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

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Message-ID: <CAADnVQKGm-t2SdN_vFVMn0tNiQ5Fs6FutD2Au-jO69aGdhKS7Q@mail.gmail.com>
Date: Fri, 9 Jan 2026 14:18:36 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 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 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.