Re: [PATCH V4 08/13] ftrace: Add perf text poke events for ftrace trampolines

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Mar 04, 2020 at 11:06:28AM +0200, Adrian Hunter wrote:
> Add perf text poke events for ftrace trampolines when created and when
> freed.

If I'm not mistaken that ends up like so:

static void ftrace_update_trampoline(struct ftrace_ops *ops)
{
+       unsigned long trampoline = ops->trampoline;
+
	arch_ftrace_update_trampoline(ops);
+       if (ops->trampoline && ops->trampoline != trampoline &&
> +	    (ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP)) {
> +		/* Add to kallsyms before the perf events */
+               ftrace_add_trampoline_to_kallsyms(ops);
> +		perf_event_ksymbol(PERF_RECORD_KSYMBOL_TYPE_OOL,
> +				   ops->trampoline, ops->trampoline_size, false,
> +				   FTRACE_TRAMPOLINE_SYM);
> +		/*
> +		 * Record the perf text poke event after the ksymbol register
> +		 * event.
> +		 */
> +		perf_event_text_poke((void *)ops->trampoline, NULL, 0,
> +				     (void *)ops->trampoline,
> +				     ops->trampoline_size);
	}
}

And afaict, that is wrong.

The thing is; arch_ftrace_update_trampoline() can actually *update* an
existing trampoline, as per the name. Yes it also creates a trampoline
if there isn't one already, but if there already is one, it will modify
it in-place.

I see the appeal of having this event in generic code; but I'm thinking
you'll need the update even in arch code anyway, at which point it'd
probably be easier to do all of this in arch code.