linux-kernel - Re: linux-next: manual merge of the rcu tree with the ftrace tree

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Message-ID: <20251114104633.0721bdbe@gandalf.local.home>
Date: Fri, 14 Nov 2025 10:46:33 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Stephen Rothwell <sfr@...b.auug.org.au>, "Paul E. McKenney"
 <paulmck@...nel.org>, Frederic Weisbecker <frederic@...nel.org>, Neeraj
 Upadhyay <neeraj.upadhyay@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 Uladzislau Rezki <urezki@...il.com>, Masami Hiramatsu
 <mhiramat@...nel.org>, Linux Kernel Mailing List
 <linux-kernel@...r.kernel.org>, Linux Next Mailing List
 <linux-next@...r.kernel.org>
Subject: Re: linux-next: manual merge of the rcu tree with the ftrace tree

On Fri, 14 Nov 2025 14:35:32 +0100
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> On 2025-11-14 07:42:55 [-0500], Steven Rostedt wrote:
> > > diff --cc kernel/trace/trace_syscalls.c
> > > index e96d0063cbcf,3f699b198c56..000000000000
> > > --- a/kernel/trace/trace_syscalls.c
> > > +++ b/kernel/trace/trace_syscalls.c
> > > @@@ -878,6 -322,8 +890,7 @@@ static void ftrace_syscall_enter(void *
> > >   	 * buffer and per-cpu data require preemption to be disabled.
> > >   	 */
> > >   	might_fault();
> > > + 	preempt_rt_guard();
> > >  -	guard(preempt_notrace)();  
> > 
> > My code made it so that preemption is not needed here but is moved later
> > down for the logic that does the reading of user space data.
> > 
> > Note, it must have preemption disabled for all configs (including RT).
> > Otherwise, the data it has can get corrupted.
> > 
> > Paul, can you change it so that you *do not* touch this file?  
> 
> Where is preempt_rt_guard() from?

Ah, it's from the patch I submitted that has this:

+/*
+ * When PREEMPT_RT is enabled, it disables migration instead
+ * of preemption. The pseudo syscall trace events need to match
+ * so that the counter logic recorded into he ring buffer by
+ * trace_event_buffer_reserve() still matches what it expects.
+ */
+#ifdef CONFIG_PREEMPT_RT
+# define preempt_rt_guard()  guard(migrate)()
+#else
+# define preempt_rt_guard()
+#endif
+

I must be getting old, as I forgot I wrote this :-p

I only saw the update from Stephen and thought it was disabling preemption.

It doesn't disable preemption, but is here to keep the latency
preempt_count counting the same in both PREEMPT_RT and non PREEMPT_RT. You
know, the stuff that shows up in the trace:

  "d..4."

Paul, never mind, this code will not affect the code I added.

-- Steve