linux-kernel - Re: [ANNOUNCE] 3.4.9-rt17

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Date:	Wed, 22 Aug 2012 13:48:05 +0200
From:	"Tim Sander" <tim.sander@....com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	"LKML" <linux-kernel@...r.kernel.org>,
	"RT" <linux-rt-users@...r.kernel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Carsten Emde" <C.Emde@...dl.org>, "John Kacur" <jkacur@...hat.com>
Subject: Re: [ANNOUNCE] 3.4.9-rt17

Dear Linux-Realtimers

I just had the time to test this realtime release. Due to time constraints 
this is the first test on the 3.4 rt series. The HW Platform i used for testing 
is a ARM PCM043 i.mx35 with some tweaks to decrease latency. One of them is 
using an interrupt to drive our system timer and not using hr timers.
But these tweaks are essentially the same between 3.0.30-rt50 and the now 
tested 3.4.9-rt17. The only thing which is not applied on 3.4.9-rt17 is 
switching the cpu to idle (which gives cold caches and bad latency), since the 
old location was removed an i didn't found the new location but pm is switch 
of in the 3.4 kernelconfig as attached.

The performance of the 3.4.9-rt17 is much much worse than the 3.0.30-rt50 
kernel. While the 3.0 kernel was able to keep up with a 2400khz interrupt 
source in usermode the 3.4 kernel drops out by a wide margin (6ms+?). I even 
thought on the first moment that i didn't enable full realtime...

So if any of you have some comments why the latency performance degraded this 
far or if you have a hint for me where to find the arch_idle function which has 
been located in arch/arm/plat-mxc/include/mach/system.h so the config between 
3.0 and 3.4 is as identical as it can be.

Attached is the kernel config for reference.

Best regards
Tim

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View attachment "kernelconfig" of type "text/plain" (52033 bytes)