Re: [PATCH] mm: mmap_lock: check trace_mmap_lock_$type_enabled() instead of regcount

[Axel Rasmussen <axelrasmussen@google.com>]

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This looks like a nice cleanup to me. Thanks Vlastimil!

Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>

On Tue, Nov 5, 2024 at 3:35 AM Vlastimil Babka <vbabka@suse.cz> wrote:

> Since 7d6be67cfdd4 ("mm: mmap_lock: replace get_memcg_path_buf() with
> on-stack buffer") we use trace_mmap_lock_reg()/unreg() only to maintain
> an atomic reg_refcount which is checked to avoid performing
> get_mm_memcg_path() in case none of the tracepoints using it is enabled.
>
> This can be achieved directly by putting all the work needed for the
> tracepoint behind the trace_mmap_lock_$type_enabled(), as suggested by
> Documentation/trace/tracepoints.rst and with the following advantages:
>
> - uses the tracepoint's static key instead of evaluating a branch
>
> - the check tracepoint specific, not shared by all of them
>
> - we can get rid of trace_mmap_lock_reg()/unreg() completely
>
> Thus use the trace_..._enabled() check and remove unnecessary code.
>
> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  include/trace/events/mmap_lock.h | 14 ++++--------
>  mm/mmap_lock.c                   | 39 +++++++-------------------------
>  2 files changed, 12 insertions(+), 41 deletions(-)
>
> diff --git a/include/trace/events/mmap_lock.h
> b/include/trace/events/mmap_lock.h
> index f2827f98a44f..bc2e3ad787b3 100644
> --- a/include/trace/events/mmap_lock.h
> +++ b/include/trace/events/mmap_lock.h
> @@ -10,9 +10,6 @@
>
>  struct mm_struct;
>
> -extern int trace_mmap_lock_reg(void);
> -extern void trace_mmap_lock_unreg(void);
> -
>  DECLARE_EVENT_CLASS(mmap_lock,
>
>         TP_PROTO(struct mm_struct *mm, const char *memcg_path, bool write),
> @@ -40,16 +37,15 @@ DECLARE_EVENT_CLASS(mmap_lock,
>  );
>
>  #define DEFINE_MMAP_LOCK_EVENT(name)                                    \
> -       DEFINE_EVENT_FN(mmap_lock, name,                                \
> +       DEFINE_EVENT(mmap_lock, name,                                   \
>                 TP_PROTO(struct mm_struct *mm, const char *memcg_path,  \
>                         bool write),                                    \
> -               TP_ARGS(mm, memcg_path, write),                         \
> -               trace_mmap_lock_reg, trace_mmap_lock_unreg)
> +               TP_ARGS(mm, memcg_path, write))
>
>  DEFINE_MMAP_LOCK_EVENT(mmap_lock_start_locking);
>  DEFINE_MMAP_LOCK_EVENT(mmap_lock_released);
>
> -TRACE_EVENT_FN(mmap_lock_acquire_returned,
> +TRACE_EVENT(mmap_lock_acquire_returned,
>
>         TP_PROTO(struct mm_struct *mm, const char *memcg_path, bool write,
>                 bool success),
> @@ -76,9 +72,7 @@ TRACE_EVENT_FN(mmap_lock_acquire_returned,
>                 __get_str(memcg_path),
>                 __entry->write ? "true" : "false",
>                 __entry->success ? "true" : "false"
> -       ),
> -
> -       trace_mmap_lock_reg, trace_mmap_lock_unreg
> +       )
>  );
>
>  #endif /* _TRACE_MMAP_LOCK_H */
> diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
> index 368b840e7508..f186d57df2c6 100644
> --- a/mm/mmap_lock.c
> +++ b/mm/mmap_lock.c
> @@ -19,43 +19,23 @@ EXPORT_TRACEPOINT_SYMBOL(mmap_lock_released);
>
>  #ifdef CONFIG_MEMCG
>
> -static atomic_t reg_refcount;
> -
>  /*
>   * Size of the buffer for memcg path names. Ignoring stack trace support,
>   * trace_events_hist.c uses MAX_FILTER_STR_VAL for this, so we also use
> it.
>   */
>  #define MEMCG_PATH_BUF_SIZE MAX_FILTER_STR_VAL
>
> -int trace_mmap_lock_reg(void)
> -{
> -       atomic_inc(&reg_refcount);
> -       return 0;
> -}
> -
> -void trace_mmap_lock_unreg(void)
> -{
> -       atomic_dec(&reg_refcount);
> -}
> -
> -#define TRACE_MMAP_LOCK_EVENT(type, mm, ...)                    \
> -       do {                                                    \
> -               char buf[MEMCG_PATH_BUF_SIZE];                  \
> -               get_mm_memcg_path(mm, buf, sizeof(buf));        \
> -               trace_mmap_lock_##type(mm, buf, ##__VA_ARGS__); \
> +#define TRACE_MMAP_LOCK_EVENT(type, mm, ...)                           \
> +       do {                                                            \
> +               if (trace_mmap_lock_##type##_enabled()) {               \
> +                       char buf[MEMCG_PATH_BUF_SIZE];                  \
> +                       get_mm_memcg_path(mm, buf, sizeof(buf));        \
> +                       trace_mmap_lock_##type(mm, buf, ##__VA_ARGS__); \
> +               }                                                       \
>         } while (0)
>
>  #else /* !CONFIG_MEMCG */
>
> -int trace_mmap_lock_reg(void)
> -{
> -       return 0;
> -}
> -
> -void trace_mmap_lock_unreg(void)
> -{
> -}
> -
>  #define TRACE_MMAP_LOCK_EVENT(type, mm, ...)
>      \
>         trace_mmap_lock_##type(mm, "", ##__VA_ARGS__)
>
> @@ -65,16 +45,13 @@ void trace_mmap_lock_unreg(void)
>  #ifdef CONFIG_MEMCG
>  /*
>   * Write the given mm_struct's memcg path to a buffer. If the path cannot
> be
> - * determined or the trace event is being unregistered, empty string is
> written.
> + * determined, empty string is written.
>   */
>  static void get_mm_memcg_path(struct mm_struct *mm, char *buf, size_t
> buflen)
>  {
>         struct mem_cgroup *memcg;
>
>         buf[0] = '\0';
> -       /* No need to get path if no trace event is registered. */
> -       if (!atomic_read(&reg_refcount))
> -               return;
>         memcg = get_mem_cgroup_from_mm(mm);
>         if (memcg == NULL)
>                 return;
> --
> 2.47.0
>
>

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