linux-kernel - Re: [PATCH v5] tracing: Guard __DECLARE_TRACE() use of __DO_TRACE

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Message-ID: <86c106f9-f968-4423-a81e-7377989321c5@efficios.com>
Date: Fri, 9 Jan 2026 17:31:39 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 Linux trace kernel <linux-trace-kernel@...r.kernel.org>,
 bpf <bpf@...r.kernel.org>, Masami Hiramatsu <mhiramat@...nel.org>,
 "Paul E. McKenney" <paulmck@...nel.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v5] tracing: Guard __DECLARE_TRACE() use of
 __DO_TRACE_CALL() with SRCU-fast

On 2026-01-09 17:18, Alexei Starovoitov wrote:
> On Fri, Jan 9, 2026 at 2:00 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>>
>> On Fri, 9 Jan 2026 13:54:34 -0800
>> Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
>>
>>> On Fri, Jan 9, 2026 at 12:21 PM Mathieu Desnoyers
>>> <mathieu.desnoyers@...icios.com> wrote:
>>>>
>>>>
>>>> * preempt disable/enable pair:                                     1.1 ns
>>>> * srcu-fast lock/unlock:                                           1.5 ns
>>>>
>>>> CONFIG_RCU_REF_SCALE_TEST=y
>>>> * migrate disable/enable pair:                                     3.0 ns
>>>
>>> .. and you're arguing that 3ns vs 1ns difference is so important
>>> for your out-of-tree tracer that in-tree tracers need to do
>>> some workarounds?! wtf
>>
>> This has nothing to do with out of tree tracers. The overhead of the
>> 22ns is for any tracepoint in an in-tree module. That's because the
>> rq->nr_pinned isn't exported for modules to use.
> 
> None of the driver's tracepoints are in the critical path.
> You perfectly know that Mathieu argued about not slowing down lttng.

My argument is about not slowing down high-throughput tracers
on preempt-rt kernels. This affects ftrace as well as lttng,
so both in-tree and OOT tracers just alike.

Are you so sure that no tracepoint instrumentation whatsoever can
be compiled into a module which is a critical path for some workload ?
That's a bold statement.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com