On Thu, 3 Nov 2016 09:59:31 -0700
"Paul E. McKenney" [off-list ref] wrote:

On Thu, Nov 03, 2016 at 05:33:27PM +0100, Sebastian Andrzej Siewior wrote:

quoted

On 2016-11-03 09:22:28 [-0700], Paul E. McKenney wrote:  

quoted

On Wed, Nov 02, 2016 at 05:30:02PM +0100, Sebastian Andrzej Siewior wrote:  

quoted

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@linutronix.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.

quoted

quoted

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 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".

-- Steve