linux-kernel - CONFIG_PEEMPT_RT_FULL vs. CONFIG_PREEMPT

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Date:	Mon, 15 Aug 2011 16:04:53 +0200
From:	"Tim Sander" <tim.sander@....com>
To:	"linux-rt-users" <linux-rt-users@...r.kernel.org>
Cc:	"Peter Zijlstra" <peterz@...radead.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"Remy Bohmer" <linux@...mer.net>
Subject: CONFIG_PEEMPT_RT_FULL vs. CONFIG_PREEMPT_RTB

Hi

I have more than twice as much cpu load when running the  
CONFIG_PREEMPT_RT_FULL=y kernel compared to CONFIG_PREEMPT_RTB=y. First i 
thought that might be the difference that one config had softirqs enabled and 
the other not. Now i realized that softirqs are already enabled with 
CONFIG_PREEMPT_RTB so i don't see a good reason why the CONFIG_PREEMPT_RT_FULL 
option should have two times more cpu load than CONFIG_PREEMPT_RTB.

The arm1136 cpu (533Mhz pcm043) has 9.6khz interrupt rate so i surely expect 
some load. But i wouldn't expect so much difference between 
CONFIG_PREEMPT_RT_FULL and CONFIG_PREEMPT_RTB. So does anybody has an idea 
what might a reason to so much more cpu load when going to full preemption?

Besides of the cpu load i also found out that with rt11 i can again wake up 
usermode from a IRQF_NO_THREAD waiting on a waitqueue. So the steps outlined 
in this mail http://www.spinics.net/lists/linux-rt-users/msg06834.html do not 
seem to be neccessary anymore? Or does this create an error and than some high 
load?

Best regards
Tim

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