From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jens Axboe Subject: Re: [PATCH] block: virtio_blk: don't hold spin lock during world switch Date: Thu, 29 May 2014 21:35:50 -0600 Message-ID: <5387FC96.4030508@kernel.dk> References: <1401418169-3361-1-git-send-email-ming.lei@canonical.com> <5387F8B2.1070509@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Ming Lei Cc: virtualization@lists.linux-foundation.org, Linux Kernel Mailing List , "Michael S. Tsirkin" List-Id: virtualization@lists.linuxfoundation.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