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authorFlorian Mickler <florian@mickler.org>2011-03-31 13:40:42 +0200
committerTejun Heo <tj@kernel.org>2011-03-31 13:40:42 +0200
commite2de9e0862778f4aba103027ce575efbddb8117f (patch)
tree22e002c6a8a56f5bcd5897fabd568ec54e243ca9 /Documentation/workqueue.txt
parent6aba74f2791287ec407e0f92487a725a25908067 (diff)
workqueue: Document debugging tricks
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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-rw-r--r--Documentation/workqueue.txt40
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diff --git a/Documentation/workqueue.txt b/Documentation/workqueue.txt
index 01c513fac40e..a0b577de918f 100644
--- a/Documentation/workqueue.txt
+++ b/Documentation/workqueue.txt
@@ -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.

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