183 lines
8.0 KiB
ReStructuredText
183 lines
8.0 KiB
ReStructuredText
.. 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
|
|
format of the ORC data is much simpler than DWARF, which in turn allows
|
|
the ORC unwinder to be much simpler and faster.
|
|
|
|
The ORC data consists of unwind tables which are generated by objtool.
|
|
They contain out-of-band data which is used by the in-kernel ORC
|
|
unwinder. Objtool generates the ORC data by first doing compile-time
|
|
stack metadata validation (CONFIG_STACK_VALIDATION). After analyzing
|
|
all the code paths of a .o file, it determines information about the
|
|
stack state at each instruction address in the file and outputs that
|
|
information to the .orc_unwind and .orc_unwind_ip sections.
|
|
|
|
The per-object ORC sections are combined at link time and are sorted and
|
|
post-processed at boot time. The unwinder uses the resulting data to
|
|
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.
|
|
|
|
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
|
|
frame pointers and enable the ORC unwinder, you get a nice performance
|
|
improvement across the board, and still have reliable stack traces.
|
|
|
|
Ingo Molnar says:
|
|
|
|
"Note that it's not just a performance improvement, but also an
|
|
instruction cache locality improvement: 3.2% .text savings almost
|
|
directly transform into a similarly sized reduction in cache
|
|
footprint. That can transform to even higher speedups for workloads
|
|
whose cache locality is borderline."
|
|
|
|
Another benefit of ORC compared to frame pointers is that it can
|
|
reliably unwind across interrupts and exceptions. Frame pointer based
|
|
unwinds can sometimes skip the caller of the interrupted function, if it
|
|
was a leaf function or if the interrupt hit before the frame pointer was
|
|
saved.
|
|
|
|
The main disadvantage of the ORC unwinder compared to frame pointers is
|
|
that it needs more memory to store the ORC unwind tables: roughly 2-4MB
|
|
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
|
|
the tracking of unnecessary registers. This allows the unwinder to be
|
|
much simpler, meaning fewer bugs, which is especially important for
|
|
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
|
|
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.)
|
|
|
|
The ORC data format does have a few downsides compared to DWARF. ORC
|
|
unwind tables take up ~50% more RAM (+1.3MB on an x86 defconfig kernel)
|
|
than DWARF-based eh_frame tables.
|
|
|
|
Another potential downside is that, as GCC evolves, it's conceivable
|
|
that the ORC data may end up being *too* simple to describe the state of
|
|
the stack for certain optimizations. But IMO this is unlikely because
|
|
GCC saves the frame pointer for any unusual stack adjustments it does,
|
|
so I suspect we'll really only ever need to keep track of the stack
|
|
pointer and the frame pointer between call frames. But even if we do
|
|
end up having to track all the registers DWARF tracks, at least we will
|
|
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
|
|
paths, and so it already has all the information it needs to be able to
|
|
generate ORC data from scratch. So it's an easy step to go from stack
|
|
validation to ORC data generation.
|
|
|
|
It should be possible to instead generate the ORC data with a simple
|
|
tool which converts DWARF to ORC data. However, such a solution would
|
|
be incomplete due to the kernel's extensive use of asm, inline asm, and
|
|
special sections like exception tables.
|
|
|
|
That could be rectified by manually annotating those special code paths
|
|
using GNU assembler .cfi annotations in .S files, and homegrown
|
|
annotations for inline asm in .c files. But asm annotations were tried
|
|
in the past and were found to be unmaintainable. They were often
|
|
incorrect/incomplete and made the code harder to read and keep updated.
|
|
And based on looking at glibc code, annotating inline asm in .c files
|
|
might be even worse.
|
|
|
|
Objtool still needs a few annotations, but only in code which does
|
|
unusual things to the stack like entry code. And even then, far fewer
|
|
annotations are needed than what DWARF would need, so they're much more
|
|
maintainable than DWARF CFI annotations.
|
|
|
|
So the advantages of using objtool to generate ORC data are that it
|
|
gives more accurate debuginfo, with very few annotations. It also
|
|
insulates the kernel from toolchain bugs which can be very painful to
|
|
deal with in the kernel since we often have to workaround issues in
|
|
older versions of the toolchain for years.
|
|
|
|
The downside is that the unwinder now becomes dependent on objtool's
|
|
ability to reverse engineer GCC code flow. If GCC optimizations become
|
|
too complicated for objtool to follow, the ORC data generation might
|
|
stop working or become incomplete. (It's worth noting that livepatch
|
|
already has such a dependency on objtool's ability to follow GCC code
|
|
flow.)
|
|
|
|
If newer versions of GCC come up with some optimizations which break
|
|
objtool, we may need to revisit the current implementation. Some
|
|
possible solutions would be asking GCC to make the optimizations more
|
|
palatable, or having objtool use DWARF as an additional input, or
|
|
creating a GCC plugin to assist objtool with its analysis. But for now,
|
|
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
|
|
tools/objtool/Documentation/objtool.txt. After analyzing all
|
|
the code paths of a .o file, it creates an array of orc_entry structs,
|
|
and a parallel array of instruction addresses associated with those
|
|
structs, and writes them to the .orc_unwind and .orc_unwind_ip sections
|
|
respectively.
|
|
|
|
The ORC data is split into the two arrays for performance reasons, to
|
|
make the searchable part of the data (.orc_unwind_ip) more compact. The
|
|
arrays are sorted in parallel at boot time.
|
|
|
|
Performance is further improved by the use of a fast lookup table which
|
|
is created at runtime. The fast lookup table associates a given address
|
|
with a range of indices for the .orc_unwind table, so that only a small
|
|
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
|
|
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
|
|
veracious ORC unwinder wastes no time or siloconic effort decoding
|
|
variable-length zero-extended unsigned-integer byte-coded
|
|
state-machine-based debug information entries.
|
|
|
|
Similar to how Orcs frequently unravel the well-intentioned plans of
|
|
their adversaries, the ORC unwinder frequently unravels stacks with
|
|
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
|