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

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Message-ID: <48709348.6000101@grandegger.com>
Date:	Sun, 06 Jul 2008 11:41:28 +0200
From:	Wolfgang Grandegger <wg@...ndegger.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	"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?

Steven Rostedt wrote:
> On Wed, 2 Jul 2008, Wolfgang Grandegger wrote:
>> 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.
> 
> This to me sounds like we have preempt_enable_noresched someplace that
> shouldn't. In otherwords, we enabled preemption without checking if
> preemption is needed, which is a serious bug in -rt.

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).

Wolfgang.
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