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

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Message-ID: <20200401100955.GY20713@hirez.programming.kicks-ass.net>
Date:   Wed, 1 Apr 2020 12:09:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 08/13] ftrace: Add perf text poke events for ftrace
 trampolines

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.