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

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Message-ID: <20080801210913.GC14851@linux.vnet.ibm.com>
Date:	Fri, 1 Aug 2008 14:09:13 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Wolfgang Grandegger <wg@...ndegger.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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?

On Wed, Jul 02, 2008 at 01:03:27PM +0200, Wolfgang Grandegger wrote:
> Wolfgang Grandegger wrote:
>> Steven Rostedt wrote:
>> There seems to be something in trace_mark() keeping latency low:
>>   http://lxr.linux.no/linux+v2.6.24.4/include/linux/marker.h#L52
>> I will follow your suggestions.
>
> I removed all "likely" and "unlikely" macros, but the latencies did not 
> improve. Then I added
>
> 	preempt_disable();
> 	preempt_enable();
>
> at the two locations mentioned above, like trace_mark() does,  and disabled 
> CONFIG_RCU_TRACE. That helped to keep the latencies low, and it did for 
> 2.6.25.8-rt7 as well. As I see it, adding preemption points seem to prevent 
> high latencies.

Interesting.  Perhaps a preempt_enable_no_resched() somewhere that
should be preempt_enable()?

							Thanx, Paul
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