Re: [RFC 00/12] locking: Separate lock tracepoints from lockdep/lock_stat (v1)

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Namhyung Kim <namhyung@kernel.org>,
	Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
	Waiman Long <longman@redhat.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Byungchul Park <byungchul.park@lge.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Radoslaw Burny <rburny@google.com>, Tejun Heo <tj@kernel.org>,
	rcu <rcu@vger.kernel.org>, cgroups <cgroups@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	intel-gfx <intel-gfx@lists.freedesktop.org>
Subject: Re: [RFC 00/12] locking: Separate lock tracepoints from lockdep/lock_stat (v1)
Date: Thu, 10 Feb 2022 11:14:04 -0800	[thread overview]
Message-ID: <20220210191404.GM4285@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <YgTXUQ9CBoo3+A+c@hirez.programming.kicks-ass.net>

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@infradead.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?  ;-)

						Thanx, Paul