linux-kernel - Re: [PATCH v1 07/12] dyndbg: repack struct

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Date:   Fri, 3 Nov 2023 19:49:02 -0600
From:   jim.cromie@...il.com
To:     Łukasz Bartosik <lb@...ihalf.com>
Cc:     Jason Baron <jbaron@...mai.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Douglas Anderson <dianders@...omium.org>,
        Guenter Roeck <groeck@...gle.com>,
        Yaniv Tzoreff <yanivt@...gle.com>,
        Benson Leung <bleung@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Sean Paul <seanpaul@...omium.org>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
        upstream@...ihalf.com
Subject: Re: [PATCH v1 07/12] dyndbg: repack struct _ddebug

On Fri, Nov 3, 2023 at 7:10 AM Łukasz Bartosik <lb@...ihalf.com> wrote:
>
> From: Jim Cromie <jim.cromie@...il.com>
>
> Move the JUMP_LABEL to the top of the struct, since theyre both
> align(8) and this closes a pahole (unfortunately trading for padding,
> but still).
>
> Signed-off-by: Jim Cromie <jim.cromie@...il.com>

let me add, I havent really tested this, nevermind thorough.
specifically, I didnt look for any offset dependence on the static-key
inside their container.
Conversely, maybe theres a free default or something in there.

> ---
>  include/linux/dynamic_debug.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
> index 497130816e9c..b9237e4ecd1b 100644
> --- a/include/linux/dynamic_debug.h
> +++ b/include/linux/dynamic_debug.h
> @@ -14,6 +14,12 @@
>   * the special section is treated as an array of these.
>   */
>  struct _ddebug {
> +#ifdef CONFIG_JUMP_LABEL
> +       union {
> +               struct static_key_true dd_key_true;
> +               struct static_key_false dd_key_false;
> +       } key;
> +#endif
>         /*
>          * These fields are used to drive the user interface
>          * for selecting and displaying debug callsites.
> @@ -53,12 +59,6 @@ struct _ddebug {
>  #define _DPRINTK_FLAGS_DEFAULT 0
>  #endif
>         unsigned int flags:8;
> -#ifdef CONFIG_JUMP_LABEL
> -       union {
> -               struct static_key_true dd_key_true;
> -               struct static_key_false dd_key_false;
> -       } key;
> -#endif
>  } __attribute__((aligned(8)));
>
>  enum class_map_type {
> --
> 2.42.0.869.gea05f2083d-goog
>