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 D1C87C433F5 for ; Fri, 17 Dec 2021 18:02:26 +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:Reply-To: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:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=W0qmeyI6ZPIGchJ0bsjba1wN+jqxAEdL44IW7rO7vzE=; b=UEc75gflL1vvjh bnL4rcBZcDkUDNviXVwZLp5mojQGomKmQgfY3np0AKFg3iluigAAMerpJEgN7qUYPYX9AYKepgzKC t5KEzibVqig6WfH81ps4L3MDGeXfV0BKQgwVTY7I96oArvvQMonxHcZLVp88MkEn9wrhxCxLSgcfu qOhUEoH/9mNgEtDyXW0BUatnZYnPR0gxuN/9oCxUI4hJchH26ltA4VaCedbS7GmqjfkHpbcDxTjrD xrPW0c5kBq2IdjBhDWhZZd9ScI5ZOs6kDITrYZkT2DehySrLOYqoweuRQPWz0tD6/VCrd4XZ5r1fk e3l+SADTi0kJPNO5LGsw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1myHXA-00Bcr8-TY; Fri, 17 Dec 2021 18:00:45 +0000 Received: from dfw.source.kernel.org ([2604:1380:4641:c500::1]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1myGMT-00BFDj-RM for linux-arm-kernel@lists.infradead.org; Fri, 17 Dec 2021 16:45:39 +0000 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 230DD622F5; Fri, 17 Dec 2021 16:45:36 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 875B4C36AE1; Fri, 17 Dec 2021 16:45:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1639759535; bh=+Z8HaNORlefmFC0iorPRnnxWsiru7mFwmG4mt8j06g4=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=Uxtf76sJEP4I9ft93wAfQue9vIwowuCrbeQOafQU8q5x25anDEvL/i0tGoheL5fIG UrUvxGSD0HapyDwZ4QB4fN5FUdjRJljktSyF2/kBTSBcVNRWcseC9Vtw0evM/f6Pyy Kx0OZlk5IGVD0g9NdVSJ/9pZKBifhSDE0VEI9m5dzhW0EuJYIyN8CQ2QvywsjjYkaG 1ON6eJPNElI0OVa6p7txk8YagpmccK7Cp104iB2f9i9TX17j2NsvjA8XIyqMzCZYWY 7PxU2dUqKUOA/8r+hXqvGh3aB/xAjxfZlmmc+JQubmkDcrDxUxbFwePVAQvLZO7/9A gps/Rws4WD2ZQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 4AEEA5C0610; Fri, 17 Dec 2021 08:45:35 -0800 (PST) Date: Fri, 17 Dec 2021 08:45:35 -0800 From: "Paul E. McKenney" To: Paolo Bonzini Cc: Mark Rutland , Nicolas Saenz Julienne , maz , frederic , linux-kernel , rcu , Thomas Gleixner , Will Deacon , kvmarm@lists.cs.columbia.edu, linux-arm-kernel Subject: Re: Possible nohz-full/RCU issue in arm64 KVM Message-ID: <20211217164535.GU641268@paulmck-ThinkPad-P17-Gen-1> References: <70f112072d9496d21901946ea82832d3ed3a8cb2.camel@redhat.com> <9ab8107f-ff41-6a9e-57e1-a261bea93aca@redhat.com> <20211217160749.GS641268@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20211217_084537_980047_A02655CB X-CRM114-Status: GOOD ( 20.66 ) 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: , Reply-To: paulmck@kernel.org 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 05:34:04PM +0100, Paolo Bonzini wrote: > On 12/17/21 17:07, Paul E. McKenney wrote: > > > 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. > > > > Do scheduling-clock interrupts from guest mode have the "user" parameter > > set? If so, that would keep RCU happy. > > No, thread is in supervisor mode. But after every interrupt (timer tick or > anything), one of three things can happen: > > * KVM will go around the execution loop and invoke rcu_note_context_switch() > again > > * or KVM will go back to user space Here "user space" is a user process as opposed to a guest OS? > * or the thread will be preempted > > and either will keep RCU happy as far as I understand. Regardless of the answer to my question above, yes, these will keep RCU happy. ;-) Thanx, Paul _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel