tracing: Do not calculate strlen() twice for __string() fields

The TRACE_EVENT() macro handles dynamic strings by having:

  TP_PROTO(struct some_struct *s),
  TP_ARGS(s),
  TP_STRUCT__entry(
        __string(my_string, s->string)
 ),
 TP_fast_assign(
        __assign_str(my_string, s->string);
 )
 TP_printk("%s", __get_str(my_string))

There's even some code that may call a function helper to find the
s->string value. The problem with the above is that the work to get the
s->string is done twice. Once at the __string() and again in the
__assign_str().

The length of the string is calculated via a strlen(), not once, but
twice. Once during the __string() macro and again in __assign_str(). But
the length is actually already recorded in the data location and here's no
reason to call strlen() again.

Just use the saved length that was saved in the __string() code for the
__assign_str() code.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: Masami Hiramatsu <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Cc: Chuck Lever <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

0 files changed, 0 insertions, 0 deletions