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

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Date:	Fri, 1 Aug 2008 14:09:45 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Wolfgang Grandegger <wg@...ndegger.com>
Cc:	Luotao Fu <l.fu@...gutronix.de>,
	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 - RCU
	issue?

On Thu, Jul 10, 2008 at 09:50:47AM +0200, Wolfgang Grandegger wrote:
> 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).

This changed -- preempt_disable()/preempt_enable() pair
was added for rcu_trace_boost_boost_called_preempt() and
rcu_trace_boost_unboost_called_preempt() later to suppress a warning
(and also make that statistic accurate in face of preemption).

						Thanx, Paul

>> 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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