Re: Realtime Preemption, 2.6.12, Beginners Guide?

[Alistair John Strachan]

On Wednesday 06 Jul 2005 18:01, Ingo Molnar wrote:
> * Alistair John Strachan <s0348365@xxxxxxxxxxxx> wrote:
> > > I'm beginning to understand the issue, and I see why you think the
> > > proposed patch fixes it. I'll compile and boot V0.7.51-05 now.
> >
> > Indeed, this seems to have fixed it.
> >
> > ( softirq-timer/0-3    |#0): new 8 us maximum-latency wakeup.
> > ( softirq-timer/0-3    |#0): new 9 us maximum-latency wakeup.
> > ( softirq-timer/0-3    |#0): new 9 us maximum-latency wakeup.
> > ( softirq-timer/0-3    |#0): new 9 us maximum-latency wakeup.
> > ( softirq-timer/0-3    |#0): new 10 us maximum-latency wakeup.
> > ( softirq-timer/0-3    |#0): new 14 us maximum-latency wakeup.
>
> great! Do the softlockup warnings still occur?

Yes, but in no greater a number.

[root] 18:09 [~] uptime
 18:09:39 up 19 min,  4 users,  load average: 0.36, 0.29, 0.16

[root] 18:09 [~] dmesg | grep BUG: | wc -l
5

So far, however, there have been no lockups! The previous kernels would die 
very obviously within a couple of minutes.

I wonder if the ACPI problem was causing lockups (one thought I had was that 
the "ondemand" cpufreq governor was generating more ACPI events than usual, 
as the BIOS stepped through the different CPU speeds).

>
> > Find attached another trace (only 33us this time).
>
> the main latency comes from here:
> >    <...>-3485  0Dnh2   13us : enqueue_task (__schedule)
> >    <...>-3485  0Dnh2   14us+: trace_array (__schedule)
> >    <...>-3485  0Dnh2   18us : trace_array <softirq--3> (69 6e)
> >    <...>-3485  0Dnh2   18us : trace_array <<...>-3485> (76 78)
> >    <...>-3485  0Dnh2   20us+: trace_array (__schedule)
> > softirq--3     0Dnh2   28us+: __switch_to (__schedule)
>
> trace_array() can be quite expensive (it generates a trace entry of
> every runnable task, with interrupts and preemption disabled). It is
> disabled if RT_DEADLOCK_DETECT is disabled. For pure wakeup latency
> tracing, the most optimal combination of options is:
>
[snip]
>
> such a kernel will still be able to generate /proc/latency_trace traces,
> but has much lower runtime overhead than your current kernel. (But you
> should probably keep all debugging enabled until all of the current
> problems have been resolved.)
>
> 	Ingo

Well, thanks for the info. As you said, when the remaining issues have been 
resolved, I'll need to step up to a more efficient kernel, because I require 
extremely low kernel latency for the software I'm writing (this was not an 
idle patch fest).

-- 
Cheers,
Alistair.

personal:   alistair()devzero!co!uk
university: s0348365()sms!ed!ac!uk
student:    CS/CSim Undergraduate
contact:    1F2 55 South Clerk Street,
            Edinburgh. EH8 9PP.
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