Re: RCU related performance regression in 3.3

[Pascal Chapperon <pascal.chapperon@wanadoo.fr>]

> Message du 05/04/12 16:40
> De : "Paul E. McKenney" 
> A : "Pascal CHAPPERON" 
> Copie à : "Josh Boyer" , linux-kernel@vger.kernel.org, kernel-team@fedoraproject.org
> Objet : Re: RCU related performance regression in 3.3
> 
> On Thu, Apr 05, 2012 at 04:15:33PM +0200, Pascal CHAPPERON wrote:
> > Hello,
> > 
> > I didn't notice any significant slowdown while the system is up and running. 
A full kernel compilation (make -j 16) takes 14mn with both 3.2.10 and 3.3.0.
> 
> OK, so the natural approach is to disable CONFIG_RCU_FAST_NO_HZ at
> boot time. Unfortunately, you appear to need it to remain disabled
> through at least filesystem mounting, which if I understand correctly
> happens long after system_state gets set to SYSTEM_RUNNING.
> 
In fact, I need it to remain disable until all the systemd units are completed. 
Some units, such as NetworkManager can take longer time to complete with
RCU_FAST_NO_HZ enabled.
And i need it to be disabled at shutdown, as umounting cgroups, sysfs, etc.
plus old-root mounting can take one plain second for each umounting.

> If RCU has some way to find out when init is complete, I can easily
> make it so that CONFIG_RCU_FAST_NO_HZ optimizes for speed during boot
> and energy efficiency during runtime.
> 
I said that I didn't noticed significant slowdown during runtime, but my
laptop usage is basic. Some specific tasks similar to systemd may 
perhaps be impacted by this feature.
I can test a task/program that could stress RCU_FAST_NO_HZ if any ?

> One thing I could easily do would be to provide a sysfs parameter or
> some such that allows the boot process to enable energy-efficiency
> mode at runtime. I would much prefer to make this automatic, though.
> 
So the feature is disabled until you trigger a sysfs parameter, and can be
disabled before shutdown ? It would be fair, at least for hardware like my
own.

> Other thoughts?
> 
Do you think that the culprit is a buggy hardware in my laptop, or the 
number of cpu/threads ?

> Thanx, Paul
> 

Resent in plain text because rejected. Sorry, i forgot the rules.

Pascal