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

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Message-ID: <20260112153128.GO830755@noisy.programming.kicks-ass.net>
Date: Mon, 12 Jan 2026 16:31:28 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.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>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5] tracing: Guard __DECLARE_TRACE() use of
 __DO_TRACE_CALL() with SRCU-fast

On Fri, Jan 09, 2026 at 04:02:02PM -0500, Steven Rostedt wrote:
> On Fri, 9 Jan 2026 15:21:19 -0500
> 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
> > * calls to migrate disable/enable pair within noinline functions: 17.0 ns
> > 
> > CONFIG_RCU_REF_SCALE_TEST=m
> > * migrate disable/enable pair:                                    22.0 ns
> 
> OUCH! So migrate disable/enable has a much larger overhead when executed in
> a module than in the kernel? This means all spin_locks() in modules
> converted to mutexes in PREEMPT_RT are taking this hit!

Not so, the migrate_disable() for PREEMPT_RT is still in core code --
kernel/locking/spinlock_rt.c is very much not build as a module.