From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-185.mta0.migadu.com (out-185.mta0.migadu.com [91.218.175.185]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1A0CC30B502; Fri, 31 Jul 2026 02:22:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.185 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785464524; cv=none; b=hG78TlHTDdqs6SPd9CyhD7QaY01807XGoSMehXPINBkHIITDuTmALxwaOj3Us3e78WAiklbNfw2sFbggYM91h944mOejjJrmX7R9WP+2tJgCT+mbjEqjTFMYvowkyxDAZMPKvA7AMlr2FfCdjQ2g3GEk0q7enhyK9fXh1TwA8dU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785464524; c=relaxed/simple; bh=3ds8w1yZdDY6SnmL5w25+/boYExqVHlnd/fcLD3jbuc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=mpe4gO8v+9LMVZIlBr8NTp3R6/4Zr1hwAbozdvKDmOGzeH8iq1vs6vTdLQ3LGt6ZQdsPmM/Fg33dXK9wh4jZ0pciGHsI9A/JAjCTsMj4lcIuZdhiXi8JPClHSTke0kHDuJAQ3z7jj9QUFSz5yWWhv7bncxM5JifYHSxrsUEDCtA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=l1ZbtrjK; arc=none smtp.client-ip=91.218.175.185 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="l1ZbtrjK" Message-ID: <93b6496e-8f52-4983-95e1-01db68609bc0@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1785464519; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=s6dMstA3s1q3jz7gMCf0YyeW1NWmEDHVWbTYZcaJQdY=; b=l1ZbtrjKAzRkvs24cBTwpKonWTyd14o0F7EvbfY9RMtZNkyM6mNCdhteXhYFZn5SoA5r+N FHyWJR7YEFRe3GkWTgUCgWID775ogz3/PPAm7Uv9qslKbxqVY2lO/PBxHajRosk6aQVe+m lSgONwJ/8oDN3jEppAMDdVl1XfKJ3Lc= Date: Fri, 31 Jul 2026 10:21:37 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH 1/2] arm64: ftrace: enable single ftrace_ops for direct calls To: Ihor Solodrai , Catalin Marinas , Will Deacon , Steven Rostedt , Masami Hiramatsu , Mark Rutland , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eduard Zingerman , Kumar Kartikeya Dwivedi , Martin KaFai Lau , Song Liu , Yonghong Song , Jiri Olsa , Emil Tsalapatis , Shuah Khan , "Jose Fernandez (Anthropic)" Cc: bpf@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20260727142844.21212-1-leon.hwang@linux.dev> <20260727142844.21212-2-leon.hwang@linux.dev> <77edc25d-a521-4f65-84ed-dbb016593902@linux.dev> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Leon Hwang In-Reply-To: <77edc25d-a521-4f65-84ed-dbb016593902@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT 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@linux.dev/ > [2] https://lore.kernel.org/all/amjnf5gz0xP5PTSB@J2N7QTR9R3/ > >> >> Assisted-by: Codex:gpt-5.6-sol >> Signed-off-by: Leon Hwang >> --- >> 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@linux.dev/ > >> 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; >