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 AE52EC433EF for ; Tue, 11 Jan 2022 12:23:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239847AbiAKMXQ (ORCPT ); Tue, 11 Jan 2022 07:23:16 -0500 Received: from foss.arm.com ([217.140.110.172]:45610 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237953AbiAKMXP (ORCPT ); Tue, 11 Jan 2022 07:23:15 -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 A0AEE1FB; Tue, 11 Jan 2022 04:23:14 -0800 (PST) Received: from FVFF77S0Q05N (unknown [10.57.1.156]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id BD14E3F774; Tue, 11 Jan 2022 04:23:12 -0800 (PST) Date: Tue, 11 Jan 2022 12:23:10 +0000 From: Mark Rutland To: Nicolas Saenz Julienne Cc: Paolo Bonzini , 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> <399d8805ca09f7d3c905b8c653abf60dd7141574.camel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <399d8805ca09f7d3c905b8c653abf60dd7141574.camel@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 11, 2022 at 12:32:38PM +0100, Nicolas Saenz Julienne wrote: > Hi Mark, > > On Tue, 2022-01-04 at 16:39 +0000, Mark Rutland wrote: > > 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 > > Had a look at the patches and they seeem OK to me. > > Thanks! Cool. FWIW I have an updated version at: https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=kvm/entry-rework ... which is largely the same approach, but the helpers got renamed, the lockdep/tracing bits got fixed, and I've aligned mips, riscv, and x86 on the same approach. Once I get a free hour or so I intend to rebase that atop v5.16 and post that out. I'll start a new thread with that, and rope in the relevant arch maintainers (since e.g. I'm not sure what to do for ppc and s390). Thanks, Mark. > > -- > Nicolás Sáenz >