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This patch enables support for DYNAMIC_FTRACE_WITH_CALL_OPS on RISC-V. This allows each ftrace callsite to provide an ftrace_ops to the common ftrace trampoline, allowing each callsite to invoke distinct tracer functions without the need to fall back to list processing or to allocate custom trampolines for each callsite. This significantly speeds up cases where multiple distinct trace functions are used and callsites are mostly traced by a single tracer. The idea and most of the implementation is taken from the ARM64's implementation of the same feature. The idea is to place a pointer to the ftrace_ops as a literal at a fixed offset from the function entry point, which can be recovered by the common ftrace trampoline. We use -fpatchable-function-entry to reserve 8 bytes above the function entry by emitting 2 4 byte or 4 2 byte nops depending on the presence of CONFIG_RISCV_ISA_C. These 8 bytes are patched at runtime with a pointer to the associated ftrace_ops for that callsite. Functions are aligned to 8 bytes to make sure that the accesses to this literal are atomic. This approach allows for directly invoking ftrace_ops::func even for ftrace_ops which are dynamically-allocated (or part of a module), without going via ftrace_ops_list_func. We've benchamrked this with the ftrace_ops sample module on Spacemit K1 Jupiter: Without this patch: baseline (Linux rivos 6.14.0-09584-g7d06015d936c #3 SMP Sat Mar 29 +-----------------------+-----------------+----------------------------+ | Number of tracers | Total time (ns) | Per-call average time | |-----------------------+-----------------+----------------------------| | Relevant | Irrelevant | 100000 calls | Total (ns) | Overhead (ns) | |----------+------------+-----------------+------------+---------------| | 0 | 0 | 1357958 | 13 | - | | 0 | 1 | 1302375 | 13 | - | | 0 | 2 | 1302375 | 13 | - | | 0 | 10 | 1379084 | 13 | - | | 0 | 100 | 1302458 | 13 | - | | 0 | 200 | 1302333 | 13 | - | |----------+------------+-----------------+------------+---------------| | 1 | 0 | 13677833 | 136 | 123 | | 1 | 1 | 18500916 | 185 | 172 | | 1 | 2 | |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.