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@xxxxxxxxxxx> CC: Tejun Heo <tj@xxxxxxxxxx> --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
