linux-kernel - Re: [PATCH] rcu: update: make RCU_EXPEDITE

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Message-ID: <20161103163326.jkjbncoz7a5oriy5@linutronix.de>
Date:   Thu, 3 Nov 2016 17:33:27 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Julia Cartwright <julia@...com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        linux-rt-users@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: update: make RCU_EXPEDITE_BOOT default

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@...utronix.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