Re: [PATCH v7 bpf-next 06/10] tracepoint: compute num_args at build time

Mathieu Desnoyers
2018-03-28T19:47:56Z

----- On Mar 28, 2018, at 3:38 PM, rostedt rostedt@goodmis.org wrote:

> On Wed, 28 Mar 2018 15:32:20 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
>> -#define __DO_TRACE(tp, proto, args, cond, rcucheck) \
>> +#define __DO_TRACE(name, proto, args, cond, rcucheck) \
>> do { \
>> struct tracepoint_func *it_func_ptr; \
>> void *it_func; \
>> @@ -140,7 +140,7 @@ extern void syscall_unregfunc(void);
>> if (rcucheck) \
>> rcu_irq_enter_irqson(); \
>> rcu_read_lock_sched_notrace(); \
>> - it_func_ptr = rcu_dereference_sched((tp)->funcs); \
>> + it_func_ptr = rcu_dereference_sched(__trace_##name##_funcs); \
>
> What we lose in data size, we may make up for in text (which is even
> more important). This will remove a dereference in the hot path.
>
> I'll make a few builds and run size on the vmlinux images to see how
> this pans out.

I like the approach. If it passes testing, I think it's a valuable improvement
to lessen cache footprint of tracepoint when tracing is active.

Thanks!

Mathieu

>
> -- Steve
>
>
>> if (it_func_ptr) { \
> > do { \

--
Mathieu Desnoyers
EfficiOS Inc.