Re: [PATCHv3 bpf-next 2/7] uprobe: Add uretprobe syscall to speed up return probe

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

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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