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SubjectRe: [PATCHv3 bpf-next 2/7] uprobe: Add uretprobe syscall to speed up return probe
On Sun, Apr 21, 2024 at 12:42 PM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding uretprobe syscall instead of trap to speed up return probe.
>
> At the moment the uretprobe setup/path is:
>
> - install entry uprobe
>
> - when the uprobe is hit, it overwrites probed function's return address
> on stack with address of the trampoline that contains breakpoint
> instruction
>
> - the breakpoint trap code handles the uretprobe consumers execution and
> jumps back to original return address
>
> This patch replaces the above trampoline's breakpoint instruction with new
> ureprobe syscall call. This syscall does exactly the same job as the trap
> with some more extra work:
>
> - syscall trampoline must save original value for rax/r11/rcx registers
> on stack - rax is set to syscall number and r11/rcx are changed and
> used by syscall instruction
>
> - the syscall code reads the original values of those registers and
> restore those values in task's pt_regs area
>
> - only caller from trampoline exposed in '[uprobes]' is allowed,
> the process will receive SIGILL signal otherwise
>
> Even with some extra work, using the uretprobes syscall shows speed
> improvement (compared to using standard breakpoint):
>
> On Intel (11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz)
>
> current:
> uretprobe-nop : 1.498 ± 0.000M/s
> uretprobe-push : 1.448 ± 0.001M/s
> uretprobe-ret : 0.816 ± 0.001M/s
>
> with the fix:
> uretprobe-nop : 1.969 ± 0.002M/s < 31% speed up
> uretprobe-push : 1.910 ± 0.000M/s < 31% speed up
> uretprobe-ret : 0.934 ± 0.000M/s < 14% speed up
>
> On Amd (AMD Ryzen 7 5700U)
>
> current:
> uretprobe-nop : 0.778 ± 0.001M/s
> uretprobe-push : 0.744 ± 0.001M/s
> uretprobe-ret : 0.540 ± 0.001M/s
>
> with the fix:
> uretprobe-nop : 0.860 ± 0.001M/s < 10% speed up
> uretprobe-push : 0.818 ± 0.001M/s < 10% speed up
> uretprobe-ret : 0.578 ± 0.000M/s < 7% speed up
>
> The performance test spawns a thread that runs loop which triggers
> uprobe with attached bpf program that increments the counter that
> gets printed in results above.
>
> The uprobe (and uretprobe) kind is determined by which instruction
> is being patched with breakpoint instruction. That's also important
> for uretprobes, because uprobe is installed for each uretprobe.
>
> The performance test is part of bpf selftests:
> tools/testing/selftests/bpf/run_bench_uprobes.sh
>
> Note at the moment uretprobe syscall is supported only for native
> 64-bit process, compat process still uses standard breakpoint.
>
> Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> arch/x86/kernel/uprobes.c | 115 ++++++++++++++++++++++++++++++++++++++
> include/linux/uprobes.h | 3 +
> kernel/events/uprobes.c | 24 +++++---
> 3 files changed, 135 insertions(+), 7 deletions(-)
>

LGTM as far as I can follow the code

Acked-by: Andrii Nakryiko <andrii@kernel.org>

[...]

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