Messages in this thread Patch in this message | | | Date | Wed, 11 Feb 2015 06:03:36 +0008 | From | Jason Wang <> | Subject | Re: [PATCH RFC v5 net-next 2/6] virtio_ring: try to disable event index callbacks in virtqueue_disable_cb() |
| |
On Tue, Feb 10, 2015 at 6:24 PM, Michael S. Tsirkin <mst@redhat.com> wrote: > 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 <jasowang@redhat.com> > > 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.
It was used to disable tx interrupt during start_xmit(). > > 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?
Yes. It does reduce, see the following test results on multiple sessions of TCP_RR
Datacopy: /size/sessions/+-throughput%/+-cpu%/+-per_cpu_throughput%/+-tx_interrupts%/ /1/50/+0.1/+0.7/-0.6/-72.3/ /64/50/+1.7/+1.0/+0.7/-72.2/ /256/50/+0.0/-0.8/+0.9/-72.4/
Zerocopy: /size/sessions/+-throughput%/+-cpu%/+-per_cpu_throughput%/+-tx_interrupts%/ /1/50/+3.2/-1.3/+4.6/-71.7/ /64/50/+1.9/-0.2/+2.1/-70.8/ /256/50/+4.4/+0.6/+3.9/-84.2/
No obvious changes for stream rx and tx. > > > >> --- >> 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?
We need to try best to avoid any interrupt after this and virtqueue_disable_cb() were always need virtqueue_enable_cb() or virtqueue_enable_cb_delayed() afterwards. Those two function will check the pending used buffers and set a proper used event. > > > 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?
I tested avail->idx + vq.num but not obvious changes in the performance compared to just use avail->idx here. So 0xffff probably won't help much. Since we call virtqueue_enable_cb() or virtqueue_disable_cb_delayed() afterwards, it doesn't matter that whether avail->idx or avail->idx - 1 is used. > > > > >> } >> EXPORT_SYMBOL_GPL(virtqueue_disable_cb); >> >> -- >> 1.8.3.1 > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html
| |