On Thu, Feb 19, 2009 at 7:11 AM, Thomas Pfaff <tpfaff@xxxxxxx> wrote:
> While testing the 2.6.26.8-rt16 kernel on an AT91SAM9260-EK Arm9 board i noticed
> a small regression to 2.6.24.7-rt27.
>
> The kernel compiles and boots fine but the average load when the cpu is idle
> tends to be 1, it starts at 0 when the system boots and goes to 1 or above after
> some time.
>
> All kernels have the at91 patches from http://maxim.org.za/sam9.html
>
> Following kernels are ok:
>
> 2.6.24.7-rt27
> 2.6.26.8 without rt
>
> With the rt patch i have tested several combinations of the PREEMPT options:
>
> Without CONFIG_PREEMPT_HARDIRQ i get an oops on boot (see next mail).
>
> All other CONFIG_PREEMPT_[NONE|VOLUNTARY|DESKTOP|RT] options are working but
> with the wrong average load.
Please test the following patch. I think you should find it fixes the problem.
Paul.
From 10906efab35b6f561defce7a90a2967a547de5f2 Mon Sep 17 00:00:00 2001
From: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
Date: Mon, 26 Jan 2009 21:15:32 -0500
Subject: [PATCH] softirq loadavg: one less active task in PREEMPT_SOFTIRQ loadavg calc
Building with a typical RT .config, but with just PREEMPT_RT disabled,
one gets a kernel that constantly shows a loadavg of one.
When RT is enabled, we subtract one from the nr_active since there is
a loadavg thread which calls calc_load(). But in the PREEMPT_SOFTIRQ
case, we should also subtract one, since we are still calling it from
a thread, as it is called as follows:
init_timers()
\_ open_softirq(..., run_timer_softirq, ...)
run_timer_softirq()
\_ update_times()
\_ ifndef PREEMPT_RT calc_load()
\_ count_active_tasks()
\_ nr_active(), or nr_active()-1 for RT.
For reference, the original RT patch that introduces the -1 is
preempt-realtime-timer.patch, then bz235099-idle-load-fix.patch
updates it to add/mention the threaded calculation for RT.
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---
kernel/timer.c | 7 +++++--
1 files changed, 5 insertions(+), 2 deletions(-)
diff --git a/kernel/timer.c b/kernel/timer.c
index 4df2d50..eae813a 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1065,9 +1065,12 @@ static unsigned long count_active_tasks(void)
{
/*
* On PREEMPT_RT, we are running in the loadavg thread,
- * so consider 1 less running tasks:
+ * and similarly if we are only running PREEMPT_SOFTIRQS we
+ * are running in the run_timer_softirq thread, so consider
+ * one less running task in these two cases. (PREEMPT_RT
+ * selects PREEMPT_SOFTIRQ, so we get both via the latter)
*/
-#ifdef CONFIG_PREEMPT_RT
+#ifdef CONFIG_PREEMPT_SOFTIRQS
return (nr_active() - 1) * FIXED_1;
#else
return nr_active() * FIXED_1;
--
1.6.0.4
