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

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Message-ID: <20161107173546.t7aokzncidauio43@x>
Date:   Mon, 7 Nov 2016 09:35:46 -0800
From:   Josh Triplett <josh@...htriplett.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Julia Cartwright <julia@...com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        linux-rt-users@...r.kernel.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 Mon, Nov 07, 2016 at 06:30:30PM +0100, Sebastian Andrzej Siewior wrote:
> On 2016-11-07 12:19:39 [-0500], Steven Rostedt wrote:
> > I agree, but if this creates a boot time regression in large machines,
> > it may not be warranted.
> > 
> > I know Linus usually doesn't like options with default y, but this may
> > be one of those exceptions. Perhaps we should make it on by default and
> > say in the config "if you have a machine with 100s or 1000s of CPUs,
> > you may want to disable this".
> 
> The default could change if we know where the limit is. I have access to
> a box with approx 140 CPUs so I could check there if it is already bad.
> But everything above that / in the 1000 range is a different story.

Right; if we can characterize what machines it benefits and what
machines it hurts, we can automatically detect and run the appropriate
case with no configuration option needed.