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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id E6446C433EF for ; Tue, 4 Jan 2022 16:40:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=EwuPLG6Yts2h/KFtkNa7FUeKUUaSYiiu5ssmvEycous=; b=bpjk0/AG5geTDJ VJF8GVVxYFYnS5ZpMmGELolC+kUoXUqa6MmSmxKW38jzlnVs8CVx3VjJ73UwUuSjfErrT4pGZcvgT 9+ygHCVlHQG8OFYKV2T39sFW/XWdBXfHiNqVMKhUGZlgxKKiy5vEF9zMEIDY2UT0s1rDkcpDpy7QB VjYZb4BB4rlOMP1Rg661QJ2/Ss8woei+NFi++H64wETIAUYK4I04XBAoV2KVj8F5C/hNXK4rPUX6L 3Kku86ZecVLlOqPJ3yf4PnjKojPANIy3FZY44pvB8LIiYxoUIeE0zDE3LMbJqxXcnykgrUSqRNZU5 er9OkLnVtN7gZp+mzMKg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1n4mqW-00CHeJ-Pr; Tue, 04 Jan 2022 16:39:36 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1n4mqS-00CHcC-PT for linux-arm-kernel@lists.infradead.org; Tue, 04 Jan 2022 16:39:34 +0000 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-Disposition: inline In-Reply-To: <9ab8107f-ff41-6a9e-57e1-a261bea93aca@redhat.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220104_083932_902301_CBC8BC99 X-CRM114-Status: GOOD ( 17.50 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.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. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel