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

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Date:   Thu, 13 Jul 2017 07:22:59 +0200
From:   Mike Galbraith <efault@....de>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, 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>
Subject: Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

On Wed, 2017-07-12 at 21:40 -0700, Andi Kleen wrote:
> On Thu, Jul 13, 2017 at 06:28:43AM +0200, Mike Galbraith wrote:
> > On Wed, 2017-07-12 at 21:15 -0700, Andi Kleen wrote:
> > > On Thu, Jul 13, 2017 at 05:03:00AM +0200, Mike Galbraith wrote:
> > > > On Wed, 2017-07-12 at 15:30 -0700, Andi Kleen wrote:
> > > > > Josh Poimboeuf <jpoimboe@...hat.com> writes:
> > > > > >
> > > > > > The ORC data format does have a few downsides compared to DWARF.  The
> > > > > > ORC unwind tables take up ~1MB more memory than DWARF eh_frame tables.
> > > > > >
> > > > > Can we have an option to just use dwarf instead? For people
> > > > > who don't want to waste a MB+ to solve a problem that doesn't
> > > > > exist (as proven by many years of opensuse kernel experience)
> > > > 
> > > > Sure the dwarf unwinder works well for crashes, but at the price of
> > > > demolishing ftrace/perf utility.
> > > 
> > > You mean the unwind performance?
> > 
> > Yeah, it hurts.. massively, has even been known to kill big boxen.
> 
> Why was that?

Presuming you mean the big box bit, danged if I know, I haven't
personally met that, only the massive overhead.

> > > That's a valid concern, but neither ORC nor dwarf are likely
> > > to address it. However most usages of ftrace/perf shouldn't be that
> > > depending on unwind performance -- just lower the frequency of your
> > > events. 
> > > 
> > > The only possible win is if the win from not using FP code is
> > > significant enough. On the x86 side the only modern CPUs that should really
> > > care about this are Atoms.
> > 
> > Nope, they all care.  Measure performance delta of fast/light stuff.
> 
> Well if your test cares that much about function overhead you may want to try
> LTO. It can get rid of a lot of functions by doing cross file
> inlining.
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/ak/linux-misc.git/log/?h=lto-411-2
> 
> > Maybe I'm expecting too much good stuff to follow, but don't spoil it
> > for me, I think I'm looking at a real winner :)
> 
> It's somewhat surprising. It would be good to under stand why that
> happens. Is it icache misses, data cache misses for the stack, or
> simply more instructions executed, or worse tail calls?

No idea.  It was speculated that it was register loss, but I played
with that, saw nearly zero delta until I stole too many.

	-Mike