Re: [PATCH 1/2] arm64: ftrace: enable single ftrace_ops for direct calls

[Leon Hwang]

On 31/7/26 07:03, Ihor Solodrai wrote:
> On 7/27/26 7:28 AM, Leon Hwang wrote:
>> The BPF tracing multi link updates several direct-call sites through one
>> ftrace_ops. Its implementation is therefore gated by
>> HAVE_SINGLE_FTRACE_DIRECT_OPS in addition to
>> DYNAMIC_FTRACE_WITH_DIRECT_CALLS.
>>
>> Select HAVE_SINGLE_FTRACE_DIRECT_OPS whenever arm64 enables dynamic ftrace
>> direct calls. This enables BPF tracing multi links on arm64. Also
>> generalize the unreachable-trampoline comment because the single-ops path
>> does not use ops->direct_call.
> 
> Hi Leon,
> 
> I don't think this change can land as is yet. The series doesn't even
> apply cleanly to bpf-next, but that's minor.
> 
> More importantly, it depends on Jose's series [1], which is not in the
> mainline yet. And there Mark has raised performance concerns [2] and
> the discussion still seems to be open.
> 
> [1] https://lore.kernel.org/all/20260609-arm64-ftrace-direct-calls-v1-0-4a46f266697f@xxxxxxxxx/
> [2] https://lore.kernel.org/all/amjnf5gz0xP5PTSB@J2N7QTR9R3/
> 
>>
>> Assisted-by: Codex:gpt-5.6-sol
>> Signed-off-by: Leon Hwang <leon.hwang@xxxxxxxxx>
>> ---
>>  arch/arm64/Kconfig         | 2 ++
>>  arch/arm64/kernel/ftrace.c | 3 +--
>>  2 files changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 0de419ed780f..c98dca76859b 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -188,6 +188,8 @@ config ARM64
>>  		    CLANG_SUPPORTS_DYNAMIC_FTRACE_WITH_ARGS)
>>  	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS \
>>  		if DYNAMIC_FTRACE_WITH_ARGS
>> +	select HAVE_SINGLE_FTRACE_DIRECT_OPS \
>> +		if DYNAMIC_FTRACE_WITH_DIRECT_CALLS\
> 
> The select is only conditional on DYNAMIC_FTRACE_WITH_DIRECT_CALLS, so
> it can be set along with HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS. And AFAIU
> this would make the fast path effectively dead: every BPF direct call
> routed through ftrace_caller now goes through the slow path.
> 
> As Mark noted in the other thread, on arm64 trampolines come from
> EXECMEM_BPF, so they always land out of BL range (chance of landing in
> range is 256M/terabytes).
> 
> I vibe-slop-coded a benchmark and ran it on a Neoverse V2 machine, and
> toggling your config change seems to be causing a 1.3x regression in
> the tracing overhead:
> 
>   do-nothing fentry (r0=0; exit) on __arm64_sys_getpid, 20 M calls, min-of-N, several boots. Results:
> 
>               untraced (base)   traced        overhead
>   Kernel A    ~128 ns/call      ~145.5 ns     ~17.6 ns   (fast path: br x17)
>   Kernel B    ~127 ns/call      ~150.4 ns     ~23.4 ns   (slow path: save regs + call_direct_funcs + hash)


Thanks for your testing.

> 
> This confirms Jiri's suspicion.

True.

> 
> However my understanding is the regression should mostly disappear in
> case some version of in-range trampoline allocation on arm64 lands.
> 
> So, I think the landing sequence should be something like follows:
>   * in-range BPF-trampoline allocation that Jose proposed [3]
>   * then HAVE_SINGLE_FTRACE_DIRECT_OPS selection
> 
> After all of that reaches mainline, then a selftest patch can go
> through the bpf-next.

Sounds reasonable.

This series is based on Jose's series and is intended for the arm64
tree, rather than bpf-next. Like Jose's series, this series aims to
enable BPF tracing_multi link on arm64. And yes, with in-range
BPF-trampoline allocation, the regression should mostly disappear.

I'll follow the sequence and repost the patches afterward.

Thanks,
Leon

> 
> [3] https://lore.kernel.org/all/amn7cBQ-cqs2XlKV@xxxxxxxxx/
> 
>>  	select HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS \
>>  		if (DYNAMIC_FTRACE_WITH_ARGS && !CFI && \
>>  		    (CC_IS_CLANG || !CC_OPTIMIZE_FOR_SIZE))
>> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
>> index e1a3c0b3a051..56ba72a87dfa 100644
>> --- a/arch/arm64/kernel/ftrace.c
>> +++ b/arch/arm64/kernel/ftrace.c
>> @@ -301,8 +301,7 @@ static bool ftrace_find_callable_addr(struct dyn_ftrace *rec,
>>  
>>  	/*
>>  	 * If a custom trampoline is unreachable, rely on the ftrace_caller
>> -	 * trampoline which knows how to indirectly reach that trampoline
>> -	 * through ops->direct_call.
>> +	 * trampoline which knows how to indirectly reach that trampoline.
>>  	 */
>>  	if (*addr != FTRACE_ADDR && !reachable_by_bl(*addr, pc))
>>  		*addr = FTRACE_ADDR;
>