linux-kernel - Re: RCU related performance regression in 3.3

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Date:	Fri, 18 May 2012 16:48:12 +0200
From:	Pascal Chapperon <pascal.chapperon@...adoo.fr>
To:	paulmck@...ux.vnet.ibm.com
CC:	Josh Boyer <jwboyer@...hat.com>, linux-kernel@...r.kernel.org,
	kernel-team@...oraproject.org
Subject: Re: RCU related performance regression in 3.3

Le 18/05/2012 14:14, Paul E. McKenney a écrit :
> On Fri, May 18, 2012 at 01:01:41PM +0200, Pascal Chapperon wrote:
>> Le 15/05/2012 00:32, Paul E. McKenney a écrit :
>>> On Fri, May 04, 2012 at 04:14:42PM -0700, Paul E. McKenney wrote:
>>>> On Fri, May 04, 2012 at 11:41:13PM +0200, Pascal Chapperon wrote:
>>>>> Le 04/05/2012 17:04, Paul E. McKenney a écrit :
>>>>>> On Fri, May 04, 2012 at 04:42:54PM +0200, Pascal Chapperon wrote:
>>>>>>> Le 01/05/2012 17:45, Paul E. McKenney a écrit :
>>>>>>>
>>>>>>>> Here is my RCU_FAST_NO_HZ patch stack on top of v3.4-rc4.
>>>>>>>>
>>>>>>>> Or you can pull branch fnh.2012.05.01a from:
>>>>>>>>
>>>>>>>> 	git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
>>>>>>>>
>>>>>>>> 							Thanx, Paul
>>>>>>>>
>>>>>>> I applied your global patch on top of v3.4-rc4. But the slowdown is
>>>>>>> worse than before : boot sequence took 80s instead 20-30s (12s for
>>>>>>> initramfs instead of 2s).
>>>>>>>
>>>>>>> I'll send you rcu tracing log in a second mail.
>>>>>>
>>>>>> Hmmm...  Well, I guess I am glad that I finally did something that
>>>>>> had an effect, but I sure wish that the effect had been in the other
>>>>>> direction!
>>>>>>
>>>>>> Just to make sure I understand: the difference between the 20-30s and
>>>>>> the 80s is exactly the patch I sent you?
>>>>>>
>>>>>> 							Thanx, Paul
>>>>>>
>>>>>>
>>>>> Yes. Exactly same kernel config as in previous results, I applied
>>>>> your patch against v3.4-rc4, and sorry, the result is exactly what I
>>>>> said;
>>>>> I saw that your global patch was quite huge, and addresses things which
>>>>> are not directly related with the initial patch (commit
>>>>> 7cb92499000e3c86dae653077b1465458a039ef6); maybe a side effect?
>>>>>
>>>>> However, I'm ready to try this patch on my smaller laptop which
>>>>> supports well CONFIG_FAST_NO_HZ=y and systemd, if you think it can
>>>>> help ?
>>>>>
>>>>> Another thought: this issue as nothing to do with i7 Hyper-threading
>>>>> capacities ? (as I test core2duo, Pentium ulv in same conditions and I
>>>>> don't encountered any slowdown ?)
>>>>
>>>> Well, one possibility is that your setup starts the jiffies counter
>>>> at some interesting value.  The attached patch (also against v3.4-rc4)
>>>> applies a bit more paranoia to the initialization to handle this
>>>> and other possibilities.
>>>
>>> This patchset fixes the problem where RCU_FAST_NO_HZ's timers were
>>> being ignored due to the dyntick-idle code having already calculated
>>> the CPU's wakeup time (which I sent earlier, mistakenly offlist), but
>>> also fixes a botched check in my workaround.
>>>
>>> Could you please try it out?  This patch is against 3.4-rc4.
>>>
>>> 							Thanx, Paul
>>>
>> Hi Paul,
>>
>> <  +     if (!rcu_cpu_has_nonlazy_callbacks(cpu))
>> ---
>>> +     if (rcu_cpu_has_nonlazy_callbacks(cpu))
>>
>> I was a little disappointed by the previous patch (boot sequence still
>> took 72 s.), but this one makes a huge difference ;-)
>> Slowdown during boot or shutdown with CONFIG_RCU_FAST_NO_HZ has
>> disappeared (~ 10 attempts) :
>> # systemd-analyze
>> Startup finished in 1990ms (kernel) + 1174ms (initramfs) + 3121ms
>> (userspace) = 6285ms
>> .
>
> Very good!  And thank you very much for all your testing efforts and
> for bearing with me through this!
>
> Does this mean that I can add your Tested-by?

Yes: the results are good and stable, at least for my hardware.
I tried with both a standard fedora 16 kernel configuration and a custom
one (hardware optimized, preempted, etc...) and this on over more 20
attempts.
With or without FAST_NO_HZ makes no difference now.

>
>> Do you want the rcu tracing log for this patch ?
>
> Could you please?  Just in case there is some other surprise that
> I should know about that might not be visible.  ;-)
>
> 							Thanx, Paul
I'll send you the logs in a second mail (offlist).

Pascal


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