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

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Message-ID: <Pine.LNX.4.58.0807052041350.8466@gandalf.stny.rr.com>
Date:	Sat, 5 Jul 2008 20:42:44 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Wolfgang Grandegger <wg@...ndegger.com>
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?


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.

-- Steve

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