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

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Message-ID: <52de2e14-33d9-bdda-4b37-3e72ae9954c7@redhat.com>
Date:   Thu, 10 Feb 2022 14:27:11 -0500
From:   Waiman Long <longman@...hat.com>
To:     paulmck@...nel.org, Peter Zijlstra <peterz@...radead.org>
Cc:     Namhyung Kim <namhyung@...nel.org>, 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>,
        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>
Subject: Re: [RFC 00/12] locking: Separate lock tracepoints from
 lockdep/lock_stat (v1)

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@...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.
>> 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.

Cheers,
Longman