From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 A54884657FB for ; Tue, 4 Aug 2026 13:43:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785851022; cv=none; b=E9EiDmMQrovy49c6sKDfjkH5bD+FMo/2ca0sf7DmCPNca3yk0hLDPOruicjH2t1Nu7BM+/bwnEqBiac4hOHalWW/uuIxx1xnB++7iqmREQyZE+yWsheLUy8+LAK44vPHrbIi5xZarMtSnkMccwxzKE692jg9OTQEWGC5jF0yd/8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785851022; c=relaxed/simple; bh=Vaw+9i1uithhOakmcBKkqbyk6oCmIlY1rHLZ1/Ayhsc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ahahyQeY7OVadV9j1JlKcVlrTMXkueGJ8m+xMJIDIQzFS5FTC5GcCV4GUuf9eaO0I/7Svm9zMUgWeM5O8MfchbiC4NezeECRTjbaa7VER7N1IeEZ1yaams6qXuda2sLm8dRe94/JoIMVtAJ4Daz1Ac2qJpvhBHNOcydO1qNzo3g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=AHtmCdds; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="AHtmCdds" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8FD5B1F000E9; Tue, 4 Aug 2026 13:43:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785851021; bh=IBH/Dc/ZJz+4sJLW+ut9cuYtgG9NRpWhNfYqB994G2g=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=AHtmCddswf2PGZfFcl62pyqqu04/E7F86tIAV9KM56Q5mQvysgWALmoW8qvGMEPtR HBMDSSXBJflPvUTOlLnjZsqOjrTFtum7s4cbYYKas1fQt27U2HnbRya71c0Wo6USc3 Iv4IVqCrmNgfTlzkcOAnqGtaLsglgfasO9QF8P8MeugPQ4SYyGRlLckNrPHrBHswFE 8GdjBCg3n3Bt/9axjacpHDRnn1VJ6UBN4RlVOPAy6ckT1FsVG/ZewYkSURKS1V8gjS 6yjV+Fvo5mDBJZE/7cKa86Z5QntI9ieyTI2glVuRYHV7dePDLx/UtbUPoVx3UneCvh 8W9330z+eNB1Q== Date: Tue, 4 Aug 2026 14:43:35 +0100 From: Will Deacon To: Naman Jain Cc: Anshuman Khandual , Jinjie Ruan , Catalin Marinas , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Marc Zyngier , Thomas Huth , Fuad Tabba , Thomas Gleixner , Pengjie Zhang , mrigendrachaubey , Saurabh Sengar Subject: Re: [PATCH] arm64: smp: distinguish secondary CPUs that hang after reaching head.S Message-ID: References: <20260722113044.1835365-1-namjain@linux.microsoft.com> <6e13c10f-ccc2-48c3-bf8a-d65133a33e17@huawei.com> <70e8596f-7316-4cc9-90e8-ff06a94c4616@arm.com> <45ab9640-14b7-48d7-89e2-109aa3935539@linux.microsoft.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <45ab9640-14b7-48d7-89e2-109aa3935539@linux.microsoft.com> On Mon, Jul 27, 2026 at 11:08:05AM +0530, Naman Jain wrote: > On 7/26/2026 7:16 PM, Will Deacon wrote: > > On Fri, Jul 24, 2026 at 10:16:03AM +0530, Naman Jain wrote: > > > On 7/23/2026 11:26 AM, Anshuman Khandual wrote: > > > > On 23/07/26 8:19 AM, Jinjie Ruan wrote: > > > > > 在 2026/7/22 19:30, Naman Jain 写道: > > > > > > When a secondary CPU fails to come online, __cpu_up() falls back to > > > > > > __early_cpu_boot_status, but boot status 0x0 is ambiguous: it cannot > > > > > > distinguish a CPU that never executed head.S (firmware/hypervisor never > > > > > > dispatched it, so it never ran a single instruction) from one that > > > > > > entered head.S, started executing, and then got stuck somewhere in kernel > > > > > > bring-up. Add a change to let us tell those two cases apart, which > > > > > > narrows down where to look when a CPU goes missing during boot. > > > > > I previously encountered this issue when debugging the parallel startup > > > > > of ARM64 secondary cores. It is difficult for the kernel to determine > > > > > whether the secondary core is hung in the firmware or whether it has not > > > > > executed a single instruction. So I think this motive is reasonable. > > > > > > > > Why should kernel determine the difference here ? Would not the firmware > > > > know if it has started any secondary CPU for the kernel which must have > > > > come inside head.S ? If the cpu gets hung inside firmware while starting > > > > up then the debug responsibilities belong there instead. > > > > > > > > Still wondering what's the rationale for this change. > > > > > > Hello Anshuman, > > > This sounds fair to me. Let me elaborate the problem, beyond the scope of > > > this patch. In production, we occasionally see these crashes where one of > > > the CPU fails to bring up online, with 0x0 status code. Hypervisor may be > > > missing the telemetry, but the problem is that we don't know if the > > > secondary CPU ever started executing the instructions or is stuck somewhere > > > between the start of head.S and marking itself online at the end of > > > secondary_start_kernel(). > > > There are couple of places, where we get those other status codes, but not > > > everywhere. If the issue is not easily reproducible, experiments on local > > > setups do not yield anything. That's where I am attempting to add some more > > > information in kernel to debug these issues. > > > > I think this is a game of diminishing returns. There's a lot of stuff > > that the firmware/hypervisor can get wrong here and trying to detect or > > handle that in Linux is going to be a real mess. For example, if it > > enters the kernel at the wrong address, or in the wrong mode, or with > > the MMU enabled etc. It sounds like you don't have much idea about > > what happens in the failure case, so it might not even execute the code > > that you're adding correctly. > > That is true. I agree. While we cannot and should not worry about adding > logs for each of these firmware failure points in kernel, do you see any > merit in adding any of this information to the kernel to at least narrow > down the problem? Possibly, but I'd prefer to land the parallel bringup work first and then see whether this still makes sense. Will