Subject: Re: [PATCH v3 net-next 4/4] net: switchdev: Add tracepoints

From: Tobias Waldekranz <[email-masked]>
Date: Mon, 26 Feb 2024 14:05:20 +0100

On fre, feb 23, 2024 at 10:38, Steven Rostedt <[email-masked]> wrote:
> On Fri, 23 Feb 2024 12:44:53 +0100
> Tobias Waldekranz <[email-masked]> wrote:
>
>> 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 <[email-masked]>
>> ---
>>  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?

I would say the typical message is around 60-80 bytes. Some common examples:

    PORT_OBJ_ADD PORT_VLAN(flags 0x0 orig br0) vid 1 flags 0x27
    PORT_OBJ_DEL HOST_MDB(flags 0x0 orig br0) vid 100 addr 33:33:ff:ff:00:09

The worst case I can think of currently is 95 characters:

    VXLAN_FDB_ADD_TO_DEVICE vid 1000 addr de:ad:be:ef:00:01 added_by_user is_local locked offloaded

>> +
>> +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? There's helpers to pass strings
> around (namely a struct trace_seq *p).

I'm a complete novice when it comes to tracepoint internals. Am I right
in assuming that TP_printk may execute at a much later time than
TP_fast_assign? E.g., the object referenced by __entry->info may very
well have been freed by that time? That at least seems to be what my
naive refactor to replace __entry->msg with __get_buf() suggests :)

If so, the layout of the switchdev_notifier_* structs makes it a bit
cumbersome to clone the notification in the assign phase, as the size of
a specific notification (e.g., switchdev_notifier_port_obj_info) is not
known by the common notification (switchdev_notifier_info). In the case
of switchdev objects, the problem repeats a second time. E.g., the size
of switchdev_obj_port_vlan is not known by switchdev_obj.

> It would require a plugin for libtraceevent if you want to expose it to
> user space tools for trace-cmd and perf though.
>
> Another possibility is if this event will not race with other events on he
> same CPU, you could create a per-cpu buffer, write into that, and then use
> __string() and __assign_str() to save it, as traces happen with preemption
> disabled.

But bottom halves are still enabled I suppose? Notifications can be
generated both from process context (e.g., users configuring the bridge
with iproute2), and from bridge packet processing (e.g., adding new
neighbors to the FDB). So I don't think that would work in this case.

> -- Steve

Thanks for taking the time!

>> +	),
>> +
>> +	TP_printk("dev %s %s -> %d", __get_str(dev), __entry->msg, __entry->err)
>> +);
>> +
>> +DEFINE_EVENT(switchdev_call, switchdev_defer,
>> +	TP_PROTO(unsigned long val,
>> +		 const struct switchdev_notifier_info *info,
>> +		 int err),
>> +
>> +	TP_ARGS(val, info, err)
>> +);
>> +
>> +DEFINE_EVENT(switchdev_call, switchdev_call_atomic,
>> +	TP_PROTO(unsigned long val,
>> +		 const struct switchdev_notifier_info *info,
>> +		 int err),
>> +
>> +	TP_ARGS(val, info, err)
>> +);
>> +
>> +DEFINE_EVENT(switchdev_call, switchdev_call_blocking,
>> +	TP_PROTO(unsigned long val,
>> +		 const struct switchdev_notifier_info *info,
>> +		 int err),
>> +
>> +	TP_ARGS(val, info, err)
>> +);
>> +
>> +DEFINE_EVENT(switchdev_call, switchdev_call_replay,
>> +	TP_PROTO(unsigned long val,
>> +		 const struct switchdev_notifier_info *info,
>> +		 int err),
>> +
>> +	TP_ARGS(val, info, err)
>> +);
>> +
>> +#endif /* _TRACE_SWITCHDEV_H */
>> +