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

From: Paul E. McKenney
Date: Mon Nov 07 2016 - 13:29:53 EST

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