183 lines
8.0 KiB
ReStructuredText
183 lines
8.0 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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============
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ORC unwinder
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============
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Overview
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========
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The kernel CONFIG_UNWINDER_ORC option enables the ORC unwinder, which is
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similar in concept to a DWARF unwinder. The difference is that the
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format of the ORC data is much simpler than DWARF, which in turn allows
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the ORC unwinder to be much simpler and faster.
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The ORC data consists of unwind tables which are generated by objtool.
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They contain out-of-band data which is used by the in-kernel ORC
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unwinder. Objtool generates the ORC data by first doing compile-time
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stack metadata validation (CONFIG_STACK_VALIDATION). After analyzing
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all the code paths of a .o file, it determines information about the
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stack state at each instruction address in the file and outputs that
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information to the .orc_unwind and .orc_unwind_ip sections.
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The per-object ORC sections are combined at link time and are sorted and
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post-processed at boot time. The unwinder uses the resulting data to
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correlate instruction addresses with their stack states at run time.
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ORC vs frame pointers
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=====================
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With frame pointers enabled, GCC adds instrumentation code to every
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function in the kernel. The kernel's .text size increases by about
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3.2%, resulting in a broad kernel-wide slowdown. Measurements by Mel
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Gorman [1]_ have shown a slowdown of 5-10% for some workloads.
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In contrast, the ORC unwinder has no effect on text size or runtime
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performance, because the debuginfo is out of band. So if you disable
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frame pointers and enable the ORC unwinder, you get a nice performance
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improvement across the board, and still have reliable stack traces.
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Ingo Molnar says:
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"Note that it's not just a performance improvement, but also an
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instruction cache locality improvement: 3.2% .text savings almost
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directly transform into a similarly sized reduction in cache
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footprint. That can transform to even higher speedups for workloads
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whose cache locality is borderline."
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Another benefit of ORC compared to frame pointers is that it can
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reliably unwind across interrupts and exceptions. Frame pointer based
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unwinds can sometimes skip the caller of the interrupted function, if it
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was a leaf function or if the interrupt hit before the frame pointer was
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saved.
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The main disadvantage of the ORC unwinder compared to frame pointers is
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that it needs more memory to store the ORC unwind tables: roughly 2-4MB
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depending on the kernel config.
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ORC vs DWARF
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============
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ORC debuginfo's advantage over DWARF itself is that it's much simpler.
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It gets rid of the complex DWARF CFI state machine and also gets rid of
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the tracking of unnecessary registers. This allows the unwinder to be
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much simpler, meaning fewer bugs, which is especially important for
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mission critical oops code.
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The simpler debuginfo format also enables the unwinder to be much faster
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than DWARF, which is important for perf and lockdep. In a basic
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performance test by Jiri Slaby [2]_, the ORC unwinder was about 20x
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faster than an out-of-tree DWARF unwinder. (Note: That measurement was
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taken before some performance tweaks were added, which doubled
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performance, so the speedup over DWARF may be closer to 40x.)
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The ORC data format does have a few downsides compared to DWARF. ORC
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unwind tables take up ~50% more RAM (+1.3MB on an x86 defconfig kernel)
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than DWARF-based eh_frame tables.
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Another potential downside is that, as GCC evolves, it's conceivable
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that the ORC data may end up being *too* simple to describe the state of
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the stack for certain optimizations. But IMO this is unlikely because
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GCC saves the frame pointer for any unusual stack adjustments it does,
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so I suspect we'll really only ever need to keep track of the stack
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pointer and the frame pointer between call frames. But even if we do
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end up having to track all the registers DWARF tracks, at least we will
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still be able to control the format, e.g. no complex state machines.
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ORC unwind table generation
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===========================
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The ORC data is generated by objtool. With the existing compile-time
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stack metadata validation feature, objtool already follows all code
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paths, and so it already has all the information it needs to be able to
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generate ORC data from scratch. So it's an easy step to go from stack
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validation to ORC data generation.
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It should be possible to instead generate the ORC data with a simple
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tool which converts DWARF to ORC data. However, such a solution would
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be incomplete due to the kernel's extensive use of asm, inline asm, and
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special sections like exception tables.
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That could be rectified by manually annotating those special code paths
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using GNU assembler .cfi annotations in .S files, and homegrown
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annotations for inline asm in .c files. But asm annotations were tried
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in the past and were found to be unmaintainable. They were often
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incorrect/incomplete and made the code harder to read and keep updated.
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And based on looking at glibc code, annotating inline asm in .c files
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might be even worse.
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Objtool still needs a few annotations, but only in code which does
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unusual things to the stack like entry code. And even then, far fewer
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annotations are needed than what DWARF would need, so they're much more
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maintainable than DWARF CFI annotations.
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So the advantages of using objtool to generate ORC data are that it
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gives more accurate debuginfo, with very few annotations. It also
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insulates the kernel from toolchain bugs which can be very painful to
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deal with in the kernel since we often have to workaround issues in
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older versions of the toolchain for years.
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The downside is that the unwinder now becomes dependent on objtool's
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ability to reverse engineer GCC code flow. If GCC optimizations become
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too complicated for objtool to follow, the ORC data generation might
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stop working or become incomplete. (It's worth noting that livepatch
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already has such a dependency on objtool's ability to follow GCC code
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flow.)
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If newer versions of GCC come up with some optimizations which break
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objtool, we may need to revisit the current implementation. Some
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possible solutions would be asking GCC to make the optimizations more
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palatable, or having objtool use DWARF as an additional input, or
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creating a GCC plugin to assist objtool with its analysis. But for now,
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objtool follows GCC code quite well.
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Unwinder implementation details
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===============================
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Objtool generates the ORC data by integrating with the compile-time
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stack metadata validation feature, which is described in detail in
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tools/objtool/Documentation/objtool.txt. After analyzing all
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the code paths of a .o file, it creates an array of orc_entry structs,
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and a parallel array of instruction addresses associated with those
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structs, and writes them to the .orc_unwind and .orc_unwind_ip sections
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respectively.
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The ORC data is split into the two arrays for performance reasons, to
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make the searchable part of the data (.orc_unwind_ip) more compact. The
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arrays are sorted in parallel at boot time.
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Performance is further improved by the use of a fast lookup table which
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is created at runtime. The fast lookup table associates a given address
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with a range of indices for the .orc_unwind table, so that only a small
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subset of the table needs to be searched.
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Etymology
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=========
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Orcs, fearsome creatures of medieval folklore, are the Dwarves' natural
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enemies. Similarly, the ORC unwinder was created in opposition to the
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complexity and slowness of DWARF.
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"Although Orcs rarely consider multiple solutions to a problem, they do
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excel at getting things done because they are creatures of action, not
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thought." [3]_ Similarly, unlike the esoteric DWARF unwinder, the
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veracious ORC unwinder wastes no time or siloconic effort decoding
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variable-length zero-extended unsigned-integer byte-coded
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state-machine-based debug information entries.
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Similar to how Orcs frequently unravel the well-intentioned plans of
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their adversaries, the ORC unwinder frequently unravels stacks with
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brutal, unyielding efficiency.
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ORC stands for Oops Rewind Capability.
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.. [1] https://lore.kernel.org/r/20170602104048.jkkzssljsompjdwy@suse.de
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.. [2] https://lore.kernel.org/r/d2ca5435-6386-29b8-db87-7f227c2b713a@suse.cz
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.. [3] http://dustin.wikidot.com/half-orcs-and-orcs
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