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

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Message-ID: <20080708150826.GA30586@pengutronix.de>
Date:	Tue, 8 Jul 2008 17:08:26 +0200
From:	Luotao Fu <l.fu@...gutronix.de>
To:	Wolfgang Grandegger <wg@...ndegger.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Luotao Fu <l.fu@...gutronix.de>,
	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 - RCU
	issue?

Hi,

I found some time and played with my mpc5200B Board again on this issue
(2.6.25.8-rt7, same test environment)

On Sun, Jul 06, 2008 at 11:41:28AM +0200, Wolfgang Grandegger wrote:
....
> Yep, in 2.6.25.8-rt7 trace_mark() is not used any more, maybe for that  
> reason. Nevertheless, this "bug" kept the latency on my MPC5200 box low.
> I will try with the old tracer to understand what the real cause of the  
> high latencies is (with CONFIG_RCU_TRACE disabled).
>
As you said, in 2.6.25.8-rt7 there're rcu_trace_boost_unboost_called() and
rcu_trace_boost_unboosted() are used instead of trace_mark. I still haven't had
time to take a closer look waht these routines exactly do. Seemed however to me,
that they possibly do the same thing. I played a little bit around with
the RCU_TRACE and RCU_BOOST Option. Also I tried to comment out the
rcu_trace_boost_unboost_called() and rcu_trace_boost_unboosted() calls. My Tests
last appr. 30 Minutes with non-rt Payload "while [ 1 ]; do ls /;done". None of
my test showed extraordinary results. My worstcase values stay at about 220 us
with all my test combinations. I will attache a config with RCU_BOOST and
RCU_TRACE turned off to this mail, which provide a worst case value at 223 us
after about 40 Minutes test run on my board.

Are you using the FEC all a pci network card?

cheers
Luotao Fu
-- 
   Dipl.-Ing. Luotao Fu | Phone: +49-5121-206917-3
Pengutronix - Linux Solutions for Science and Industry
Entwicklungszentrum Nord     http://www.pengutronix.de


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