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workqueue: Document debugging tricks

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It is not obvious how to debug run-away workers.

These are some tips given by Tejun on lkml.

Signed-off-by: Florian Mickler <florian@mickler.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
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Florian Mickler 2011-03-31 13:40:42 +02:00 committed by Tejun Heo
parent 6aba74f279
commit e2de9e0862
1 changed files with 40 additions and 0 deletions
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Documentation/workqueue.txt
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@ -12,6 +12,7 @@ CONTENTS
4. Application Programming Interface (API)
5. Example Execution Scenarios
6. Guidelines
7. Debugging
1. Introduction
@ -379,3 +380,42 @@ If q1 has WQ_CPU_INTENSIVE set,
* Unless work items are expected to consume a huge amount of CPU
cycles, using a bound wq is usually beneficial due to the increased
level of locality in wq operations and work item execution.
7. Debugging
Because the work functions are executed by generic worker threads
there are a few tricks needed to shed some light on misbehaving
workqueue users.
Worker threads show up in the process list as:
root 5671 0.0 0.0 0 0 ? S 12:07 0:00 [kworker/0:1]
root 5672 0.0 0.0 0 0 ? S 12:07 0:00 [kworker/1:2]
root 5673 0.0 0.0 0 0 ? S 12:12 0:00 [kworker/0:0]
root 5674 0.0 0.0 0 0 ? S 12:13 0:00 [kworker/1:0]
If kworkers are going crazy (using too much cpu), there are two types
of possible problems:
1. Something beeing scheduled in rapid succession
2. A single work item that consumes lots of cpu cycles
The first one can be tracked using tracing:
$ echo workqueue:workqueue_queue_work > /sys/kernel/debug/tracing/set_event
$ cat /sys/kernel/debug/tracing/trace_pipe > out.txt
(wait a few secs)
^C
If something is busy looping on work queueing, it would be dominating
the output and the offender can be determined with the work item
function.
For the second type of problems it should be possible to just check
the stack trace of the offending worker thread.
$ cat /proc/THE_OFFENDING_KWORKER/stack
The work item's function should be trivially visible in the stack
trace.