From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A12DE1EB2A for ; Mon, 1 Jul 2024 13:09:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719839345; cv=none; b=ZPSXb7qCrQKO9lYv5Da8s/5JSUUpNlznYhbLY3A6KB2HY2mHgNAEgAUqRtBNvs109Eb11Zfkowft3S6OOiEd+KGK8mwYXrxoPuK9SAv/L+o+ZXh9sdajUqw4+nw3pCIFXBiCoE0Le6SZLGqeU1aiHF5OLOdAu6ANtG03BPz0TnU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719839345; c=relaxed/simple; bh=AKOtvwBE+iAb1YgrZ9LxJ7T3pHNxgFd6TqsDNeKoAAY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=azLiuhUxayf6vd5WraU/oMRJOnCm21PawH9JD9rytN7pvUUXbg2wv+ssYiNgNKec+E6vtt1meSub9ergT9URmiVMw6DVgHmebtwbxRRLZSaM9eC3SCFSe9JoT/c5YaHs8DLX8xWKp0bRXSgB3Mq7aQPx2y6E3udByzbLokG2TME= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=Ed7aI9RS; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Ed7aI9RS" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1719839342; 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:in-reply-to:in-reply-to: references:references; bh=HDghtGgi9SYDQpIXIitUpG8W1XNWFlatyKAnpi2QbPA=; b=Ed7aI9RSiMLXwACPB19mzXAc6/FWw2FjXz5jGre6+y3tdU45tN1jT6xTlkw7XjlR7jRWKE cDIsN9f0nvqEuGUAwoh4ewdhLa4joafQ6UzaGRoV9ukQZaifLfmbs1gMGVB+cztX413h/9 iO2gLKfqtn1PrvokMP9SbTgwtoo0UIs= Received: from mx-prod-mc-02.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-47-y6oZN0DaMpK2Pn4NV72BXA-1; Mon, 01 Jul 2024 09:09:00 -0400 X-MC-Unique: y6oZN0DaMpK2Pn4NV72BXA-1 Received: from mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.15]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 684571944DDB; Mon, 1 Jul 2024 13:08:59 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.124]) by mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 5E7C01956089; Mon, 1 Jul 2024 13:08:54 +0000 (UTC) Date: Mon, 1 Jul 2024 14:08:50 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Mikulas Patocka Cc: Tejun Heo , Lai Jiangshan , Waiman Long , Mike Snitzer , Laurence Oberman , Jonathan Brassow , Ming Lei , Ondrej Kozina , Milan Broz , linux-kernel@vger.kernel.org, dm-devel@lists.linux.dev, users@lists.libvirt.org Subject: Re: dm-crypt performance regression due to workqueue changes Message-ID: Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= References: <32fd8274-d5f-3eca-f5d2-1a9117fd8edb@redhat.com> Precedence: bulk X-Mailing-List: dm-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/2.2.12 (2023-09-09) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.15 On Sun, Jun 30, 2024 at 08:49:48PM +0200, Mikulas Patocka wrote: > > > On Sun, 30 Jun 2024, Tejun Heo wrote: > > > Hello, > > > > On Sat, Jun 29, 2024 at 08:15:56PM +0200, Mikulas Patocka wrote: > > > > > With 6.5, we get 3600MiB/s; with 6.6 we get 1400MiB/s. > > > > > > The reason is that virt-manager by default sets up a topology where we > > > have 16 sockets, 1 core per socket, 1 thread per core. And that workqueue > > > patch avoids moving work items across sockets, so it processes all > > > encryption work only on one virtual CPU. > > > > The performance degradation may be fixed with "echo 'system' > > > >/sys/module/workqueue/parameters/default_affinity_scope" - but it is > > > regression anyway, as many users don't know about this option. > > > > > > How should we fix it? There are several options: > > > 1. revert back to 'numa' affinity > > > 2. revert to 'numa' affinity only if we are in a virtual machine > > > 3. hack dm-crypt to set the 'numa' affinity for the affected workqueues > > > 4. any other solution? > > > > Do you happen to know why libvirt is doing that? There are many other > > implications to configuring the system that way and I don't think we want to > > design kernel behaviors to suit topology information fed to VMs which can be > > arbitrary. > > > > Thanks. > > I don't know why. I added users@lists.libvirt.org to the CC. > > How should libvirt properly advertise "we have 16 threads that are > dynamically scheduled by the host kernel, so the latencies between them > are changing and unpredictable"? NB, libvirt is just control plane, the actual virtual hardware exposed is implemented across QEMU and the KVM kernel mod. Guest CPU topology and/or NUMA cost information is the responsibility of QEMU. When QEMU's virtual CPUs are floating freely across host CPUs there's no perfect answer. The host admin needs to make a tradeoff in their configuration They can optimize for density, by allowing guest CPUs to float freely and allow CPU overcommit against host CPUs, and the guest CPU topology is essentially a lie. They can optimize for predictable performance, by strictly pinning guest CPUs 1:1 to host CPUs, and minimize CPU overcommit, and have the guest CPU topology 1:1 match the host CPU topology. 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 :|