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/[email protected]/ [2] https://lore.kernel.org/all/amjnf5gz0xP5PTSB@J2N7QTR9R3/ > > Assisted-by: Codex:gpt-5.6-sol > Signed-off-by: Leon Hwang <[email protected]> > --- > 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) This confirms Jiri's suspicion. 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. [3] https://lore.kernel.org/all/[email protected]/ > 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;
