It looks like in ARM the cpu_idle() (and default_idle) get traced . So
for instance you'll get a situation when preemption is off, interrupts
are off, and then the cpu runs something like halt on x86. Then an
interrupt wakes up the cpu.
I attached a patch that might fix the tracing. I tried to prevent the
tracing around when the cpu halts.
Daniel
On Fri, 2005-12-30 at 13:40 +0100, kus Kusche Klaus wrote:
> > From: Ingo Molnar
> > there seem to be leaked preempt counts:
> >
> > <idle>-0 0.n.1 8974us : touch_critical_timing (cpu_idle)
> >
> > we should never have preemption disabled in cpu_idle(). To
> > debug leaked
> > preemption counts, enable CONFIG_DEBUG_PREEMPT.
>
> Something really fishy is going on here:
> That 9 ms latency seems to be really *idle* time!
>
> * If the box is idle, I get that trace almost immediately, and
> almost always with close to 9 ms (system clock is 100 Hz,
> i.e. 10 ms tick period).
>
> * If the box is 100 % loaded, I don't get that trace. I get
> different traces from different processes, mostly shorter than
> 9 ms.
>
> * If I load the box with work at regular intervals
> and idle time in between, I get traces
> identical to the 9 ms idle trace, but consistently shorter.
> If I throw a flood ping with 1000 pkt/s against my box, the idle
> trace shows up with 800 or 900 microseconds, i.e. the idle time
> between packets.
>
> Now, is the tracer wrong, or has the idle time a wrong status?
>
> By the way, I had one trace today where the cat /proc/latency_trace
> itself showed up:
>
> \ / ||||| \ | /
> cat-3129 0D... 1us!: preempt_schedule_irq (svc_preempt)
> cat-3129 0.... 5502us+: rt_up (l_start)
> cat-3129 0D..1 5511us+: check_raw_flags (rt_up)
> cat-3129 0...1 5514us+: rt_up (l_start)
> cat-3129 0...1 5518us : sub_preempt_count_ti (rt_up)
>
> What's happening in those 5 ms?
>
Index: linux-2.6.14/arch/arm/kernel/process.c
===================================================================
--- linux-2.6.14.orig/arch/arm/kernel/process.c
+++ linux-2.6.14/arch/arm/kernel/process.c
@@ -89,12 +89,12 @@ void default_idle(void)
if (hlt_counter)
cpu_relax();
else {
- raw_local_irq_disable();
+ __raw_local_irq_disable();
if (!need_resched()) {
timer_dyn_reprogram();
arch_idle();
}
- raw_local_irq_enable();
+ __raw_local_irq_enable();
}
}
@@ -121,8 +121,10 @@ void cpu_idle(void)
if (!idle)
idle = default_idle;
leds_event(led_idle_start);
+ __preempt_enable_no_resched();
while (!need_resched())
idle();
+ preempt_disable();
leds_event(led_idle_end);
__preempt_enable_no_resched();
__schedule();
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