Re: [PATCH] tracepoints: Do not punish non static call users

Mathieu Desnoyers
2021-02-04T19:59:45Z

----- On Feb 4, 2021, at 2:17 PM, rostedt rostedt@goodmis.org wrote:

> With static calls, a tracepoint can call the callback directly if there is
> only one callback registered to that tracepoint. When there is more than
> one, the static call will call the tracepoint's "iterator" function, which
> needs to reload the tracepoint's "funcs" array again, as it could have
> changed since the first time it was loaded.
>
> But an arch without static calls is punished by having to load the
> tracepoint's "funcs" array twice. Once in the DO_TRACE macro, and once
> again in the iterator macro.

In addition to loading it, it needs to test it against NULL twice.

>
> For archs without static calls, there's no reason to load the array macro
> in the first place, since the iterator function will do it anyway.
>
> Change the __DO_TRACE_CALL() macro to do the double call only for

Do you mean "double call" or "double load and NULL check" ?

> architectures with static calls, and just call the iterator function
> directly for architectures without static calls.
>
> [ Tested only on architectures with static calls, will test on those
> without later ]
>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index dc1d4c612cc3..966bfa6a861c 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -152,9 +152,18 @@ static inline struct tracepoint
> *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> #ifdef TRACEPOINTS_ENABLED
>
> #ifdef CONFIG_HAVE_STATIC_CALL
> -#define __DO_TRACE_CALL(name) static_call(tp_func_##name)
> +#define __DO_TRACE_CALL(name, args) \
> + do { \
> + struct tracepoint_func *it_func_ptr; \
> + it_func_ptr = \
> + rcu_dereference_raw((&__tracepoint_##name)->funcs); \
> + if (it_func_ptr) { \
> + __data = (it_func_ptr)->data; \
> + static_call(tp_func_##name)(args); \
> + } \
> + } while (0)
> #else
> -#define __DO_TRACE_CALL(name) __traceiter_##name
> +#define __DO_TRACE_CALL(name, args) __traceiter_##name(args)

Also, we may want to comment or annotate the "void *data" argument of
__traceiter_##_name() to state that it is unused.

Thanks,

Mathieu

> #endif /* CONFIG_HAVE_STATIC_CALL */
>
> /*
> @@ -168,7 +177,6 @@ static inline struct tracepoint
> *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> */
> #define __DO_TRACE(name, proto, args, cond, rcuidle) \
> do { \
> - struct tracepoint_func *it_func_ptr; \
> int __maybe_unused __idx = 0; \
> void *__data; \
> \
> @@ -190,12 +198,7 @@ static inline struct tracepoint
> *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> rcu_irq_enter_irqson(); \
> } \
> \
> - it_func_ptr = \
> - rcu_dereference_raw((&__tracepoint_##name)->funcs); \
> - if (it_func_ptr) { \
> - __data = (it_func_ptr)->data; \
> - __DO_TRACE_CALL(name)(args); \
> - } \
> + __DO_TRACE_CALL(name, TP_ARGS(args)); \
> \
> if (rcuidle) { \
> rcu_irq_exit_irqson(); \

--
Mathieu Desnoyers
EfficiOS Inc.