[PATCH] workqueue: document debugging tricks

Florian Mickler
2011-03-31T10:37:48Z

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>
CC: Tejun Heo <tj@kernel.org>
---
Documentation/workqueue.txt | 38 ++++++++++++++++++++++++++++++++++++++
1 files changed, 38 insertions(+), 0 deletions(-)

diff --git a/Documentation/workqueue.txt b/Documentation/workqueue.txt
index 01c513f..cdbc3c6 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 +380,40 @@ 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 problem 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.
--
1.7.4.1