linux-kernel - Re: 2.6.24-rc8-rt1: Strange latencies on mpc5200 powerpc

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Date:	Mon, 28 Jan 2008 16:38:57 +0100
From:	Wolfgang Grandegger <wg@...ndegger.com>
To:	Luotao Fu <l.fu@...gutronix.de>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: 2.6.24-rc8-rt1: Strange latencies on mpc5200 powerpc

Hi Fu,

Luotao Fu wrote:
> Hi,
> 
> I took some time today and went through Wolfgangs scenarios partly. Now
> some results from my side. I ran my tests on a 2.6.24-rt1
> 
> Wolfgang Grandegger wrote:
>  > I also did some more measurements and made, by chance, interesting
>> observations. I will summarize in more detail later on. Here are some
>> preliminary results. My high latencies of up to 570us (without latency
>> tracer) seem to be caused mainly by the following setting:
>>
>>   CONFIG_RCU_TRACE=m
>>
> 
> I also got high latencies without CONFIG_RCU_TRACE set at all. setting
> CONFIG_RCU_TRACE to y or m causes also high latency though in my test
> runs. Hence I doubt the rcu Tracer is really the problem.
> 
> As I mentioned in my last mail. Only thing I could reproduce reliably is
> that the measurement results depend heavily on kind of the non-rt
> Workload. For with hackbench or cache calibrator I couldn't produce
> abnormal high latencies. Nor could I produce the hight latencies on a
> system booted via flash. Hence my suspects stays on the fec irq thread
> and filesystem access routines.
> 
>> With CONFIG_NO_HZ=y or CONFIG_PPC_BESTCOMM_GEN_BD=y the latency
>> increases by approx. 100..150us, each.
>>
> 
> Since I was producing high latencies independently from the rcu
> settings. I didn't spend a lot of time playing around with the GEN_BD
> and dynamic clock. All I can say is that disabling them also cause high
> latency. ;-) The average results don't differ significantly in my test
> runs.

In the meantime I have measured the impact more carefully and posted the
results to the list:

- CONFIG_RCU_TRACE or CONFIG_PREEMPT_RCU_BOOST not set causes high
  latencies up to 500..600us.

- CONFIG_NO_HZ=y increases the latency by 90 us, at least.

- CONFIG_PPC_BESTCOMM_GEN_BD=y seems not to harm.

Do you still get high latencies with:

  CONFIG_PREEMPT_RCU_BOOST=y
  CONFIG_RCU_TRACE=y
  CONFIG_NO_HZ is not set

With this setting I have not yet realized latencies > 150us. Could you
please give it a try? If I change one of the parameters above, latency
increases in short time.

Thanks.

Wolfgang.


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