linux-kernel - Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

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Message-ID: <20170712225402.q45uhl4s6hpzajgg@alap3.anarazel.de>
Date:   Wed, 12 Jul 2017 15:54:02 -0700
From:   Andres Freund <andres@...razel.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>, Jiri Slaby <jslaby@...e.cz>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

On 2017-07-12 17:40:45 -0500, Josh Poimboeuf wrote:
> On Wed, Jul 12, 2017 at 03:36:05PM -0700, Andres Freund wrote:
> > Hi,
> > 
> > On 2017-07-12 17:32:25 -0500, Josh Poimboeuf wrote:
> > > If you want perf to be able to use ORC instead of DWARF for user space
> > > binaries, that's not currently possible, though I don't see any
> > > technical blockers for doing so.  Perf would need to be taught to read
> > > ORC data.
> > 
> > Right, that's what I was hoping for.
> > 
> > 
> > > And I think it should be possible to convert DWARF to ORC, assuming the
> > > DWARF data is trusted.  We could probably add an objtool subcommand for
> > > that.
> > 
> > That'd be pretty helpful.
> 
> Can I ask why?  Is DWARF too slow, or is it something else?

Both. Dwarf is really slow and uses a lot of space - on a bigger machine
it's often nearly unusable. Secondly dwarf isn't available for BPF based
stuff, IIUC because the kernel has to create a full backtrace there
(rather than saving enough data that userland can do so). Which wasn't
"allowed" to be done in-kernel w/ dwarf, just fp so far.

- Andres