Messages in this thread Patch in this message |  | | Date | Mon, 16 Jan 2023 23:40:14 -0500 | | From | Steven Rostedt <> | | Subject | [PATCH 4/5] tracing/histogram: Document variable stacktrace |
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From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
Add a little documentation (and a useful example) of how a stacktrace can be used within a histogram variable.
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org> --- Documentation/trace/histogram.rst | 49 +++++++++++++++++++++++++++++++ 1 file changed, 49 insertions(+)
diff --git a/Documentation/trace/histogram.rst b/Documentation/trace/histogram.rst index f95459aa984f..5b7f985f35d9 100644 --- a/Documentation/trace/histogram.rst +++ b/Documentation/trace/histogram.rst @@ -1786,6 +1786,8 @@ or assigned to a variable and referenced in a subsequent expression:: # echo 'hist:keys=next_pid:us_per_sec=1000000 ...' >> event/trigger # echo 'hist:keys=next_pid:timestamp_secs=common_timestamp/$us_per_sec ...' >> event/trigger +Variables can even hold stacktraces, which are useful with synthetic events. + 2.2.2 Synthetic Events ---------------------- @@ -1940,6 +1942,53 @@ the ".buckets" modifier and specify a size (in this case groups of 10). Entries: 16 Dropped: 0 +To save stacktraces, create a synthetic event with a field of type "unsigned long[]" +or even just "long[]". For example, to see how long a task is blocked in an +uninterruptible state: + + # cd /sys/kernel/tracing + # echo 's:block_lat pid_t pid; u64 delta; unsigned long[] stack;' > dynamic_events + # echo 'hist:keys=next_pid:ts=common_timestamp.usecs,st=stacktrace if prev_state == 2' >> events/sched/sched_switch/trigger + # echo 'hist:keys=prev_pid:delta=common_timestamp.usecs-$ts,s=$st:onmax($delta).trace(block_lat,prev_pid,$delta,$s)' >> events/sched/sched_switch/trigger + # echo 1 > events/synthetic/block_lat/enable + # 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 + 2.2.3 Hist trigger 'handlers' and 'actions' ------------------------------------------- -- 2.39.0
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