Reading EeePC900 battery info causes stalls (was Re: How how latent should non-preemptive scheduling be?)

[Sitsofe Wheeler <sitsofe@yahoo.com>]

Peter Zijlstra wrote:
> Its actually function tracer output I'm interested in.. that shows what
> all its doing to make it take 120+ms.

I switched the tracer to ftrace and waited for the problem to occur.
When stall did happen it was far worse than usual with the tracing on
(instead of looping sound of < 0.5 second it looped it for about 2-3
seconds). Looking atthe trace it was filled with hald events. Killing 
off all of hald made the 30 second stalls go away.

A quick strace showed that what hal was doing every 30 seconds was
reading various battery stats from /sys. Doing a simple
cat /sys/class/power_supply/BAT0/manufacturer
is enough to provoke a stall. The stalls only occur if you are on 
battery or are on AC and the battery is not reporting that it is 100% 
charged (but in the latter case the stalls are less pronounced).

I can reliably hear the stalls at runlevel 1 by running
speaker-test -b75000
and
watch --interval=1 cat /proc/acpi/battery/BAT0/info
within separate terminals within screen.

A 5 second ftrace of the stall being provoked is provided on 
<http://sucs.org/~sits/test/eeepc-ftrace.txt.gz> (it's 6Mbytes 
uncompressed).

Putting the alsa buffer size up to 100000 allows you to still hear
stalls but far less frequently. Putting to 150000 will stop you from
hearing stalls. Even though xruns is set to 2 alsa not report any buffer
underruns to syslog.

Another way of seeing the stalls is to run glxgears and every second the
gears' spinning will jump (even on an unloaded system).

(I guess this works as the stalls are over 100ms)

The xandros provided 2.6.21.4 kernel does not exhibit this problem at 
all but my hand compiled 2.6.24something and ubuntu 2.6.24 kernels do.

For some reason latencytop did not really finger ACPI as a cause of
stalls (although some acpi stuff does show up but never in the top
spot). Is this simply a part of the kernel that latencytop does not trace?

> I thought we had a wakeup latency tracer exacty like we have preempt and
> irq off latency tracers, Steve, where'd that go?

I rebuilt my kernel after a make clean and wakeup was still not there. 
It might be a good idea to modify the kernel documentation currently
provided with 2.6.27 if it has gone away for now (or document the extra
switches needed to turn it on if that's why it didn't show up for me)...

--
Sitsofe | http://sucs.org/~sits/