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

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Message-ID: <20260109173915.1e8a784e@fedora>
Date: Fri, 9 Jan 2026 17:39:15 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
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, 9 Jan 2026 17:33:26 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> How is this about lttng? Sure he cares about that, but even tracepoints
> that lttng uses doesn't get affected any more than ftrace or bpf.
> Because lttng is one of the callbacks. The migrate disable happens in
> the in-tree portion of the code.
> 
> So you are saying that all the tracepoints for xfs are not in a fastpath?

Regardless of tracing. I now have my RT hat on. The spin_locks that are
converted to mutex use migrate disable. The fact that migrate_disable
in modules are close to 10x slower than the same code in-kernel is
troubling to say the least. It means that modules in RT take a hit
every time they take a spin_lock().

The migrate disable being slow for modules is no longer just a tracing
issue. It's a PREEMPT_RT issue.

-- Steve