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-rw-r--r--Documentation/workqueue.txt40
-rw-r--r--kernel/workqueue.c8
2 files changed, 47 insertions, 1 deletions
diff --git a/Documentation/workqueue.txt b/Documentation/workqueue.txt
index 01c513fac40..a0b577de918 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.
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 8859a41806d..e3378e8d3a5 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1291,8 +1291,14 @@ __acquires(&gcwq->lock)
return true;
spin_unlock_irq(&gcwq->lock);
- /* CPU has come up in between, retry migration */
+ /*
+ * We've raced with CPU hot[un]plug. Give it a breather
+ * and retry migration. cond_resched() is required here;
+ * otherwise, we might deadlock against cpu_stop trying to
+ * bring down the CPU on non-preemptive kernel.
+ */
cpu_relax();
+ cond_resched();
}
}