linux-kernel - Re: [PATCH] perf trace: Handle legacy syscalls

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Date:	Fri, 20 Mar 2015 10:12:33 -0600
From:	David Ahern <dsahern@...il.com>
To:	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
CC:	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH] perf trace: Handle legacy syscalls

On 3/20/15 10:06 AM, Arnaldo Carvalho de Melo wrote:
> Em Fri, Mar 20, 2015 at 09:48:10AM -0600, David Ahern escreveu:
>> On 3/20/15 9:32 AM, Arnaldo Carvalho de Melo wrote:
>>> But then I don't think we need to do that, i.e. we can just have a
>>> boolean we set at some point to tell that we need to skip the first
>>> entry.
>>>
>>> I'll try to cook up a patch for that.
>>
>> why have a boolean that is checked every time through the loop when its
>> value will always be the same for a give run? why not just remove the entry
>> as I suggested?
>
> First gut reaction was: hey, we got that from a library, libtraceevent,
> that could have other users, etc, better not to change it behind its
> back.
>
> I.e. something like this, based on your approach, but not modifying the
> data structures received from libtraceevent:
>
> diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> index 6af6bcec930e..001c6ae9a1b1 100644
> --- a/tools/perf/builtin-trace.c
> +++ b/tools/perf/builtin-trace.c
> @@ -1135,6 +1135,8 @@ static struct syscall_fmt *syscall_fmt__find(const char *name)
>
>   struct syscall {
>   	struct event_format *tp_format;
> +	int		    nr_args;
> +	struct format_field *args;
>   	const char	    *name;
>   	bool		    filtered;
>   	bool		    is_exit;
> @@ -1442,14 +1444,14 @@ static int syscall__set_arg_fmts(struct syscall *sc)
>   	struct format_field *field;
>   	int idx = 0;
>
> -	sc->arg_scnprintf = calloc(sc->tp_format->format.nr_fields - 1, sizeof(void *));
> +	sc->arg_scnprintf = calloc(sc->nr_args, sizeof(void *));
>   	if (sc->arg_scnprintf == NULL)
>   		return -1;
>
>   	if (sc->fmt)
>   		sc->arg_parm = sc->fmt->arg_parm;
>
> -	for (field = sc->tp_format->format.fields->next; field; field = field->next) {
> +	for (field = sc->args; field; field = field->next) {
>   		if (sc->fmt && sc->fmt->arg_scnprintf[idx])
>   			sc->arg_scnprintf[idx] = sc->fmt->arg_scnprintf[idx];
>   		else if (field->flags & FIELD_IS_POINTER)
> @@ -1515,6 +1517,14 @@ static int trace__read_syscall_info(struct trace *trace, int id)
>   	if (sc->tp_format == NULL)
>   		return -1;
>
> +	sc->args = sc->tp_format->format.fields;
> +	sc->nr_args = sc->tp_format->format.nr_fields;
> +	/* drop nr field - not relevant here; does not exist on older kernels */
> +	if (sc->args && strcmp(sc->args->name, "nr") == 0) {
> +		sc->args = sc->args->next;
> +		--sc->nr_args;
> +	}
> +
>   	sc->is_exit = !strcmp(name, "exit_group") || !strcmp(name, "exit");
>
>   	return syscall__set_arg_fmts(sc);
> @@ -1537,7 +1547,7 @@ static size_t syscall__scnprintf_args(struct syscall *sc, char *bf, size_t size,
>   	unsigned char *p;
>   	unsigned long val;
>
> -	if (sc->tp_format != NULL) {
> +	if (sc->args != NULL) {
>   		struct format_field *field;
>   		u8 bit = 1;
>   		struct syscall_arg arg = {
> @@ -1547,7 +1557,7 @@ static size_t syscall__scnprintf_args(struct syscall *sc, char *bf, size_t size,
>   			.thread = thread,
>   		};
>
> -		for (field = sc->tp_format->format.fields->next; field;
> +		for (field = sc->args; field;
>   		     field = field->next, ++arg.idx, bit <<= 1) {
>   			if (arg.mask & bit)
>   				continue;
>

Seems reasonable.

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