From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932208AbaE3Dfy (ORCPT ); Thu, 29 May 2014 23:35:54 -0400 Received: from mail-pd0-f181.google.com ([209.85.192.181]:34897 "EHLO mail-pd0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932065AbaE3Dfx (ORCPT ); Thu, 29 May 2014 23:35:53 -0400 Message-ID: <5387FC96.4030508@kernel.dk> Date: Thu, 29 May 2014 21:35:50 -0600 From: Jens Axboe User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0 MIME-Version: 1.0 To: Ming Lei CC: Linux Kernel Mailing List , Rusty Russell , "Michael S. Tsirkin" , virtualization@lists.linux-foundation.org Subject: Re: [PATCH] block: virtio_blk: don't hold spin lock during world switch References: <1401418169-3361-1-git-send-email-ming.lei@canonical.com> <5387F8B2.1070509@kernel.dk> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2014-05-29 21:34, Ming Lei wrote: > On Fri, May 30, 2014 at 11:19 AM, Jens Axboe wrote: >> On 2014-05-29 20:49, Ming Lei wrote: >>> >>> Firstly, it isn't necessary to hold lock of vblk->vq_lock >>> when notifying hypervisor about queued I/O. >>> >>> Secondly, virtqueue_notify() will cause world switch and >>> it may take long time on some hypervisors(such as, qemu-arm), >>> so it isn't good to hold the lock and block other vCPUs. >>> >>> On arm64 quad core VM(qemu-kvm), the patch can increase I/O >>> performance a lot with VIRTIO_RING_F_EVENT_IDX enabled: >>> - without the patch: 14K IOPS >>> - with the patch: 34K IOPS >> >> >> Patch looks good to me. I don't see a hit on my qemu-kvm testing, but it >> definitely makes sense and I can see it hurting in other places. > > It isn't easy to observe the improvement on x86 VM, especially > with few vCPUs, because qemu-system-x86_64 only takes > several microseconds to handle the notification, but on arm64, it > may take hundreds of microseconds, so the improvement is > obvious on arm VM. > > I hope this patch can be merged, at least arm VM can benefit > from it. If Rusty agrees, I'd like to add it for 3.16 with a stable marker. -- Jens Axboe