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Date: Thu, 10 Feb 2022 12:10:58 -0800 From: "Paul E. McKenney" <paulmck@...nel.org> To: Waiman Long <longman@...hat.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>, 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 Thu, Feb 10, 2022 at 02:27:11PM -0500, Waiman Long wrote: > 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. Plus I was too lazy to go look at lore. ;-) For whatever it is worth, we did something similar in DYNIX/ptx, whose spinlocks were limited to a single byte. But it does have its drawbacks. Thanx, Paul
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