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

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Message-ID: <20240223103815.35fdf430@gandalf.local.home>
Date: Fri, 23 Feb 2024 10:38:15 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Tobias Waldekranz <tobias@...dekranz.com>
Cc: davem@...emloft.net, kuba@...nel.org, roopa@...dia.com,
 razor@...ckwall.org, bridge@...ts.linux.dev, netdev@...r.kernel.org,
 jiri@...nulli.us, ivecera@...hat.com, mhiramat@...nel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v3 net-next 4/4] net: switchdev: Add tracepoints

On Fri, 23 Feb 2024 12:44:53 +0100
Tobias Waldekranz <tobias@...dekranz.com> 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 <tobias@...dekranz.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?

> +
> +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).

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.

-- Steve

> +	),
> +
> +	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 */
> +