[PATCH] powerpc: Fix accounting of softirq time when idle

Anton Blanchard anton at samba.org
Mon Mar 21 12:28:03 EST 2011

commit cf9efce0ce31 (powerpc: Account time using timebase rather
than PURR) used in_irq() to detect if the time was spent in
interrupt processing. This only catches hardirq context so if we
are in softirq context and in the idle loop we end up accounting it
as idle time. If we instead use in_interrupt() we catch both softirq
and hardirq time.

The issue was found when running a network intensive workload. top
showed the following:

0.0%us,  1.1%sy,  0.0%ni, 85.7%id,  0.0%wa,  9.9%hi,  3.3%si,  0.0%st

85.7% idle. But this was wildly different to the perf events data.
To confirm the suspicion I ran something to keep the core busy:

# yes > /dev/null &

8.2%us,  0.0%sy,  0.0%ni,  0.0%id,  0.0%wa, 10.3%hi, 81.4%si,  0.0%st

We only got 8.2% of the CPU for the userspace task and softirq has
shot up to 81.4%.

With the patch below top shows the correct stats:

0.0%us,  0.0%sy,  0.0%ni,  5.3%id,  0.0%wa, 13.3%hi, 81.3%si,  0.0%st

Signed-off-by: Anton Blanchard <anton at samba.org>
Cc: stable at kernel.org

Index: linux-2.6/arch/powerpc/kernel/time.c
--- linux-2.6.orig/arch/powerpc/kernel/time.c	2011-03-21 12:05:12.056482258 +1100
+++ linux-2.6/arch/powerpc/kernel/time.c	2011-03-21 12:05:18.516721851 +1100
@@ -356,7 +356,7 @@ void account_system_vtime(struct task_st
 	get_paca()->user_time_scaled += user_scaled;
-	if (in_irq() || idle_task(smp_processor_id()) != tsk) {
+	if (in_interrupt() || idle_task(smp_processor_id()) != tsk) {
 		account_system_time(tsk, 0, delta, sys_scaled);
 		if (stolen)

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