linux-kernel - Re: [PATCHv6 bpf-next 1/9] ftrace,bpf: Remove FTRACE_OPS_FL_JMP ftrace

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Message-ID: <CAEf4BzY6vVCk_XohdYVmjma4k=QvGb+h+rzOe7hmRLoUL7ZXOg@mail.gmail.com>
Date: Fri, 9 Jan 2026 16:36:34 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jiri Olsa <jolsa@...nel.org>
Cc: Steven Rostedt <rostedt@...nel.org>, Florent Revest <revest@...gle.com>, 
	Mark Rutland <mark.rutland@....com>, bpf@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Menglong Dong <menglong8.dong@...il.com>, Song Liu <song@...nel.org>
Subject: Re: [PATCHv6 bpf-next 1/9] ftrace,bpf: Remove FTRACE_OPS_FL_JMP
 ftrace_ops flag

On Tue, Dec 30, 2025 at 6:50 AM Jiri Olsa <jolsa@...nel.org> wrote:
>
> At the moment the we allow the jmp attach only for ftrace_ops that
> has FTRACE_OPS_FL_JMP set. This conflicts with following changes
> where we use single ftrace_ops object for all direct call sites,
> so all could be be attached via just call or jmp.
>
> We already limit the jmp attach support with config option and bit
> (LSB) set on the trampoline address. It turns out that's actually
> enough to limit the jmp attach for architecture and only for chosen
> addresses (with LSB bit set).
>
> Each user of register_ftrace_direct or modify_ftrace_direct can set
> the trampoline bit (LSB) to indicate it has to be attached by jmp.
>
> The bpf trampoline generation code uses trampoline flags to generate
> jmp-attach specific code and ftrace inner code uses the trampoline
> bit (LSB) to handle return from jmp attachment, so there's no harm
> to remove the FTRACE_OPS_FL_JMP bit.
>
> The fexit/fmodret performance stays the same (did not drop),
> current code:
>
>   fentry         :   77.904 ± 0.546M/s
>   fexit          :   62.430 ± 0.554M/s
>   fmodret        :   66.503 ± 0.902M/s
>
> with this change:
>
>   fentry         :   80.472 ± 0.061M/s
>   fexit          :   63.995 ± 0.127M/s
>   fmodret        :   67.362 ± 0.175M/s
>
> Fixes: 25e4e3565d45 ("ftrace: Introduce FTRACE_OPS_FL_JMP")
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  include/linux/ftrace.h  |  1 -
>  kernel/bpf/trampoline.c | 32 ++++++++++++++------------------
>  kernel/trace/ftrace.c   | 14 --------------
>  3 files changed, 14 insertions(+), 33 deletions(-)
>

I don't see anything wrong with this from BPF side

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

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