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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (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 2466DC433F5 for ; Thu, 12 May 2022 08:26:32 +0000 (UTC) Received: from localhost ([::1]:59742 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1np49X-0003Rt-3t for qemu-devel@archiver.kernel.org; Thu, 12 May 2022 04:26:31 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:52276) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1np3yy-0001CV-Rg for qemu-devel@nongnu.org; Thu, 12 May 2022 04:15:39 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:20531) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1np3yt-0003kt-PY for qemu-devel@nongnu.org; Thu, 12 May 2022 04:15:34 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1652343329; h=from:from:reply-to: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=0hPRLMc2ErNYW/vKtm79G/gK7PHjvaR2XhEwVjwJ+JA=; b=ZL+EhC6UIvVDx6yy475TeqawARuMpbxwlGx704RyOwTN/vqcOxvFGwWzn+NpJGXkkzew1G Os/43QCbxe9iaGCK37vVkdz76f79y6nUeZisPNTu4R/LqKqZVN1I3ra6kf6M9QIJnFJirL tX1Rk+XSLR92pqDZmHtnFwRREbaEpV4= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-197-hSylL1pIN463hP70glpiMg-1; Thu, 12 May 2022 04:15:28 -0400 X-MC-Unique: hSylL1pIN463hP70glpiMg-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 443A4101A52C for ; Thu, 12 May 2022 08:15:28 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.134]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5E44141617B; Thu, 12 May 2022 08:15:26 +0000 (UTC) Date: Thu, 12 May 2022 09:15:22 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Paolo Bonzini Cc: David Hildenbrand , Michal Privoznik , qemu-devel@nongnu.org Subject: Re: [PATCH] util: NUMA aware memory preallocation Message-ID: References: <8a6b84ed-50bc-8f6e-4b71-7e15247c4ac0@redhat.com> <5374a249-8389-3d11-1b30-b0b6e6910a51@redhat.com> <04938ba0-7ff4-df3c-348d-b679eac4fbac@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <04938ba0-7ff4-df3c-348d-b679eac4fbac@redhat.com> User-Agent: Mutt/2.2.1 (2022-02-19) X-Scanned-By: MIMEDefang 2.85 on 10.11.54.10 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, May 12, 2022 at 09:41:29AM +0200, Paolo Bonzini wrote: > On 5/11/22 18:54, Daniel P. Berrangé wrote: > > On Wed, May 11, 2022 at 01:07:47PM +0200, Paolo Bonzini wrote: > > > On 5/11/22 12:10, Daniel P. Berrangé wrote: > > > > I expect creating/deleting I/O threads is cheap in comparison to > > > > the work done for preallocation. If libvirt is using -preconfig > > > > and object-add to create the memory backend, then we could have > > > > option of creating the I/O threads dynamically in -preconfig mode, > > > > create the memory backend, and then delete the I/O threads again. > > > > > > I think this is very overengineered. Michal's patch is doing the obvious > > > thing and if it doesn't work that's because Libvirt is trying to micromanage > > > QEMU. > > > > Calling it micromanaging is putting a very negative connotation on > > this. What we're trying todo is enforce a host resource policy for > > QEMU, in a way that a compromised QEMU can't escape, which is a > > valuable protection. > > I'm sorry if that was a bit exaggerated, but the negative connotation was > intentional. > > > > As mentioned on IRC, if the reason is to prevent moving around threads in > > > realtime (SCHED_FIFO, SCHED_RR) classes, that should be fixed at the kernel > > > level. > > > > We use cgroups where it is available to us, but we don't always have > > the freedom that we'd like. > > I understand. I'm thinking of a new flag to sched_setscheduler that fixes > the CPU affinity and policy of the thread and prevents changing it in case > QEMU is compromised later. The seccomp/SELinux sandboxes can prevent > setting the SCHED_FIFO class without this flag. > > In addition, my hunch is that this works only because the RT setup of QEMU > is not safe against priority inversion. IIRC the iothreads are set with a > non-realtime priority, but actually they should have a _higher_ priority > than the CPU threads, and the thread pool I/O bound workers should have an > even higher priority; otherwise you have a priority inversion situation > where an interrupt is pending that would wake up the CPU, but the iothreads > cannot process it because they have a lower priority than the CPU. At least for RHEL deployments of KVM-RT, IIC the expectation is that the VCPUs with RT priority never do I/O, and that there is at least 1 additional non-RT vCPU from which the OS performs I/O. IOW, the RT VCPU works in a completely self contained manner with no interaction to any other QEMU threads. If that's not the case, then you would have to make sure those other threads have priority / schedular adjustments to avoid priority inversion With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|