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 0DB9AC433F5 for ; Fri, 17 Dec 2021 16:34:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235639AbhLQQeM (ORCPT ); Fri, 17 Dec 2021 11:34:12 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:31470 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231193AbhLQQeM (ORCPT ); Fri, 17 Dec 2021 11:34:12 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1639758851; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ChIkAA1P0rhxmMj+OSv7E9eITrJWi4wnxfRUd873BMw=; b=CPd1BWE6OLW6SVUtnbR0MlqmxFBZNBNn9TTR2FqUcxr6iLSE/uRPshQrQptexjsKCngcii qqakzCoK2xx1tXpugALuqC5jezmuVRnJZhLxfNh/CcQLnXo8q0m2DlSq0xSGKDvtuquI8k P8roqy9OvmyotpJqtGXg9ADaq/ftmBU= Received: from mail-wr1-f69.google.com (mail-wr1-f69.google.com [209.85.221.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-112-1ipTiB3QMzqnXnsq-M88Nw-1; Fri, 17 Dec 2021 11:34:10 -0500 X-MC-Unique: 1ipTiB3QMzqnXnsq-M88Nw-1 Received: by mail-wr1-f69.google.com with SMTP id y4-20020adfd084000000b00186b16950f3so775933wrh.14 for ; Fri, 17 Dec 2021 08:34:10 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:to:cc:references:from:in-reply-to :content-transfer-encoding; bh=ChIkAA1P0rhxmMj+OSv7E9eITrJWi4wnxfRUd873BMw=; b=atpNFIyT8aq87e/+lY4upC5URTftR45xhekuU6uS+veuxGRGkRMy30sc6vuBzS38wq OTFTHqMjmDZXxkDfS/q7y6U1SzPOZqduT4pl+FF4MuoeKLG8YQ7qw5dvf5RySIraEvF4 mSRoxcB9JaxLZWC4UvKCh0v6hnNZon7uRYjvqBmcNVhSV+wDHheQQoK5iN0oLuF2Lg9w +ZEVgRikbOWRscbRJqPDeI8b9QBXpe07zcNzPgBI0Ngcs6PRSRzUv3R20pwSLAub9lpf ywjzWvSXlgulc4yQBckuUC6ilEDEpn+ZVUuXFXoZdsFyjY/geSQ1bI7BKGFV+TR6pI0A 5K6Q== X-Gm-Message-State: AOAM530ym+bGW2a86wwjnMn67h9MsYfN1fBQIIlZ6nrQw/fY7PDGKVQm cUdtvjdApyDU83Bs4NnqBkUT0Ab8EC07tpdz3SLMQfJ1+0d9fj7tdSEErKFDGq6C0lBwAEY5lu6 aTWp3zKhd9KNb X-Received: by 2002:adf:fb05:: with SMTP id c5mr3071898wrr.497.1639758849203; Fri, 17 Dec 2021 08:34:09 -0800 (PST) X-Google-Smtp-Source: ABdhPJzDLzLvZGGbxXRIrttXF2p3HboGOachfgkml4ByJfP3WTVmRAGVDmx2EyyY0y0qV0GcR/afCw== X-Received: by 2002:adf:fb05:: with SMTP id c5mr3071876wrr.497.1639758849019; Fri, 17 Dec 2021 08:34:09 -0800 (PST) Received: from ?IPV6:2001:b07:6468:f312:48f9:bea:a04c:3dfe? ([2001:b07:6468:f312:48f9:bea:a04c:3dfe]) by smtp.googlemail.com with ESMTPSA id h204sm7828484wmh.33.2021.12.17.08.34.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 17 Dec 2021 08:34:08 -0800 (PST) Message-ID: Date: Fri, 17 Dec 2021 17:34:04 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: Possible nohz-full/RCU issue in arm64 KVM Content-Language: en-US To: paulmck@kernel.org Cc: Mark Rutland , Nicolas Saenz Julienne , maz , frederic , linux-kernel , rcu , Thomas Gleixner , Will Deacon , kvmarm@lists.cs.columbia.edu, linux-arm-kernel References: <70f112072d9496d21901946ea82832d3ed3a8cb2.camel@redhat.com> <9ab8107f-ff41-6a9e-57e1-a261bea93aca@redhat.com> <20211217160749.GS641268@paulmck-ThinkPad-P17-Gen-1> From: Paolo Bonzini In-Reply-To: <20211217160749.GS641268@paulmck-ThinkPad-P17-Gen-1> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org 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 * or the thread will be preempted and either will keep RCU happy as far as I understand. Paolo