1 files changed, 57 insertions, 16 deletions

diff --git a/Documentation/workqueue.txt b/Documentation/workqueue.txt
index e4498a2872c3..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
@@ -190,17 +191,17 @@ resources, scheduled and executed.
         * Long running CPU intensive workloads which can be better
           managed by the system scheduler.
 
-  WQ_FREEZEABLE
+  WQ_FREEZABLE
 
-        A freezeable wq participates in the freeze phase of the system
+        A freezable wq participates in the freeze phase of the system
         suspend operations.  Work items on the wq are drained and no
         new work item starts execution until thawed.
 
-  WQ_RESCUER
+  WQ_MEM_RECLAIM
 
         All wq which might be used in the memory reclaim paths _MUST_
-        have this flag set.  This reserves one worker exclusively for
-        the execution of this wq under memory pressure.
+        have this flag set.  The wq is guaranteed to have at least one
+        execution context regardless of memory pressure.
 
   WQ_HIGHPRI
 
@@ -356,11 +357,11 @@ If q1 has WQ_CPU_INTENSIVE set,
 
 6. Guidelines
 
-* Do not forget to use WQ_RESCUER if a wq may process work items which
-  are used during memory reclaim.  Each wq with WQ_RESCUER set has one
-  rescuer thread reserved for it.  If there is dependency among
-  multiple work items used during memory reclaim, they should be
-  queued to separate wq each with WQ_RESCUER.
+* Do not forget to use WQ_MEM_RECLAIM if a wq may process work items
+  which are used during memory reclaim.  Each wq with WQ_MEM_RECLAIM
+  set has an execution context reserved for it.  If there is
+  dependency among multiple work items used during memory reclaim,
+  they should be queued to separate wq each with WQ_MEM_RECLAIM.
 
 * Unless strict ordering is required, there is no need to use ST wq.
 
@@ -368,13 +369,53 @@ If q1 has WQ_CPU_INTENSIVE set,
   recommended.  In most use cases, concurrency level usually stays
   well under the default limit.
 
-* A wq serves as a domain for forward progress guarantee (WQ_RESCUER),
-  flush and work item attributes.  Work items which are not involved
-  in memory reclaim and don't need to be flushed as a part of a group
-  of work items, and don't require any special attribute, can use one
-  of the system wq.  There is no difference in execution
-  characteristics between using a dedicated wq and a system wq.
+* A wq serves as a domain for forward progress guarantee
+  (WQ_MEM_RECLAIM, flush and work item attributes.  Work items which
+  are not involved in memory reclaim and don't need to be flushed as a
+  part of a group of work items, and don't require any special
+  attribute, can use one of the system wq.  There is no difference in
+  execution characteristics between using a dedicated wq and a system
+  wq.
 
 * 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.