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/[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)
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/[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; >
