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

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Date:	Thu, 10 Jul 2008 09:50:47 +0200
From:	Wolfgang Grandegger <wg@...ndegger.com>
To:	Wolfgang Grandegger <wg@...ndegger.com>,
	Luotao Fu <l.fu@...gutronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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?

Luotao Fu wrote:
> Hi Wolfgang,
> On Wed, Jul 09, 2008 at 03:15:01PM +0200, Wolfgang Grandegger wrote:
>> Hi Fu (without n)
>>
> ....
>> OK, in the past you have been able to reproduce the high latencies with  
>> 2.6.24-rt1 and CONFIG_RCU_TRACE disabled, IIRC. Did you use a different  
>> toolchain at that time?
>>
> 
> Nope. As mentioned above, trace_mark() does some "real" works (what ever it is.),
> while the new mechahnismen use flags to remember the state of preemption. Maybe

I don't known what you refer to, but in __rcu_preempt_unboost() of 2.6.25.8-rt7, 
the trace code simply increments a counter:

        static void rcu_trace_boost_##type(struct rcu_boost_dat *rbd)   \
        {                                                               \
                rbd->rbs_stat_##type++;                                 \
        }

and that's the reason why latency is not affected by switching CONFIG_RCU_TRACE
on (while trace_mark uses preempt_disable/preempt_enable around).

> something here got optimized away? I take for grant, that you use gcc in your
> toolchain. Which version do you have?

The ELDK v4.2 uses:

  ppc_6xx-gcc (GCC) 4.2.2

and 

  GLIBC v2.6

But I measured the same latencies with ELDK v4.1:

  ppc_6xx-gcc (GCC) 4.0.0 (DENX ELDK 4.1 4.0.0)

  GLIBC v2.3.5

Wolfgang.

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