linux-kernel - Re: [PATCH 2/2] clk: Add trace events for rate requests

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Date:   Mon, 24 Oct 2022 12:19:26 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org
Subject: Re: [PATCH 2/2] clk: Add trace events for rate requests

On Tue, 18 Oct 2022 15:56:42 +0200
Maxime Ripard <maxime@...no.tech> wrote:

I know I reviewed this but looking at this again, I noticed:


> +DECLARE_EVENT_CLASS(clk_rate_request,
> +
> +	TP_PROTO(struct clk_rate_request *req),
> +
> +	TP_ARGS(req),
> +
> +	TP_STRUCT__entry(
> +		__string(        name, req->core ? req->core->name : "none")
> +		__field(unsigned long,           min                       )
> +		__field(unsigned long,           max                       )
> +		__string(       pname, req->best_parent_hw ? clk_hw_get_name(req->best_parent_hw) : "none" )

It may be best to move the two __string() declarations together. The reason
is that dynamic strings (which __string() is) uses 4 bytes embedded in the
first part of the event. Two bytes for offset, where the dynamic string
exists, and two bytes for the strings length.

On 64 bit machines the above has:

	__string()		4 bytes
	__field(unsigned long)	8 bytes
	__field(unsigned long)	8 bytes
	__string()		4 bytes

and then another unsigned long field below, which is another 8 bytes.

> +		__field(unsigned long,           prate                     )
> +	),
> +
> +	

As compilers tend to use word alignment, the above turns into:

	__string()		4 bytes

	__PADDING__		4 bytes

	__field(unsigned long)	8 bytes
	__field(unsigned long)	8 bytes
	__string()		4 bytes

	__PADDING__		4 bytes

	__field(unsigned long)	8 bytes


Where there will be 8 bytes of padding in that event that wastes precious
ring buffer space. By changing the event to:

	TP_STRUCT__entry(
		__string(        name, req->core ? req->core->name : "none")
		__string(       pname, req->best_parent_hw ? clk_hw_get_name(req->best_parent_hw) : "none" )
		__field(unsigned long,           min                       )
		__field(unsigned long,           max                       )
		__field(unsigned long,           prate                     )
	),

It will turn the size into:

	__string()		4 bytes
	__string()		4 bytes
	__field(unsigned long)	8 bytes
	__field(unsigned long)	8 bytes
	__field(unsigned long)	8 bytes

With no padding and no wasted space.

I would suggest changing this.

-- Steve