From: Ingo Molnar <mingo@kernel.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
live-patching@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
Andy Lutomirski <luto@kernel.org>, Jiri Slaby <jslaby@suse.cz>,
"H. Peter Anvin" <hpa@zytor.com>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2 0/8] x86: undwarf unwinder
Date: Thu, 29 Jun 2017 09:55:47 +0200 [thread overview]
Message-ID: <20170629075547.y24s7aq4nqwt2rll@gmail.com> (raw)
In-Reply-To: <cover.1498659915.git.jpoimboe@redhat.com>
* Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> Undwarf vs frame pointers
> -------------------------
>
> With frame pointers enabled, GCC adds instrumentation code to every
> function in the kernel. The kernel's .text size increases by about
> 3.2%, resulting in a broad kernel-wide slowdown. Measurements by Mel
> Gorman [1] have shown a slowdown of 5-10% for some workloads.
>
> In contrast, the undwarf unwinder has no effect on text size or runtime
> performance, because the debuginfo is out of band. So if you disable
> frame pointers and enable undwarf, you get a nice performance
> improvement across the board, and still have reliable stack traces.
>
> Another benefit of undwarf compared to frame pointers is that it can
> reliably unwind across interrupts and exceptions. Frame pointer based
> unwinds can skip the caller of the interrupted function if it was a leaf
> function or if the interrupt hit before the frame pointer was saved.
>
> The main disadvantage of undwarf compared to frame pointers is that it
> needs more memory to store the undwarf table: roughly 3-5MB depending on
> the kernel config.
Note that it's not just a performance improvement, but also an instruction cache
locality improvement: 3.2% .text savings almost directly transform into a
similarly sized reduction in cache footprint. That can transform to even higher
speedups for workloads whose cache locality is borderline.
I _really_ like this feature, and the independence of the debuginfo data format.
Logistically it's too bad we are 3 days away from the merge window to be able to
pick this up:
> 56 files changed, 3466 insertions(+), 1765 deletions(-)
OTOH most of the diffstat is in objtool.
Any objections to applying the first 3 objtool patches straight away and see
whether anything breaks? That would significantly reduce the size of the rest of
the patch set.
> I'm not tied to the 'undwarf' name, other naming ideas are welcome.
Ha, a new bike shed painting job! ;-)
I think 'undwarf' isn't a bad name, it's short, catchy and describes the purpose
of the effort.
But I cannot resist some other suggestions, after 'elf' and 'dwarf' the obvious
candidates from the peoples of Middle-earth would be:
- 'Hobbit'
- 'Eagle'
- 'Ent'
- 'Dragon'
- 'Troll'
- 'Ainur'
'struct troll_entry' has a certain charm to it.
'Eagle' is even nicer IMHO: larger than a dwarf but so much faster - and eagles
are beautiful! Plus the name is 2 letters shorter than 'unwdwarf', win-win.
Thanks,
Ingo
next prev parent reply other threads:[~2017-06-29 7:56 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-28 15:11 [PATCH v2 0/8] x86: undwarf unwinder Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 1/8] objtool: move checking code to check.c Josh Poimboeuf
2017-06-30 13:12 ` [tip:core/objtool] objtool: Move " tip-bot for Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 2/8] objtool, x86: add several functions and files to the objtool whitelist Josh Poimboeuf
2017-06-30 13:12 ` [tip:core/objtool] objtool, x86: Add " tip-bot for Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 3/8] objtool: stack validation 2.0 Josh Poimboeuf
2017-06-30 8:32 ` Ingo Molnar
2017-06-30 13:23 ` Josh Poimboeuf
2017-06-30 13:26 ` Josh Poimboeuf
2017-06-30 14:09 ` [PATCH] objtool: silence warnings for functions which use iret Josh Poimboeuf
2017-06-30 17:49 ` [tip:core/objtool] objtool: Silence warnings for functions which use IRET tip-bot for Josh Poimboeuf
2017-06-30 13:13 ` [tip:core/objtool] objtool: Implement stack validation 2.0 tip-bot for Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 4/8] objtool: add undwarf debuginfo generation Josh Poimboeuf
2017-06-29 7:14 ` Ingo Molnar
2017-06-29 13:40 ` Josh Poimboeuf
2017-06-29 7:25 ` Ingo Molnar
2017-06-29 14:04 ` Josh Poimboeuf
2017-06-29 14:46 ` Ingo Molnar
2017-06-29 15:06 ` Josh Poimboeuf
2017-07-06 20:36 ` Josh Poimboeuf
2017-07-07 9:44 ` Ingo Molnar
2017-07-11 2:58 ` Josh Poimboeuf
2017-07-11 8:40 ` Ingo Molnar
2017-06-28 15:11 ` [PATCH v2 5/8] objtool, x86: add facility for asm code to provide unwind hints Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 6/8] x86/entry: add unwind hint annotations Josh Poimboeuf
2017-06-29 17:53 ` Josh Poimboeuf
2017-06-29 18:50 ` Andy Lutomirski
2017-06-29 19:05 ` Josh Poimboeuf
2017-06-29 21:09 ` Andy Lutomirski
2017-06-29 21:41 ` Josh Poimboeuf
2017-06-29 22:59 ` Andy Lutomirski
2017-06-30 2:12 ` Josh Poimboeuf
2017-06-30 5:05 ` Andy Lutomirski
2017-06-30 5:41 ` Andy Lutomirski
2017-06-30 13:11 ` Josh Poimboeuf
2017-06-30 15:44 ` Andy Lutomirski
2017-06-30 15:55 ` Josh Poimboeuf
2017-06-30 15:56 ` Andy Lutomirski
2017-06-30 16:16 ` Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 7/8] x86/asm: add unwind hint annotations to sync_core() Josh Poimboeuf
2017-06-28 15:11 ` [PATCH v2 8/8] x86/unwind: add undwarf unwinder Josh Poimboeuf
2017-06-29 7:55 ` Ingo Molnar [this message]
2017-06-29 14:12 ` [PATCH v2 0/8] x86: " Josh Poimboeuf
2017-06-29 19:13 ` Josh Poimboeuf
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