Re: [PATCH] tracing: probe-events: Add comments about entry data storing code

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 27 Feb 2025 09:45:50 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Add comments about entry data storing code to __store_entry_arg() and
> traceprobe_get_entry_data_size(). These are a bit complicated because of
> building the entry data storing code and scanning it.
> 
> This just add comments, no behavior change.
> 
> Reported-by: Steven Rostedt <rostedt@goodmis.org>
> Closes: https://lore.kernel.org/all/20250226102223.586d7119@gandalf.local.home/
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  kernel/trace/trace_probe.c |   28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
> index 8f58ee1e8858..2eeecb6c95ee 100644
> --- a/kernel/trace/trace_probe.c
> +++ b/kernel/trace/trace_probe.c
> @@ -770,6 +770,10 @@ static int check_prepare_btf_string_fetch(char *typename,
>  
>  #ifdef CONFIG_HAVE_FUNCTION_ARG_ACCESS_API
>  
> +/*
> + * Add the entry code to store the 'argnum'th parameter and return the offset
> + * in the entry data buffer where the data will be stored.
> + */
>  static int __store_entry_arg(struct trace_probe *tp, int argnum)
>  {
>  	struct probe_entry_arg *earg = tp->entry_arg;
> @@ -793,6 +797,20 @@ static int __store_entry_arg(struct trace_probe *tp, int argnum)
>  		tp->entry_arg = earg;
>  	}
>  
> +	/*
> +	 * The entry code array is repeating the pair of
> +	 * [FETCH_OP_ARG(argnum)][FETCH_OP_ST_EDATA(offset of entry data buffer)]
> +	 * and the rest of entries are filled with [FETCH_OP_END].
> +	 *
> +	 * To reduce the redundant function parameter fetching, we scan the entry
> +	 * code array to find the FETCH_OP_ARG which already fetches the 'argnum'
> +	 * parameter. If it doesn't match, update 'offset' to find the last
> +	 * offset.
> +	 * If we find the FETCH_OP_END without matching FETCH_OP_ARG entry, we
> +	 * will save the entry with FETCH_OP_ARG and FETCH_OP_ST_EDATA, and
> +	 * return data offset so that caller can find the data offset in the entry
> +	 * data buffer.
> +	 */

So I'm still not 100% sure what is happening here, but at least these
comments can help me figure it out if I spend some time gawking at the code
a bit more ;-)

Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve


>  	offset = 0;
>  	for (i = 0; i < earg->size - 1; i++) {
>  		switch (earg->code[i].op) {
> @@ -826,6 +844,16 @@ int traceprobe_get_entry_data_size(struct trace_probe *tp)
>  	if (!earg)
>  		return 0;
>  
> +	/*
> +	 * earg->code[] array has an operation sequence which is run in
> +	 * the entry handler.
> +	 * The sequence stopped by FETCH_OP_END and each data stored in
> +	 * the entry data buffer by FETCH_OP_ST_EDATA. The FETCH_OP_ST_EDATA
> +	 * stores the data at the data buffer + its offset, and all data are
> +	 * "unsigned long" size. The offset must be increased when a data is
> +	 * stored. Thus we need to find the last FETCH_OP_ST_EDATA in the
> +	 * code array.
> +	 */
>  	for (i = 0; i < earg->size; i++) {
>  		switch (earg->code[i].op) {
>  		case FETCH_OP_END: