Re: [PATCH] rcu: update: make RCU_EXPEDITE_BOOT default

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	Julia Cartwright <julia@ni.com>,
	Luiz Capitulino <lcapitulino@redhat.com>,
	linux-rt-users@vger.kernel.org,
	Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rcu: update: make RCU_EXPEDITE_BOOT default
Date: Thu, 3 Nov 2016 17:33:27 +0100	[thread overview]
Message-ID: <20161103163326.jkjbncoz7a5oriy5@linutronix.de> (raw)
In-Reply-To: <20161103162228.GG3716@linux.vnet.ibm.com>

On 2016-11-03 09:22:28 [-0700], Paul E. McKenney wrote:
> On Wed, Nov 02, 2016 at 05:30:02PM +0100, Sebastian Andrzej Siewior wrote:
> > RCU_EXPEDITE_BOOT should speed up the boot process by enforcing
> > synchronize_rcu_expedited() instead of synchronize_rcu() during the boot
> > process. There should be no reason why one does not want this and there
> > is no need worry about real time latency at this point.
> > Therefore make it default.
> > 
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> 
> Well, it has been awhile since I removed a Kconfig parameter.
> 
> So why could this be a bad thing?
> 
> 1.	Very large systems might see scalability issues with unconditional
> 	expediting at boot.  But if we don't try it, we won't know.

You mean we would make the boot process slower for them instead of
faster?

> 2.	People bringing up new hardware might not want quite so many
> 	IPIs.  But they can just set rcu_normal to prevent that.

I wanted to make things simple and not complicated…

> I am therefore queuing it for testiong and review.  ;-)

Okay thanks.

> 							Thanx, Paul

Sebastian