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Message-ID: <7e54c145-28d2-5175-6882-9f19e3939f13@intel.com>
Date: Wed, 1 Apr 2020 13:42:50 +0300
From: Adrian Hunter <adrian.hunter@...el.com>
To: Peter Zijlstra <peterz@...radead.org>
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 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.
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