linux-kernel - Re: [PATCH RFC ftrace] Chose RCU Tasks based on TASKS

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Date: Fri, 1 Mar 2024 15:30:01 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Ankur Arora
 <ankur.a.arora@...cle.com>, Thomas Gleixner <tglx@...utronix.de>,
 kernel-team@...a.com
Subject: Re: [PATCH RFC ftrace] Chose RCU Tasks based on TASKS_RCU rather
 than PREEMPTION

On Fri, 1 Mar 2024 12:25:10 -0800
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> > That would work for me.  If there are no objections, I will make this
> > change.  
> 
> But I did check the latency of synchronize_rcu_tasks_rude() (about 100ms)
> and synchronize_rcu() (about 20ms).  This is on a 80-hardware-thread
> x86 system that is being flooded with calls to one or the other of
> these two functions, but is otherwise idle.  So adding that unnecessary
> synchronize_rcu() adds about 20% to that synchronization delay.
> 
> Which might still be OK, but...  In the immortal words of MS-DOS,
> "Are you sure?".  ;-)

It's just safe to keep it. It's definitely not a fast path.

-- Steve