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

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Message-Id: <20161107180513.GQ24166@linux.vnet.ibm.com>
Date:   Mon, 7 Nov 2016 10:05:13 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        "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 09:35:46AM -0800, Josh Triplett wrote:
> 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.

I very much like this approach!  Anyone have access to large systems on
which this experiment could be carried out?  In the absence of new data,
I would just set the cutoff at 256 CPUs, as I have done in the past.

							Thanx, Paul