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Menglong Dong says: ==================== bpf trampoline support "jmp" mode For now, the bpf trampoline is called by the "call" instruction. However, it break the RSB and introduce extra overhead in x86_64 arch. For example, we hook the function "foo" with fexit, the call and return logic will be like this: call foo -> call trampoline -> call foo-body -> return foo-body -> return foo As we can see above, there are 3 call, but 2 return, which break the RSB balance. We can pseudo a "return" here, but it's not the best choice, as it will still cause once RSB miss: call foo -> call trampoline -> call foo-body -> return foo-body -> return dummy -> return foo The "return dummy" doesn't pair the "call trampoline", which can also cause the RSB miss. Therefore, we introduce the "jmp" mode for bpf trampoline, as advised by Alexei in [1]. And the logic will become this: call foo -> jmp trampoline -> call foo-body -> return foo-body -> return foo As we can see above, the RSB is totally balanced after this series. In this series, we introduce the FTRACE_OPS_FL_JMP for ftrace to make it use the "jmp" instruction instead of "call". And we also do some adjustment to bpf_arch_text_poke() to allow us specify the old and new poke_type. For the BPF_TRAMP_F_SHARE_IPMODIFY case, we will fallback to the "call" mode, as it need to get the function address from the stack, which is not supported in "jmp" mode. Before this series, we have the following performance with the bpf benchmark: $ cd tools/testing/selftests/bpf $ ./benchs/run_bench_trigger.sh usermode-count : 890.171 ± 1.522M/s kernel-count : 409.184 ± 0.330M/s syscall-count : 26.792 ± 0.010M/s fentry : 171.242 ± 0.322M/s fexit : 80.544 ± 0.045M/s fmodret : 78.301 ± 0.065M/s rawtp : 192.906 ± 0.900M/s tp : 81.883 ± 0.209M/s kprobe : 52.029 ± 0.113M/s kprobe-multi : 62.237 ± 0.060M/s kprobe-multi-all: 4.761 ± 0.014M/s kretprobe : 23.779 ± 0.046M/s kretprobe-multi: 29.134 ± 0.012M/s kretprobe-multi-all: 3.822 ± 0.003M/ And after this series, we have the following performance: usermode-count : 890.443 ± 0.307M/s kernel-count : 416.139 ± 0.055M/s syscall-count : 31.037 ± 0.813M/s fentry : 169.549 ± 0.519M/s fexit : 136.540 ± 0.518M/s fmodret : 159.248 ± 0.188M/s rawtp : 194.475 ± 0.144M/s tp : 84.505 ± 0.041M/s kprobe : 59.951 ± 0.071M/s kprobe-multi : 63.153 ± 0.177M/s kprobe-multi-all: 4.699 ± 0.012M/s kretprobe : 23.740 ± 0.015M/s kretprobe-multi: 29.301 ± 0.022M/s kretprobe-multi-all: 3.869 ± 0.005M/s As we can see above, the performance of fexit increase from 80.544M/s to 136.540M/s, and the "fmodret" increase from 78.301M/s to 159.248M/s. Link: https://lore.kernel.org/bpf/20251117034906.32036-1-dongml2@chinatelecom.cn/ Changes since v2: * reject if the addr is already "jmp" in register_ftrace_direct() and __modify_ftrace_direct() in the 1st patch. * fix compile error in powerpc in the 5th patch. * changes in the 6th patch: - fix the compile error by wrapping the write to tr->fops->flags with CONFIG_DYNAMIC_FTRACE_WITH_JMP - reset BPF_TRAMP_F_SKIP_FRAME when the second try of modify_fentry in bpf_trampoline_update() Link: https://lore.kernel.org/bpf/20251114092450.172024-1-dongml2@chinatelecom.cn/ Changes since v1: * change the bool parameter that we add to save_args() to "u32 flags" * rename bpf_trampoline_need_jmp() to bpf_trampoline_use_jmp() * add new function parameter to bpf_arch_text_poke instead of introduce bpf_arch_text_poke_type() * rename bpf_text_poke to bpf_trampoline_update_fentry * remove the BPF_TRAMP_F_JMPED and check the current mode with the origin flags instead. Link: https://lore.kernel.org/bpf/CAADnVQLX54sVi1oaHrkSiLqjJaJdm3TQjoVrgU-LZimK6iDcSA@mail.gmail.com/[1] ==================== Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org> Link: https://patch.msgid.link/20251118123639.688444-1-dongml2@chinatelecom.cn Signed-off-by: Alexei Starovoitov <ast@kernel.org> |
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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.