linux-kernel - Re: [PATCH V6 08/15] ftrace: Add perf text poke events for ftrace trampolines

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Date:   Wed, 22 Apr 2020 13:46:59 +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 V6 08/15] ftrace: Add perf text poke events for ftrace
 trampolines

On Wed, Apr 22, 2020 at 10:39:06AM +0300, Adrian Hunter wrote:
> On 21/04/20 4:45 pm, Peter Zijlstra wrote:
> > On Sun, Apr 05, 2020 at 11:13:20PM +0300, Adrian Hunter wrote:
> >> Add perf text poke events for ftrace trampolines when created and when
> >> freed.
> > 
> > Maybe also put in a little more detail on the various events. Because
> > arch_ftrace_update_trampoline() can also generate text_poke_bp() events,
> > to update an existing trampoline.
> > 
> > A diagram, like with the kprobes thing perhaps.
> 
> How about adding this:
> 
> There can be 3 text_poke events for ftrace trampolines:
> 
> 1. NULL -> trampoline
>    By ftrace_update_trampoline() when !ops->trampoline
>    Trampoline created
> 
> 2. [e.g. on x86] CALL rel32 -> CALL rel32
>    By arch_ftrace_update_trampoline() when ops->trampoline and
>                         ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP
>    [e.g. on x86] via text_poke_bp() which generates text poke events
>    Trampoline-called function target updated
> 
> 3. trampoline -> NULL
>    By ftrace_trampoline_free() when ops->trampoline and
>                  ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP
>    Trampoline freed

Yes, very nice. Thanks!