sched: Automatically namespace schedulers' implementation functions - src

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author	Olivier Certner <olce@FreeBSD.org>	2026-10-09 14:29:06 +0000
committer	Olivier Certner <olce@FreeBSD.org>	2026-11-03 17:42:22 +0000
commit	510331fa376309ae0860b91bd501c9c5a73e86a7 (patch)
tree	777a2c3a46b627432b47dd96206aced1d894cf37 /sys
parent	28c31242758323499eaf8af1f03fc4ae4a960205 (diff)

sched: Automatically namespace schedulers' implementation functions

This renames back scheduler implementation functions to their old names, i.e., the same name as the interface functions, in the source code, while providing the infrastructure to still produce from them C symbols namespaced by the scheduler implementation, in order to avoid symbol collisions with interface dispatching functions and other scheduler implementations when multiple ones are compiled in. In a nutshell, the implementation consists in re-routing interface names to implementation names via the C preprocessor. The main reasons for this change are that different implementation names: 1. Break navigating code with ctags-like tools like in-tree ctags, Global and Universal Ctags. 2. Spoils inlining opportunities in internal scheduler code if not careful. This alternatively could be regained by changing call points to use scheduler-specific names, but that does not prevent people from inadvertently adding more calls to interface functions. Also, having to namespace calls with the implementation name in the implementation itself can be seen as code clutter. 3. Introduces avoidable cognitive burden, for example in the form of having to ask oneself whether internal scheduler functions should be namespaced or not, and then when calling them which their actual names are. However, C symbol collisions create difficulties with debugging and ELF manipulation tools, including when printing backtraces (which scheduler implementation is in use?). For example, as of this writing, 'disassemble' in GDB will choose one of the existing functions and disassemble it without mentioning the existing others; our ddb(4) will set a breakpoint on one of these functions only (which should be improved); etc. So keeping namespaced symbol names was desirable, which is why symbol re-routing via the C preprocessor was chosen. People wanting to extract information from ELF files will have to use full symbol names instead of source code names. This is certainly a drawback, but we expect that people will spend by far more time on the code rather than trying to disassemble some particular implementation function, so the inconvenience will barely occur in practice. Also, DWARF information matching machine code to source code location stays undisturbed and gdb will continue to display matching source code correctly. ctags-like tool are likely to never implement macro substitution when definitions are in other files, or if they do to never enable it by default, as this would considerably slow down source parsing (Universal Ctags reports a 2x slowdown just by enabling expansions for definitions that are in the *same* file). LSPs have not been tested; we however expect this change will cause nothing but minor annoyances when using them. Writing new calls to scheduler functions should work as usual in terms of help/completion (including in implementations, as corresponding implementation and interface functions have same signature). Even automatically producing implementation function signatures when adding a new definition is expected to work. Please report if not the case. The implementation of the machinery to substitute interface names by implementation names has two parts. First, a new generated per-implementation-specific header has to be included in each implementation ("sched_<NAME>_ns.h"). It contains simple C preprocessor substitutions, such as, taking the example of ULE: #define sched_clock sched_ule_clock #define sched_switch sched_ule_switch allowing to use the interface names in place of the implementation names in the implementation itself. It also checks that an implementation actually defined SCHED_IMPLEMENTATION and to the same value that was used to produce it. These headers are generated from the pattern header 'sys/kern/sched_ns.h' through directives in 'sys/conf/files', via the C preprocessor using the already existing X macros and some light post-processing by the new 'sys/tools/sched_impl_ns.awk' script. Second, <sys/sched.h> declares all implementation functions when included from an implementation, so that out-of-order references within the implementation do not cause problems, as it was before multiple schedulers could coexist. Implementations must define SCHED_IMPLEMENTATION to their name used as the namespacing symbol part (e.g., ule or 4bsd), which used to be the name passed as the first argument to DECLARE_SCHEDULER(). This argument is now redundant and has been removed. That same macro with same value has to be defined in the directives from 'sys/conf/files' for each implementation's namespacing header to be produced properly. For added safety, <sys/sched.h> does not provide interface functions' declarations when SCHED_IMPLEMENTATION is defined anymore, so that implementations can never inadvertently call these functions. Reviewed by: mchoo MFC after: 3 days Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D60644

Diffstat (limited to 'sys')

-rw-r--r--

sys/conf/files

-rw-r--r--

sys/kern/sched_4bsd.c

101

-rw-r--r--

sys/kern/sched_ns.h

-rw-r--r--

sys/kern/sched_shim.c

-rw-r--r--

sys/kern/sched_ule.c

107

-rw-r--r--

sys/sys/sched.h

-rw-r--r--

sys/tools/sched_impl_ns.awk

7 files changed, 264 insertions, 111 deletions


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