[PATCH 0/5] tracing/histograms: Pass stacktrace from synthetic start event to end event

I finally got around to implementing what I've been wanting to do for a long
time! That is to pass stacktraces from the start to the end event of a
synthetic event. Specifically, I need to get the stacktrace of a task as it
schedules out, but I don't want to see it until it schedules back in and I
check the timings to show that it is longer than normal. That is now done
with:

 # echo 's:block_lat pid_t pid; u64 delta; unsigned long[] stack;' > dynamic_events
 # echo 'hist:keys=next_pid:ts=common_timestamp.usecs,st=stacktrace  if prev_state == 2' >> events/sched/sched_switch/trigger
 # echo 'hist:keys=prev_pid:delta=common_timestamp.usecs-$ts,s=$st:onmax($delta).trace(block_lat,prev_pid,$delta,$s)' >> events/sched/sched_switch/trigger

The above creates a synthetic event that will trigger on the max delta of a
task blocked in an uninterruptible state. It will show you the stack trace
of where that occurred!

 # echo 1 > events/synthetic/block_lat/enable
 # ls -lR > /dev/null
 # cat trace

# tracer: nop
#
# entries-in-buffer/entries-written: 2/2   #P:8
#
#                                _-----=> irqs-off/BH-disabled
#                               / _----=> need-resched
#                              | / _---=> hardirq/softirq
#                              || / _--=> preempt-depth
#                              ||| / _-=> migrate-disable
#                              |||| /     delay
#           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
#              | |         |   |||||     |         |
          <idle>-0       [005] d..4.   521.164922: block_lat: pid=0 delta=8322 stack=STACK:
=> __schedule+0x448/0x7b0
=> schedule+0x5a/0xb0
=> io_schedule+0x42/0x70
=> bit_wait_io+0xd/0x60
=> __wait_on_bit+0x4b/0x140
=> out_of_line_wait_on_bit+0x91/0xb0
=> jbd2_journal_commit_transaction+0x1679/0x1a70
=> kjournald2+0xa9/0x280
=> kthread+0xe9/0x110
=> ret_from_fork+0x2c/0x50

           <...>-2       [004] d..4.   525.184257: block_lat: pid=2 delta=76 stack=STACK:
=> __schedule+0x448/0x7b0
=> schedule+0x5a/0xb0
=> schedule_timeout+0x11a/0x150
=> wait_for_completion_killable+0x144/0x1f0
=> __kthread_create_on_node+0xe7/0x1e0
=> kthread_create_on_node+0x51/0x70
=> create_worker+0xcc/0x1a0
=> worker_thread+0x2ad/0x380
=> kthread+0xe9/0x110
=> ret_from_fork+0x2c/0x50

Steven Rostedt (Google) (5):
      tracing: Simplify calculating entry size using struct_size()
      tracing: Allow stacktraces to be saved as histogram variables
      tracing: Allow synthetic events to pass around stacktraces
      tracing/histogram: Document variable stacktrace
      tracing/histogram: Add simple tests for stacktrace usage of synthetic events

----
 Documentation/trace/histogram.rst                  | 49 +++++++++++++
 kernel/trace/trace.c                               |  2 +-
 kernel/trace/trace.h                               |  4 ++
 kernel/trace/trace_events_hist.c                   | 42 ++++++++---
 kernel/trace/trace_events_synth.c                  | 83 +++++++++++++++++++++-
 kernel/trace/trace_synth.h                         |  1 +
 .../inter-event/trigger-synthetic-event-stack.tc   | 24 +++++++
 .../inter-event/trigger-synthetic-event-syntax.tc  |  6 ++
 8 files changed, 196 insertions(+), 15 deletions(-)
 create mode 100644 tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-synthetic-event-stack.tc