Reading EeePC900 battery info causes stalls

[Sitsofe Wheeler <sitsofe@yahoo.com>]

(Long thread that was originally posted over here: 
http://tinyurl.com/4akxa5 )

I've found that when running on battery (and the battery is not full) 
the system will stall while battery information is read when using a non 
preemptive kernel.

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.

I have ftraces of the stalls but the traces become large very quickly. 
To that end I have disabled the tracing of certain functions to allow 
part of the traces to be produced.

By doing counts across multiple runs I would say that the most 
frequently called functions are the following (in most frequently called 
order). The counts are definitely approximate but are reasonable 
relative to each other.

  475325 acpi_os_release_object (acpi_ut_delete_generic_state)
  406895 kmem_cache_free (acpi_os_release_object)
  402838 kmem_cache_alloc (acpi_ut_create_generic_state)
  132968 acpi_ut_update_ref_count (acpi_ut_update_object_reference)
  131041 acpi_ut_pop_generic_state (acpi_ut_update_object_reference)
  131036 acpi_ut_delete_generic_state (acpi_ut_update_object_reference)
  131025 acpi_ut_create_generic_state (acpi_ut_create_update_state)
  131023 acpi_ut_create_update_state_and_push 
(acpi_ut_update_object_reference)
  131020 acpi_ut_create_update_state (acpi_ut_create_update_state_and_push)
  131018 acpi_ut_push_generic_state (acpi_ut_create_update_state_and_push)
   60147 acpi_ns_get_next_node (acpi_ns_delete_namespace_by_owner)
   28974 acpi_ns_get_next_valid_node (acpi_ns_get_next_node)


Here's the command line I used to disable the tracing of certain 
frequent functions:

echo acpi_os_release_object > set_ftrace_notrace && echo kmem_cache_* >> 
set_ftrace_notrace && echo acpi_ut_* >> set_ftrace_notrace

Logs with the filtering on can be seen here (15Mbytes decompressed each):
http://sucs.org/~sits/test/eeepc-debug/20080923/latency_trace.gz
http://sucs.org/~sits/test/eeepc-debug/20080923/trace.txt.gz

I guess the aim is to find a good point to put cond_resched() or 
otherwise solve the latency issue.

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