linux-kernel - Re: [PATCH 11/20 v2] tracing: Add symbol type to function based events

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Date:   Thu, 8 Feb 2018 12:20:31 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        linux-rt-users@...r.kernel.org, linux-trace-users@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Clark Williams <williams@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH 11/20 v2] tracing: Add symbol type to function based
 events

On Wed, Feb 07, 2018 at 03:24:13PM -0500, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> Add a special type "symbol" that will use %pS to display the field of a
> function based event.
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>  Documentation/trace/function-based-events.rst | 26 +++++++++++++++++++++++++-
>  kernel/trace/trace_event_ftrace.c             | 13 ++++++++++---
>  2 files changed, 35 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/trace/function-based-events.rst b/Documentation/trace/function-based-events.rst
> index bdb28f433bfb..f18c8f3ef330 100644
> --- a/Documentation/trace/function-based-events.rst
> +++ b/Documentation/trace/function-based-events.rst
> @@ -98,7 +98,8 @@ as follows:
>   ATOM := 'u8' | 'u16' | 'u32' | 'u64' |
>           's8' | 's16' | 's32' | 's64' |
>           'x8' | 'x16' | 'x32' | 'x64' |
> -         'char' | 'short' | 'int' | 'long' | 'size_t'
> +         'char' | 'short' | 'int' | 'long' | 'size_t' |
> +	 'symbol'
>  
>   FIELD := <name> | <name> INDEX | <name> OFFSET | <name> OFFSET INDEX
>  
> @@ -243,3 +244,26 @@ The above will take the parameter value, add it by 4, then index it by two
>  8 byte words. It's the same in C as: (u64 *)((void *)param + 4)[2]
>  
>   Note: "int skb[32]" is the same as "int skb+4[31]".
> +
> +
> +Symbols (function names)
> +========================
> +
> +To display kallsyms "%pS" type of output, use the special type "symbol".
> +
> +Again, using gdb to find the offset of the "func" field of struct work_struct
> +
> +(gdb) printf "%d\n", &((struct work_struct *)0)->func
> +24

you could also use Arnaldo's pahole for this, seems like less typing:

  $ pahole ./vmlinux -C work_struct
  die__process_function: tag not supported (INVALID)!
  struct work_struct {
          atomic_long_t              data;                 /*     0     8 */
          struct list_head           entry;                /*     8    16 */
          work_func_t                func;                 /*    24     8 */

it's in 'dwarves' package

jirka