On Fri, 2024-02-23 at 10:38 -0500, Steven Rostedt wrote:

On Fri, 23 Feb 2024 12:44:53 +0100
Tobias Waldekranz [off-list ref] wrote:

quoted

Add a basic set of tracepoints:

- switchdev_defer: Fires whenever an operation is enqueued to the
  switchdev workqueue for deferred delivery.

- switchdev_call_{atomic,blocking}: Fires whenever a notification is
  sent to the corresponding switchdev notifier chain.

- switchdev_call_replay: Fires whenever a notification is sent to a
  specific driver's notifier block, in response to a replay request.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
---
 include/trace/events/switchdev.h | 74 ++++++++++++++++++++++++++++++++
 net/switchdev/switchdev.c        | 71 +++++++++++++++++++++++++-----
 2 files changed, 135 insertions(+), 10 deletions(-)
 create mode 100644 include/trace/events/switchdev.h

diff --git a/include/trace/events/switchdev.h b/include/trace/events/switchdev.h
new file mode 100644
index 000000000000..dcaf6870d017
--- /dev/null
+++ b/include/trace/events/switchdev.h

@@ -0,0 +1,74 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM	switchdev
+
+#if !defined(_TRACE_SWITCHDEV_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_SWITCHDEV_H
+
+#include <linux/tracepoint.h>
+#include <net/switchdev.h>
+
+#define SWITCHDEV_TRACE_MSG_MAX 128

128 bytes is awfully big to waste on the ring buffer. What's the average
size of a string?

quoted

+
+DECLARE_EVENT_CLASS(switchdev_call,
+	TP_PROTO(unsigned long val,
+		 const struct switchdev_notifier_info *info,
+		 int err),
+
+	TP_ARGS(val, info, err),
+
+	TP_STRUCT__entry(
+		__field(unsigned long, val)
+		__string(dev, info->dev ? netdev_name(info->dev) : "(null)")
+		__field(const struct switchdev_notifier_info *, info)
+		__field(int, err)
+		__array(char, msg, SWITCHDEV_TRACE_MSG_MAX)
+	),
+
+	TP_fast_assign(
+		__entry->val = val;
+		__assign_str(dev, info->dev ? netdev_name(info->dev) : "(null)");
+		__entry->info = info;
+		__entry->err = err;
+		switchdev_notifier_str(val, info, __entry->msg, SWITCHDEV_TRACE_MSG_MAX);

Is it possible to just store the information in the trace event and then
call the above function in the read stage?

I agree with Steven: it looks like that with the above code the
tracepoint itself will become measurably costily in terms of CPU
cycles: we want to avoid that.

Perhaps using different tracepoints with different notifier_block type
would help? so that each trace point could just copy a few specific
fields.

Cheers,

Paolo