workqueue: Automatically mark CPU-hogging work items CPU_INTENSIVE

If a per-cpu work item hogs the CPU, it can prevent other work items from
starting through concurrency management. A per-cpu workqueue which intends
to host such CPU-hogging work items can choose to not participate in
concurrency management by setting %WQ_CPU_INTENSIVE; however, this can be
error-prone and difficult to debug when missed.

This patch adds an automatic CPU usage based detection. If a
concurrency-managed work item consumes more CPU time than the threshold
(10ms by default) continuously without intervening sleeps, wq_worker_tick()
which is called from scheduler_tick() will detect the condition and
automatically mark it CPU_INTENSIVE.

The mechanism isn't foolproof:

* Detection depends on tick hitting the work item. Getting preempted at the
  right timings may allow a violating work item to evade detection at least
  temporarily.

* nohz_full CPUs may not be running ticks and thus can fail detection.

* Even when detection is working, the 10ms detection delays can add up if
  many CPU-hogging work items are queued at the same time.

However, in vast majority of cases, this should be able to detect violations
reliably and provide reasonable protection with a small increase in code
complexity.

If some work items trigger this condition repeatedly, the bigger problem
likely is the CPU being saturated with such per-cpu work items and the
solution would be making them UNBOUND. The next patch will add a debug
mechanism to help spot such cases.

v4: Documentation for workqueue.cpu_intensive_thresh_us added to
    kernel-parameters.txt.

v3: Switch to use wq_worker_tick() instead of hooking into preemptions as
    suggested by Peter.

v2: Lai pointed out that wq_worker_stopping() also needs to be called from
    preemption and rtlock paths and an earlier patch was updated
    accordingly. This patch adds a comment describing the risk of infinte
    recursions and how they're avoided.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>

1 files changed, 12 insertions, 1 deletions

diff --git a/tools/workqueue/wq_monitor.py b/tools/workqueue/wq_monitor.py
index fc1643ba06b3..7c6f523b9164 100644
--- a/tools/workqueue/wq_monitor.py
+++ b/tools/workqueue/wq_monitor.py
@@ -11,6 +11,11 @@ https://github.com/osandov/drgn.
 
   infl     The number of currently in-flight work items.
 
+  CPUitsv  The number of times a concurrency-managed work item hogged CPU
+           longer than the threshold (workqueue.cpu_intensive_thresh_us)
+           and got excluded from concurrency management to avoid stalling
+           other work items.
+
   CMwake   The number of concurrency-management wake-ups while executing a
            work item of the workqueue.
 
@@ -53,6 +58,7 @@ WQ_MEM_RECLAIM          = prog['WQ_MEM_RECLAIM']
 
 PWQ_STAT_STARTED        = prog['PWQ_STAT_STARTED']      # work items started execution
 PWQ_STAT_COMPLETED      = prog['PWQ_STAT_COMPLETED']	# work items completed execution
+PWQ_STAT_CPU_INTENSIVE  = prog['PWQ_STAT_CPU_INTENSIVE'] # wq_cpu_intensive_thresh_us violations
 PWQ_STAT_CM_WAKEUP      = prog['PWQ_STAT_CM_WAKEUP']    # concurrency-management worker wakeups
 PWQ_STAT_MAYDAY         = prog['PWQ_STAT_MAYDAY']	# maydays to rescuer
 PWQ_STAT_RESCUED        = prog['PWQ_STAT_RESCUED']	# linked work items executed by rescuer
@@ -75,19 +81,23 @@ class WqStats:
                  'mem_reclaim'          : self.mem_reclaim,
                  'started'              : self.stats[PWQ_STAT_STARTED],
                  'completed'            : self.stats[PWQ_STAT_COMPLETED],
+                 'cpu_intensive'        : self.stats[PWQ_STAT_CPU_INTENSIVE],
                  'cm_wakeup'            : self.stats[PWQ_STAT_CM_WAKEUP],
                  'mayday'               : self.stats[PWQ_STAT_MAYDAY],
                  'rescued'              : self.stats[PWQ_STAT_RESCUED], }
 
     def table_header_str():
-        return f'{"":>24} {"total":>8} {"infl":>5} {"CMwake":>7} {"mayday":>7} {"rescued":>7}'
+        return f'{"":>24} {"total":>8} {"infl":>5} '\
+            f'{"CPUitsv":>7} {"CMwake":>7} {"mayday":>7} {"rescued":>7}'
 
     def table_row_str(self):
+        cpu_intensive = '-'
         cm_wakeup = '-'
         mayday = '-'
         rescued = '-'
 
         if not self.unbound:
+            cpu_intensive = str(self.stats[PWQ_STAT_CPU_INTENSIVE])
             cm_wakeup = str(self.stats[PWQ_STAT_CM_WAKEUP])
 
         if self.mem_reclaim:
@@ -97,6 +107,7 @@ class WqStats:
         out = f'{self.name[-24:]:24} ' \
               f'{self.stats[PWQ_STAT_STARTED]:8} ' \
               f'{max(self.stats[PWQ_STAT_STARTED] - self.stats[PWQ_STAT_COMPLETED], 0):5} ' \
+              f'{cpu_intensive:>7} ' \
               f'{cm_wakeup:>7} ' \
               f'{mayday:>7} ' \
               f'{rescued:>7} '