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

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Date:   Thu, 13 Jul 2017 10:51:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andres Freund <andres@...razel.de>, 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>, Mike Galbraith <efault@....de>
Subject: Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

On Thu, Jul 13, 2017 at 10:50:15AM +0200, Peter Zijlstra wrote:
> On Thu, Jul 13, 2017 at 09:12:53AM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 12, 2017 at 05:32:25PM -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.
> > 
> > So the problem with userspace stuff is that the unwind data isn't
> > readily available from NMI context.
> > 
> > So the kernel unwinder will trigger a fault and abort.
> > 
> > The very best we can hope for is using the EH [*] stuff that all
> > binaries actually have _and_ map. The only problem is that most programs
> > don't actually use the EH stuff much so while its mapped, its not
> > actually paged in, so we're still stuck.
> 
> One gloriously ugly hack would be to delay the userspace unwind to
> return-to-userspace, at which point we have a schedulable context and
> can take faults.
> 
> Of course, then you have to somehow identify this later unwind sample
> with all relevant prior samples and stitch the whole thing back
> together, but that should be doable.
> 
> In fact, it would be at all hard to do, just queue a task_work from the

+not

> NMI and have that do the EH based unwind.