From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754257AbbBJKYX (ORCPT ); Tue, 10 Feb 2015 05:24:23 -0500 Received: from mx1.redhat.com ([209.132.183.28]:36801 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751318AbbBJKYV (ORCPT ); Tue, 10 Feb 2015 05:24:21 -0500 Date: Tue, 10 Feb 2015 11:24:17 +0100 From: "Michael S. Tsirkin" To: Jason Wang Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, pagupta@redhat.com Subject: Re: [PATCH RFC v5 net-next 2/6] virtio_ring: try to disable event index callbacks in virtqueue_disable_cb() Message-ID: <20150210102417.GB9505@redhat.com> References: <1423471165-34243-1-git-send-email-jasowang@redhat.com> <1423471165-34243-3-git-send-email-jasowang@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1423471165-34243-3-git-send-email-jasowang@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 09, 2015 at 03:39:21AM -0500, Jason Wang wrote: > Currently, we do nothing to prevent the callbacks in > virtqueue_disable_cb() when event index is used. This may cause > spurious interrupts which may damage the performance. This patch tries > to publish avail event as the used even to prevent the callbacks. > > Signed-off-by: Jason Wang I'm surprised that this ever happens though. Normally we call this after getting an interrupt, and interrupts won't trigger again until the rings wraps around. When I tested this, touching an extra cache line was more expensive. Does this really reduce number of interrupts? Could you pls share some numbers with and without this patch? > --- > drivers/virtio/virtio_ring.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > index 545fed5..e9ffbfb 100644 > --- a/drivers/virtio/virtio_ring.c > +++ b/drivers/virtio/virtio_ring.c > @@ -539,6 +539,8 @@ void virtqueue_disable_cb(struct virtqueue *_vq) > struct vring_virtqueue *vq = to_vvq(_vq); > > vq->vring.avail->flags |= cpu_to_virtio16(_vq->vdev, VRING_AVAIL_F_NO_INTERRUPT); > + vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev, > + vq->vring.avail->idx); Hmm in fact, can't this actually cause an extra interrupt when avail->idx is completed? I think that if you really can show disabling interrupts like this helps, you should set some invalid value like 0xfffff, or move it back to vq->vring.avail->idx - 1. No? > } > EXPORT_SYMBOL_GPL(virtqueue_disable_cb); > > -- > 1.8.3.1