (b) is very similar to what is done for the kernel tracepoints. Based
> on this, implementing the init code that iterates on those sections and populates
> a hash table for use-case (a) should be easy enough.

Yes, that are good use cases for this type of functionality. I do need 
to think about how to do it for statically initialized lock first.

Thanks,
Longman

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