Performance schema introduces many conditional branches even when disabled at runtime

Unless MariaDB is built with cmake -DPLUGIN_PERFSCHEMA=NO, there will be some measurable overhead related to some frequently invoked operations, such as mysql_mutex_lock() and mysql_mutex_unlock(). Take the latter for example. It would resolve to the following inline function:

{

  int result;

#ifdef HAVE_PSI_MUTEX_INTERFACE

  if (psi_likely(that->m_psi != NULL))

    PSI_MUTEX_CALL(unlock_mutex)(that->m_psi);

#endif

#ifdef SAFE_MUTEX

  result= safe_mutex_unlock(&that->m_mutex, src_file, src_line);

#else

  result= pthread_mutex_unlock(&that->m_mutex);

#endif

  return result;

}

The call to pfs_unlock_mutex_v1() via the function pointer PSI_MUTEX_CALL(unlock_mutex), as well as the condition preceding that call are executed within the critical section of the mutex.

It would be better to make use of function pointers, say,

return PSI_MUTEX_CALL(unlock_mutex)(that);

and make the function pointer PSI_server->unlock_mutex point to either pthread_mutex_unlock or pfs_unlock_mutex(). If the server is compiled with PLUGIN_PERFSCHEMA=NO, the macro can resolve bypass the function pointer, something like this:

#ifdef HAVE_PSI_MUTEX_INTERFACE

# define mysql_mutex_unlock(M) PSI_MUTEX_CALL(unlock_mutex)(M)

#elif defined SAFE_MUTEX

# define mysql_mutex_unlock(M) \

  safe_mutex_unlock(M, __FILE__, __LINE__)

#else

# define mysql_mutex_unlock(M) pthread_mutex_unlock(M)

#endif

The function pointer would be set to pthread_mutex_unlock by static initialization, and assigned to the instrumented function at the end of pfs_init_func(). In this way, if the performance schema is disabled at runtime, mysql_mutex_unlock() would translate to a simple call to pthread_mutex_unlock() via a function pointer.

While we are at it, I would also consider removing memory bloat like this:

struct st_mysql_mutex

{

  /** The real mutex. */

#ifdef SAFE_MUTEX

  safe_mutex_t m_mutex;

#else

  pthread_mutex_t m_mutex;

#endif

  /**

    The instrumentation hook.

    Note that this hook is not conditionally defined,

    for binary compatibility of the @c mysql_mutex_t interface.

  */

  struct PSI_mutex *m_psi;

};

Which binary compatibility is this about? To my knowledge, all plugins are typically included in the MariaDB Server repository and compiled from source code.