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

Adrian Hunter
2020-04-01T10:43:40Z

On 1/04/20 1:09 pm, Peter Zijlstra wrote:
> 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.

For x86, we use text_poke_bp for updates which already does text_poke events
via text_poke_bp_batch.

It might be reasonable to assume other architectures will also need to put
updates through a common text poker which will take care of text_poke events.

The V3 patch had it in arch code but Steven Rostedt asked why it couldn't be
in ftrace_update)trampoline, so I moved it.