tracefs: Handle synthetic events with dynamic strings as fields

Commit Message

Steven Rostedt April 11, 2025, 2:04 a.m. UTC

From: "Steven Rostedt (Google)" <[e-mail]>

Dynamic strings may contain "__data_loc" in their types within the format
files, but "__data_loc" is not an acceptable type for the dynamic_events
file.

For example tracing the latency of interrupts and having the name of the
interrupt be reported:

  # sqlhist -e -n irq_lat select irq_handler_entry.name, TIMESTAMP_DELTA_USECS as delta from irq_handler_entry as start join irq_handler_exit as end ON start.irq = end.irq
  echo 's:irq_lat __data_loc char name[]; u64 delta;' >> /sys/kernel/tracing/dynamic_events
  echo 'hist:keys=irq:__arg_1193_2=name,__arg_1193_3=common_timestamp.usecs' >> /sys/kernel/tracing/events/irq/irq_handler_entry/trigger
  echo 'hist:keys=irq:__name_1193_1=$__arg_1193_2,__delta_1193_4=common_timestamp.usecs-$__arg_1193_3:onmatch(irq.irq_handler_entry).trace(irq_lat,$__name_1193_1,$__delta_1193_4)' >> /sys/kernel/tracing/events/irq/irq_handler_exit/trigger

The above writes echo 's:irq_lat __data_loc char name[]; u64 delta;' into
dynamic_events, and the "__data_loc" will cause it to fail. To show this,
adding the '-e' option will execute the command first:

  # sqlhist -e -n irq_lat select irq_handler_entry.name, TIMESTAMP_DELTA_USECS as delta from irq_handler_entry as start join irq_handler_exit as end ON start.irq = end.irq
  [23109.878734] synthetic_events: error: Invalid type
  Command: s:synthetic/irq_lat  __data_loc char name[]; u64 delta;
                                ^

The __data_loc caused the write to fail.

Strip the __data_loc off of fields so that it will produce:

  # sqlhist -e -n irq_lat select irq_handler_entry.name, TIMESTAMP_DELTA_USECS as delta from irq_handler_entry as start join irq_handler_exit as end ON start.irq = end.irq
  echo 's:irq_lat char name[]; u64 delta;' >> /sys/kernel/tracing/dynamic_events
  echo 'hist:keys=irq:__arg_1193_2=name,__arg_1193_3=common_timestamp.usecs' >> /sys/kernel/tracing/events/irq/irq_handler_entry/trigger
  echo 'hist:keys=irq:__name_1193_1=$__arg_1193_2,__delta_1193_4=common_timestamp.usecs-$__arg_1193_3:onmatch(irq.irq_handler_entry).trace(irq_lat,$__name_1193_1,$__delta_1193_4)' >> /sys/kernel/tracing/events/irq/irq_handler_exit/trigger

And not fail to execute.

Signed-off-by: Steven Rostedt (Google) <[e-mail]>
---
 src/tracefs-hist.c | 3 +++
 1 file changed, 3 insertions(+)

Patch

diff --git a/src/tracefs-hist.c b/src/tracefs-hist.c
index bfbcd65..d67f8eb 100644
--- a/src/tracefs-hist.c
+++ b/src/tracefs-hist.c
@@ -1068,6 +1068,9 @@  static int alloc_synthetic_event(struct tracefs_synth *synth)
 
 	for (i = 0; synth->synthetic_fields && synth->synthetic_fields[i]; i++) {
 		field = synth->synthetic_fields[i];
+		/* Strip off __data_loc */
+		if (!strncmp(field, "__data_loc ", sizeof("__data_loc ") - 1))
+			field += sizeof("__data_loc ") - 1;
 		format = tfs_append_string(format, i ? " " : NULL, field);
 	}