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

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Message-ID: <48987486.6010100@grandegger.com>
Date:	Tue, 05 Aug 2008 17:40:54 +0200
From:	Wolfgang Grandegger <wg@...ndegger.com>
To:	paulmck@...ux.vnet.ibm.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?

Paul E. McKenney wrote:
> 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()?

FYI, Thomas fixed the real problem a week ago and the patch went into 
2.6.24.7-rt16. See his mail with the subject "2.6.24.7-rt16".

Wolfgang.
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