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

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Message-ID: <fc59eb57-774a-417d-a818-afb5431b8e56@paulmck-laptop>
Date: Fri, 1 Mar 2024 17:54:26 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Steven Rostedt <rostedt@...dmis.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, Mar 01, 2024 at 03:30:01PM -0500, Steven Rostedt wrote:
> 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.

OK, you got it!  ;-)

							Thanx, Paul