orc-unwinder.rst (8234B)
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://lore.kernel.org/r/20170602104048.jkkzssljsompjdwy@suse.de 181.. [2] https://lore.kernel.org/r/d2ca5435-6386-29b8-db87-7f227c2b713a@suse.cz 182.. [3] http://dustin.wikidot.com/half-orcs-and-orcs