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Message-ID: <aXc4UesvE3zYfPqn@krava>
Date: Mon, 26 Jan 2026 10:48:01 +0100
From: Jiri Olsa <olsajiri@...il.com>
To: Steven Rostedt <rostedt@...nel.org>
Cc: Florent Revest <revest@...gle.com>,
Andrii Nakryiko <andrii.nakryiko@...il.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 0/9] ftrace,bpf: Use single direct ops for bpf
trampolines
hi,
gentle ping, thanks
jirka
On Thu, Jan 15, 2026 at 10:54:09AM -0800, Andrii Nakryiko wrote:
> On Tue, Dec 30, 2025 at 6:50 AM Jiri Olsa <jolsa@...nel.org> wrote:
> >
> > hi,
> > while poking the multi-tracing interface I ended up with just one ftrace_ops
> > object to attach all trampolines.
> >
> > This change allows to use less direct API calls during the attachment changes
> > in the future code, so in effect speeding up the attachment.
> >
> > In current code we get a speed up from using just a single ftrace_ops object.
> >
> > - with current code:
> >
> > Performance counter stats for 'bpftrace -e fentry:vmlinux:ksys_* {} -c true':
> >
> > 6,364,157,902 cycles:k
> > 828,728,902 cycles:u
> > 1,064,803,824 instructions:u # 1.28 insn per cycle
> > 23,797,500,067 instructions:k # 3.74 insn per cycle
> >
> > 4.416004987 seconds time elapsed
> >
> > 0.164121000 seconds user
> > 1.289550000 seconds sys
> >
> >
> > - with the fix:
> >
> > Performance counter stats for 'bpftrace -e fentry:vmlinux:ksys_* {} -c true':
> >
> > 6,535,857,905 cycles:k
> > 810,809,429 cycles:u
> > 1,064,594,027 instructions:u # 1.31 insn per cycle
> > 23,962,552,894 instructions:k # 3.67 insn per cycle
> >
> > 1.666961239 seconds time elapsed
> >
> > 0.157412000 seconds user
> > 1.283396000 seconds sys
> >
> >
> >
> > The speedup seems to be related to the fact that with single ftrace_ops object
> > we don't call ftrace_shutdown anymore (we use ftrace_update_ops instead) and
> > we skip the synchronize rcu calls (each ~100ms) at the end of that function.
> >
> > rfc: https://lore.kernel.org/bpf/20250729102813.1531457-1-jolsa@kernel.org/
> > v1: https://lore.kernel.org/bpf/20250923215147.1571952-1-jolsa@kernel.org/
> > v2: https://lore.kernel.org/bpf/20251113123750.2507435-1-jolsa@kernel.org/
> > v3: https://lore.kernel.org/bpf/20251120212402.466524-1-jolsa@kernel.org/
> > v4: https://lore.kernel.org/bpf/20251203082402.78816-1-jolsa@kernel.org/
> > v5: https://lore.kernel.org/bpf/20251215211402.353056-10-jolsa@kernel.org/
> >
> > v6 changes:
> > - rename add_hash_entry_direct to add_ftrace_hash_entry_direct [Steven]
> > - factor hash_add/hash_sub [Steven]
> > - add kerneldoc header for update_ftrace_direct_* functions [Steven]
> > - few assorted smaller fixes [Steven]
> > - added missing direct_ops wrappers for !CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
> > case [Steven]
> >
>
> So this looks good from BPF side, I think. Steven, if you don't mind
> giving this patch set another look and if everything is to your liking
> giving your ack, we can then apply it to bpf-next. Thanks!
>
> > v5 changes:
> > - do not export ftrace_hash object [Steven]
> > - fix update_ftrace_direct_add new_filter_hash leak [ci]
> >
> > v4 changes:
> > - rebased on top of bpf-next/master (with jmp attach changes)
> > added patch 1 to deal with that
> > - added extra checks for update_ftrace_direct_del/mod to address
> > the ci bot review
> >
> > v3 changes:
> > - rebased on top of bpf-next/master
> > - fixed update_ftrace_direct_del cleanup path
> > - added missing inline to update_ftrace_direct_* stubs
> >
> > v2 changes:
> > - rebased on top fo bpf-next/master plus Song's livepatch fixes [1]
> > - renamed the API functions [2] [Steven]
> > - do not export the new api [Steven]
> > - kept the original direct interface:
> >
> > I'm not sure if we want to melt both *_ftrace_direct and the new interface
> > into single one. It's bit different in semantic (hence the name change as
> > Steven suggested [2]) and I don't think the changes are not that big so
> > we could easily keep both APIs.
> >
> > v1 changes:
> > - make the change x86 specific, after discussing with Mark options for
> > arm64 [Mark]
> >
> > thanks,
> > jirka
> >
> >
> > [1] https://lore.kernel.org/bpf/20251027175023.1521602-1-song@kernel.org/
> > [2] https://lore.kernel.org/bpf/20250924050415.4aefcb91@batman.local.home/
> > ---
> > Jiri Olsa (9):
> > ftrace,bpf: Remove FTRACE_OPS_FL_JMP ftrace_ops flag
> > ftrace: Make alloc_and_copy_ftrace_hash direct friendly
> > ftrace: Export some of hash related functions
> > ftrace: Add update_ftrace_direct_add function
> > ftrace: Add update_ftrace_direct_del function
> > ftrace: Add update_ftrace_direct_mod function
> > bpf: Add trampoline ip hash table
> > ftrace: Factor ftrace_ops ops_func interface
> > bpf,x86: Use single ftrace_ops for direct calls
> >
> > arch/x86/Kconfig | 1 +
> > include/linux/bpf.h | 7 ++-
> > include/linux/ftrace.h | 31 +++++++++-
> > kernel/bpf/trampoline.c | 259 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++---------------
> > kernel/trace/Kconfig | 3 +
> > kernel/trace/ftrace.c | 406 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--------
> > 6 files changed, 632 insertions(+), 75 deletions(-)
> >
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