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 mm01.cs.columbia.edu (mm01.cs.columbia.edu [128.59.11.253]) by smtp.lore.kernel.org (Postfix) with ESMTP id ED71CC433EF for ; Fri, 17 Dec 2021 16:34:15 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 70CF94B30A; Fri, 17 Dec 2021 11:34:15 -0500 (EST) X-Virus-Scanned: at lists.cs.columbia.edu Authentication-Results: mm01.cs.columbia.edu (amavisd-new); dkim=softfail (fail, message has been altered) header.i=@redhat.com Received: from mm01.cs.columbia.edu ([127.0.0.1]) by localhost (mm01.cs.columbia.edu [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id HnZubc9oq7pu; Fri, 17 Dec 2021 11:34:14 -0500 (EST) Received: from mm01.cs.columbia.edu (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id 752554B328; Fri, 17 Dec 2021 11:34:14 -0500 (EST) Received: from localhost (localhost [127.0.0.1]) by mm01.cs.columbia.edu (Postfix) with ESMTP id B01734B316 for ; Fri, 17 Dec 2021 11:34:12 -0500 (EST) X-Virus-Scanned: at lists.cs.columbia.edu Received: from mm01.cs.columbia.edu ([127.0.0.1]) by localhost (mm01.cs.columbia.edu [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id ZCTXQvj0CsuX for ; Fri, 17 Dec 2021 11:34:11 -0500 (EST) Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by mm01.cs.columbia.edu (Postfix) with ESMTP id CC9CB4B30A for ; Fri, 17 Dec 2021 11:34:11 -0500 (EST) 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-wm1-f71.google.com (mail-wm1-f71.google.com [209.85.128.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-112-hmGwkjJzPfKS8lqgWAji9w-1; Fri, 17 Dec 2021 11:34:10 -0500 X-MC-Unique: hmGwkjJzPfKS8lqgWAji9w-1 Received: by mail-wm1-f71.google.com with SMTP id z13-20020a05600c0a0d00b003457d6619f8so799438wmp.1 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=39Mv+2T/HhcW6Dk2fkfNte8ZLJB3PgwMwIai48YGKlLsHOXS1JP4FbKepdqMt6FJB7 btemEH9Xxw76xT94Mt/bJ67H+dfYMeqU2w9yUcourM5mUVUvj7a+DZRjwM6mFoAMhhEl b6LpAWkj1w6Ag1uRaLUhbp7pFii8/8YW10SwmSfDNw6k/3QcAJg4Hro/m2VD8FTKXO3G KQ3jgQovyWFqktQGw0IUeW7NDiTBAraPmLoc6Y0nJQ6ZfIIq/JJfTbb00cx4PQ/IOnKx VIfj16hLa+ci7ACLOTLCWxGlHJR/j6pXoFVkg49ZBLWSs6hDBlZDuE53LUWZg0bSlLMT hOOg== X-Gm-Message-State: AOAM532UmlWlitosNqs2AZ6YAXpBYtSeWuE6T8a0N6+3IqUohSntu0/X U4Igz5DmpJMIlqxYpvpjEKWXeu+cag8lkcTIaEbF/Jy3WVG10EuLsTmnXezkvIqBLtOziT1KnPv PCdJIb1+kwZpUXgu0e2n3AMMV X-Received: by 2002:adf:fb05:: with SMTP id c5mr3071893wrr.497.1639758849202; 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 To: paulmck@kernel.org 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> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=pbonzini@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Cc: maz , frederic , linux-kernel , kvmarm@lists.cs.columbia.edu, rcu , Nicolas Saenz Julienne , Will Deacon , Thomas Gleixner , linux-arm-kernel X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu 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 _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm