linux-kernel - Re: [PATCH 2/2] locking: Apply contention tracepoints in the slow path

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Date:   Thu, 17 Mar 2022 12:10:55 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Byungchul Park <byungchul.park@....com>,
        paulmck <paulmck@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Radoslaw Burny <rburny@...gle.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH 2/2] locking: Apply contention tracepoints in the slow
 path

On Thu, 17 Mar 2022 09:45:28 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> > *sem, bool reader)
> > 		schedule();
> > 	}
> > 	__set_current_state(TASK_RUNNING);
> > +	trace_contention_end(sem, 0);  
> 
> So for the reader-write locks, and percpu rwlocks, the "trace contention end" will always
> have ret=0. Likewise for qspinlock, qrwlock, and rtlock. It seems to be a waste of trace
> buffer space to always have space for a return value that is always 0.
> 
> Sorry if I missed prior dicussions of that topic, but why introduce this single
> "trace contention begin/end" muxer tracepoint with flags rather than per-locking-type
> tracepoint ? The per-locking-type tracepoint could be tuned to only have the fields
> that are needed for each locking type.

per-locking-type tracepoint will also add a bigger footprint.

One extra byte is not an issue. This is just the tracepoints. You can still
attach your own specific LTTng trace events that ignores the zero
parameter, and can multiplex into specific types of trace events on your
end.

I prefer the current approach as it keeps the tracing footprint down.

-- Steve