linux-kernel - Re: [RFC PATCH v3] perf/x86: make perf callchain work without CONFIG_FRAME

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Message-ID: <CACPcB9dKmH4oNR1fAwGndoPMiZsgfFve4g2_d26pYTByGrt75w@mail.gmail.com>
Date:   Fri, 19 Apr 2019 19:47:18 +0800
From:   Kairui Song <kasong@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC PATCH v3] perf/x86: make perf callchain work without CONFIG_FRAME_POINTER

On Fri, Apr 19, 2019 at 5:43 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Apr 19, 2019 at 10:17:49AM +0800, Kairui Song wrote:
> > On Fri, Apr 19, 2019 at 8:58 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > >
> > > I still don't like using regs->bp because it results in different code
> > > paths for FP and ORC.  In the FP case, the regs are treated like real
> > > regs even though they're fake.
> > >
> > > Something like the below would be much simpler.  Would this work?  I don't
> > > know if any other code relies on the fake regs->bp or regs->sp.
> >
> > Works perfectly. My only concern is that FP path used to work very
> > well, not sure it's a good idea to change it, and this may bring some
> > extra overhead for FP path.
>
> Given Josh wrote all that code, I'm fairly sure it is still OK :-)
>
> But also looking at the code in unwind_frame.c, __unwind_start() seems
> to pretty much do what the removed caller_frame_pointer() did (when
> .regs=NULL) but better.
>

OK, with FP we will also need to do a few more extra unwinding,
previously it start directly from the frame of the trace point, now
have to trace back to the trace point first.
If that's fine I could post another update (that will be pretty much
just copy&paste from the Josh's code he posted :P , is this OK?)





--
Best Regards,
Kairui Song