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

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Message-Id: <20161103165931.GJ3716@linux.vnet.ibm.com>
Date:   Thu, 3 Nov 2016 09:59:31 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 Thu, Nov 03, 2016 at 05:33:27PM +0100, Sebastian Andrzej Siewior wrote:
> 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?

For really bit systems, quite possibly, where "really big" means
many hundreds or (more likely) thousands of CPUs.

But there are things that I can do to fix this when and if.

> > 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 know that feeling.  ;-)

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

							Thanx, Paul

> Sebastian
>