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

Steven Rostedt
2023-01-17T04:41:17Z

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