From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 90D4EC433FE for ; Tue, 4 Jan 2022 16:39:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235429AbiADQj2 (ORCPT ); Tue, 4 Jan 2022 11:39:28 -0500 Received: from foss.arm.com ([217.140.110.172]:33614 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235378AbiADQj1 (ORCPT ); Tue, 4 Jan 2022 11:39:27 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5941113A1; Tue, 4 Jan 2022 08:39:27 -0800 (PST) Received: from FVFF77S0Q05N (unknown [10.57.9.1]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 638603F774; Tue, 4 Jan 2022 08:39:25 -0800 (PST) Date: Tue, 4 Jan 2022 16:39:19 +0000 From: Mark Rutland To: Paolo Bonzini Cc: Nicolas Saenz Julienne , paulmck , maz , frederic , linux-kernel , rcu , Thomas Gleixner , Will Deacon , kvmarm@lists.cs.columbia.edu, linux-arm-kernel , Anup Patel Subject: Re: Possible nohz-full/RCU issue in arm64 KVM Message-ID: References: <70f112072d9496d21901946ea82832d3ed3a8cb2.camel@redhat.com> <9ab8107f-ff41-6a9e-57e1-a261bea93aca@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9ab8107f-ff41-6a9e-57e1-a261bea93aca@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 17, 2021 at 04:54:22PM +0100, Paolo Bonzini wrote: > On 12/17/21 15:38, Mark Rutland wrote: > > For example kvm_guest_enter_irqoff() calls guest_enter_irq_off() which calls > > vtime_account_guest_enter(), but kvm_guest_exit_irqoff() doesn't call > > guest_exit_irq_off() and the call to vtime_account_guest_exit() is open-coded > > elsewhere. Also, guest_enter_irq_off() conditionally calls > > rcu_virt_note_context_switch(), but I can't immediately spot anything on the > > exit side that corresponded with that, which looks suspicious. > > rcu_note_context_switch() is a point-in-time notification; it's not strictly > necessary, but it may improve performance a bit by avoiding unnecessary IPIs > from the RCU subsystem. > > There's no benefit from doing it when you're back from the guest, because at > that point the CPU is just running normal kernel code. I see. My main issue here was just that it's really difficult to see how the entry/exit logic is balanced, and I reckon we can solve that by splitting guest_{enter,exit}_irqoff() into helper functions to handle the vtime accounting separately from the context tracking, so that arch code can do something like: guest_timing_enter_irqoff(); guest_eqs_enter_irqoff(); < actually run vCPU here > guest_eqs_exit_irqoff(); < handle pending IRQs here > guest_timing_exit_irqoff(); ... which I hope should work for RISC-V too. I've had a go, and I've pushed out a WIP to: https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/kvm/rcu I also see we'll need to add some lockdep/irq-tracing management to arm64, and it probably makes sense to fold that into common helpers, so I'll have a play with that tomorrow. Thanks, Mark.