linux-kernel - Re: [PATCH 2/4] tracing/user_events: Introduce multi-format events

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Date: Sat, 27 Jan 2024 00:01:04 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Beau Belgrave <beaub@...ux.microsoft.com>
Cc: rostedt@...dmis.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com
Subject: Re: [PATCH 2/4] tracing/user_events: Introduce multi-format events

On Tue, 23 Jan 2024 22:08:42 +0000
Beau Belgrave <beaub@...ux.microsoft.com> wrote:

> Add a register_name (reg_name) to the user_event struct which allows for
> split naming of events. We now have the name that was used to register
> within user_events as well as the unique name for the tracepoint. Upon
> registering events ensure matches based on first the reg_name, followed
> by the fields and format of the event. This allows for multiple events
> with the same registered name to have different formats. The underlying
> tracepoint will have a unique name in the format of {reg_name}:[unique_id].
> 
> For example, if both "test u32 value" and "test u64 value" are used with
> the USER_EVENT_REG_MULTI_FORMAT the system would have 2 unique
> tracepoints. The dynamic_events file would then show the following:
>   u:test u64 count
>   u:test u32 count
> 
> The actual tracepoint names look like this:
>   test:[d5874fdac44]
>   test:[d5914662cd4]
> 
> Both would be under the new user_events_multi system name to prevent the
> older ABI from being used to squat on multi-formatted events and block
> their use.
[...]
> @@ -1923,6 +1972,39 @@ static int user_event_trace_register(struct user_event *user)
>  	return ret;
>  }
>  
> +static int user_event_set_tp_name(struct user_event *user)
> +{
> +	lockdep_assert_held(&user->group->reg_mutex);
> +
> +	if (EVENT_MULTI_FORMAT(user->reg_flags)) {
> +		char *multi_name;
> +		int len;
> +
> +		len = snprintf(NULL, 0, "%s:[%llx]", user->reg_name,
> +			       user->group->multi_id) + 1;
> +
> +		multi_name = kzalloc(len, GFP_KERNEL_ACCOUNT);
> +
> +		if (!multi_name)
> +			return -ENOMEM;
> +
> +		snprintf(multi_name, len, "%s:[%llx]", user->reg_name,
> +			 user->group->multi_id);

OK, so the each different event has suffixed name. But this will
introduce non C-variable name.

Steve, do you think your library can handle these symbols? It will
be something like "event:[1]" as the event name.
Personally I like "event.1" style. (of course we need to ensure the
user given event name is NOT including such suffix numbers)

Thank you.

-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>