1.. SPDX-License-Identifier: GPL-2.0
  2
  3============
  4ORC unwinder
  5============
  6
  7Overview
  8========
  9
 10The kernel CONFIG_UNWINDER_ORC option enables the ORC unwinder, which is
 11similar in concept to a DWARF unwinder.  The difference is that the
 12format of the ORC data is much simpler than DWARF, which in turn allows
 13the ORC unwinder to be much simpler and faster.
 14
 15The ORC data consists of unwind tables which are generated by objtool.
 16They contain out-of-band data which is used by the in-kernel ORC
 17unwinder.  Objtool generates the ORC data by first doing compile-time
 18stack metadata validation (CONFIG_STACK_VALIDATION).  After analyzing
 19all the code paths of a .o file, it determines information about the
 20stack state at each instruction address in the file and outputs that
 21information to the .orc_unwind and .orc_unwind_ip sections.
 22
 23The per-object ORC sections are combined at link time and are sorted and
 24post-processed at boot time.  The unwinder uses the resulting data to
 25correlate instruction addresses with their stack states at run time.
 26
 27
 28ORC vs frame pointers
 29=====================
 30
 31With frame pointers enabled, GCC adds instrumentation code to every
 32function in the kernel.  The kernel's .text size increases by about
 333.2%, resulting in a broad kernel-wide slowdown.  Measurements by Mel
 34Gorman [1]_ have shown a slowdown of 5-10% for some workloads.
 35
 36In contrast, the ORC unwinder has no effect on text size or runtime
 37performance, because the debuginfo is out of band.  So if you disable
 38frame pointers and enable the ORC unwinder, you get a nice performance
 39improvement across the board, and still have reliable stack traces.
 40
 41Ingo Molnar says:
 42
 43  "Note that it's not just a performance improvement, but also an
 44  instruction cache locality improvement: 3.2% .text savings almost
 45  directly transform into a similarly sized reduction in cache
 46  footprint. That can transform to even higher speedups for workloads
 47  whose cache locality is borderline."
 48
 49Another benefit of ORC compared to frame pointers is that it can
 50reliably unwind across interrupts and exceptions.  Frame pointer based
 51unwinds can sometimes skip the caller of the interrupted function, if it
 52was a leaf function or if the interrupt hit before the frame pointer was
 53saved.
 54
 55The main disadvantage of the ORC unwinder compared to frame pointers is
 56that it needs more memory to store the ORC unwind tables: roughly 2-4MB
 57depending on the kernel config.
 58
 59
 60ORC vs DWARF
 61============
 62
 63ORC debuginfo's advantage over DWARF itself is that it's much simpler.
 64It gets rid of the complex DWARF CFI state machine and also gets rid of
 65the tracking of unnecessary registers.  This allows the unwinder to be
 66much simpler, meaning fewer bugs, which is especially important for
 67mission critical oops code.
 68
 69The simpler debuginfo format also enables the unwinder to be much faster
 70than DWARF, which is important for perf and lockdep.  In a basic
 71performance test by Jiri Slaby [2]_, the ORC unwinder was about 20x
 72faster than an out-of-tree DWARF unwinder.  (Note: That measurement was
 73taken before some performance tweaks were added, which doubled
 74performance, so the speedup over DWARF may be closer to 40x.)
 75
 76The ORC data format does have a few downsides compared to DWARF.  ORC
 77unwind tables take up ~50% more RAM (+1.3MB on an x86 defconfig kernel)
 78than DWARF-based eh_frame tables.
 79
 80Another potential downside is that, as GCC evolves, it's conceivable
 81that the ORC data may end up being *too* simple to describe the state of
 82the stack for certain optimizations.  But IMO this is unlikely because
 83GCC saves the frame pointer for any unusual stack adjustments it does,
 84so I suspect we'll really only ever need to keep track of the stack
 85pointer and the frame pointer between call frames.  But even if we do
 86end up having to track all the registers DWARF tracks, at least we will
 87still be able to control the format, e.g. no complex state machines.
 88
 89
 90ORC unwind table generation
 91===========================
 92
 93The ORC data is generated by objtool.  With the existing compile-time
 94stack metadata validation feature, objtool already follows all code
 95paths, and so it already has all the information it needs to be able to
 96generate ORC data from scratch.  So it's an easy step to go from stack
 97validation to ORC data generation.
 98
 99It should be possible to instead generate the ORC data with a simple
100tool which converts DWARF to ORC data.  However, such a solution would
101be incomplete due to the kernel's extensive use of asm, inline asm, and
102special sections like exception tables.
103
104That could be rectified by manually annotating those special code paths
105using GNU assembler .cfi annotations in .S files, and homegrown
106annotations for inline asm in .c files.  But asm annotations were tried
107in the past and were found to be unmaintainable.  They were often
108incorrect/incomplete and made the code harder to read and keep updated.
109And based on looking at glibc code, annotating inline asm in .c files
110might be even worse.
111
112Objtool still needs a few annotations, but only in code which does
113unusual things to the stack like entry code.  And even then, far fewer
114annotations are needed than what DWARF would need, so they're much more
115maintainable than DWARF CFI annotations.
116
117So the advantages of using objtool to generate ORC data are that it
118gives more accurate debuginfo, with very few annotations.  It also
119insulates the kernel from toolchain bugs which can be very painful to
120deal with in the kernel since we often have to workaround issues in
121older versions of the toolchain for years.
122
123The downside is that the unwinder now becomes dependent on objtool's
124ability to reverse engineer GCC code flow.  If GCC optimizations become
125too complicated for objtool to follow, the ORC data generation might
126stop working or become incomplete.  (It's worth noting that livepatch
127already has such a dependency on objtool's ability to follow GCC code
128flow.)
129
130If newer versions of GCC come up with some optimizations which break
131objtool, we may need to revisit the current implementation.  Some
132possible solutions would be asking GCC to make the optimizations more
133palatable, or having objtool use DWARF as an additional input, or
134creating a GCC plugin to assist objtool with its analysis.  But for now,
135objtool follows GCC code quite well.
136
137
138Unwinder implementation details
139===============================
140
141Objtool generates the ORC data by integrating with the compile-time
142stack metadata validation feature, which is described in detail in
143tools/objtool/Documentation/stack-validation.txt.  After analyzing all
144the code paths of a .o file, it creates an array of orc_entry structs,
145and a parallel array of instruction addresses associated with those
146structs, and writes them to the .orc_unwind and .orc_unwind_ip sections
147respectively.
148
149The ORC data is split into the two arrays for performance reasons, to
150make the searchable part of the data (.orc_unwind_ip) more compact.  The
151arrays are sorted in parallel at boot time.
152
153Performance is further improved by the use of a fast lookup table which
154is created at runtime.  The fast lookup table associates a given address
155with a range of indices for the .orc_unwind table, so that only a small
156subset of the table needs to be searched.
157
158
159Etymology
160=========
161
162Orcs, fearsome creatures of medieval folklore, are the Dwarves' natural
163enemies.  Similarly, the ORC unwinder was created in opposition to the
164complexity and slowness of DWARF.
165
166"Although Orcs rarely consider multiple solutions to a problem, they do
167excel at getting things done because they are creatures of action, not
168thought." [3]_  Similarly, unlike the esoteric DWARF unwinder, the
169veracious ORC unwinder wastes no time or siloconic effort decoding
170variable-length zero-extended unsigned-integer byte-coded
171state-machine-based debug information entries.
172
173Similar to how Orcs frequently unravel the well-intentioned plans of
174their adversaries, the ORC unwinder frequently unravels stacks with
175brutal, unyielding efficiency.
176
177ORC stands for Oops Rewind Capability.
178
179
180.. [1] https://lkml.kernel.org/r/20170602104048.jkkzssljsompjdwy@suse.de
181.. [2] https://lkml.kernel.org/r/d2ca5435-6386-29b8-db87-7f227c2b713a@suse.cz
182.. [3] http://dustin.wikidot.com/half-orcs-and-orcs