1.. SPDX-License-Identifier: GPL-2.0
   4ORC unwinder
  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.
  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.
  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.
  28ORC vs frame pointers
  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.
  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.
  41Ingo Molnar says:
  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."
  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
  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.
  60ORC vs DWARF
  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.
  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.)
  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.
  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.
  90ORC unwind table generation
  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.
  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.
 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.
 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.
 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.
 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
 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.
 138Unwinder implementation details
 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
 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.
 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.
 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.
 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.
 173Similar to how Orcs frequently unravel the well-intentioned plans of
 174their adversaries, the ORC unwinder frequently unravels stacks with
 175brutal, unyielding efficiency.
 177ORC stands for Oops Rewind Capability.
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