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SubjectRe: RCU related performance regression in 3.3
Le 10/04/2012 18:07, Paul E. McKenney a écrit :
> On Fri, Apr 06, 2012 at 11:18:03AM +0200, Pascal Chapperon wrote:
>>> 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, good.
>
>>> 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.
>
> OK...
>
>>> 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 to try first -- could you please check boot/shutdown slowdown
> with the patch below?
>
> But yes, there are things like modifying netfilter rules and updating
> security configuration that might be affected.
>
>>> 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.
>
> That is the thought, though again I would really really prefer that this
> be automated.
>
>>> Other thoughts?
>>>
>> Do you think that the culprit is a buggy hardware in my laptop, or the
>> number of cpu/threads ?
>
> Maybe just more filesystems to mount?
>
>>> Thanx, Paul
>>>
>>
>> Resent in plain text because rejected. Sorry, i forgot the rules.
>
> No problem! Please see below for the patch.
>
> Thanx, Paul
>
RCU_IDLE_GP_DELAY=3 instead of 6 does not improve significantly
startup time. Shutdown is shorter, but there are still some delays on
umounting some sysfs or oldroot mounts (not always the same).
Startup time always varies randomly from 12s to 22s (8s stable without
RCU_FAST_NO_HZ).
The tasks taking time during startup are rarely the same from boot to
boot,and some of them run after filesystems mounting.
Example : "console-kit-system-log-start.service" systemd unit took 5s to
complete in my last try, and 1s in the previous run. This one occurs
after mountings.
I enabled CONFIG_RCU_TRACE, and hey, the result in debugfs is beyond
my knowledge :(
Do you want some data from sys/kernel/debug/rcu (rcudata, ...) ?

Pascal

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