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Documentation: x86: convert orc-unwinder.txt to reST 

This converts the plain text documentation to reStructuredText format and
add it to Sphinx TOC tree. No essential content change.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Changbin Du 2019-05-08 23:21:32 +08:00 committed by Jonathan Corbet
parent 1cd7af509d
commit 9d12f58fe9
2 changed files with 16 additions and 12 deletions
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1
Documentation/x86/index.rst
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@ -14,6 +14,7 @@ x86-specific Documentation
kernel-stacks
entry_64
earlyprintk
orc-unwinder
zero-page
tlb
mtrr

27
Documentation/x86/orc-unwinder.txt → Documentation/x86/orc-unwinder.rst
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@ -1,8 +1,11 @@
.. SPDX-License-Identifier: GPL-2.0
============
ORC unwinder
============
Overview
--------
========
The kernel CONFIG_UNWINDER_ORC option enables the ORC unwinder, which is
similar in concept to a DWARF unwinder. The difference is that the
@ -23,12 +26,12 @@ correlate instruction addresses with their stack states at run time.
ORC 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.
Gorman [1]_ have shown a slowdown of 5-10% for some workloads.
In contrast, the ORC unwinder has no effect on text size or runtime
performance, because the debuginfo is out of band. So if you disable
@ -55,7 +58,7 @@ depending on the kernel config.
ORC vs DWARF
------------
============
ORC debuginfo's advantage over DWARF itself is that it's much simpler.
It gets rid of the complex DWARF CFI state machine and also gets rid of
@ -65,7 +68,7 @@ mission critical oops code.
The simpler debuginfo format also enables the unwinder to be much faster
than DWARF, which is important for perf and lockdep. In a basic
performance test by Jiri Slaby [2], the ORC unwinder was about 20x
performance test by Jiri Slaby [2]_, the ORC unwinder was about 20x
faster than an out-of-tree DWARF unwinder. (Note: That measurement was
taken before some performance tweaks were added, which doubled
performance, so the speedup over DWARF may be closer to 40x.)
@ -85,7 +88,7 @@ still be able to control the format, e.g. no complex state machines.
ORC unwind table generation
---------------------------
===========================
The ORC data is generated by objtool. With the existing compile-time
stack metadata validation feature, objtool already follows all code
@ -133,7 +136,7 @@ objtool follows GCC code quite well.
Unwinder implementation details
-------------------------------
===============================
Objtool generates the ORC data by integrating with the compile-time
stack metadata validation feature, which is described in detail in
@ -154,7 +157,7 @@ subset of the table needs to be searched.
Etymology
---------
=========
Orcs, fearsome creatures of medieval folklore, are the Dwarves' natural
enemies. Similarly, the ORC unwinder was created in opposition to the
@ -162,7 +165,7 @@ complexity and slowness of DWARF.
"Although Orcs rarely consider multiple solutions to a problem, they do
excel at getting things done because they are creatures of action, not
thought." [3] Similarly, unlike the esoteric DWARF unwinder, the
thought." [3]_ Similarly, unlike the esoteric DWARF unwinder, the
veracious ORC unwinder wastes no time or siloconic effort decoding
variable-length zero-extended unsigned-integer byte-coded
state-machine-based debug information entries.
@ -174,6 +177,6 @@ brutal, unyielding efficiency.
ORC stands for Oops Rewind Capability.
[1] https://lkml.kernel.org/r/20170602104048.jkkzssljsompjdwy@suse.de
[2] https://lkml.kernel.org/r/d2ca5435-6386-29b8-db87-7f227c2b713a@suse.cz
[3] http://dustin.wikidot.com/half-orcs-and-orcs
.. [1] https://lkml.kernel.org/r/20170602104048.jkkzssljsompjdwy@suse.de
.. [2] https://lkml.kernel.org/r/d2ca5435-6386-29b8-db87-7f227c2b713a@suse.cz
.. [3] http://dustin.wikidot.com/half-orcs-and-orcs