Re: [PATCH 5/6] kvm: qemu: virtio-net: handle all tx in I/O thread without timer

[Mark McLoughlin <markmc@redhat.com>]

On Fri, 2008-10-31 at 09:16 +0000, Mark McLoughlin wrote:
 
> +static void virtio_net_tx_bh(void *opaque)
>  {
>      VirtIONet *n = opaque;
>  
> -    n->tx_timer_active = 0;
> -
> -    /* Just in case the driver is not ready on more */
> -    if (!(n->vdev.status & VIRTIO_CONFIG_S_DRIVER_OK))
> -        return;
> -
> -    n->tx_vq->vring.used->flags &= ~VRING_USED_F_NO_NOTIFY;
> -    virtio_net_flush_tx(n, n->tx_vq);
> +    if (!virtio_net_flush_tx(n, n->tx_vq))
> +        n->tx_vq->vring.used->flags &= ~VRING_USED_F_NO_NOTIFY;
> +    else
> +        qemu_bh_schedule(n->tx_bh);
>  }

Ouch, race condition here between our finding the ring empty and
re-enabling the notification. We need to check the ring again.

Cheers,
Mark.

From: Mark McLoughlin <markmc@redhat.com>
Subject: [PATCH] kvm: qemu: virtio-net: handle all tx in I/O thread without timer

By removing the tx timer altogether and doing all the copies in the
I/O thread, we can keep the I/O churning away in parallel with the
guest generating more I/O.

In my tests, this significantly increases guest->host throughput,
causes a minor increase in host->guest throughput, reduces CPU
utilization somewhat and greatly reduces roundtrip times.

Even aside from the benchmark results, removing the arbitrary 150us
timer is a nicer option than coming up with a heuristic to make it
vary according to load. Finally, on kernels which don't have a
suitably low posix timer latency, we won't be scuppered by effectively
having e.g. a 1ms timer.

Note, this highlights that the I/O thread may become a scalability
concern and we might want to consider e.g. an I/O thread per device.

Note also that when tuning for a specific workload, which CPU
the I/O thread is pinned to is important.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
---
 qemu/hw/virtio-net.c |   65 +++++++++++++++++++++++++------------------------
 1 files changed, 33 insertions(+), 32 deletions(-)

diff --git a/qemu/hw/virtio-net.c b/qemu/hw/virtio-net.c
index bc2ede6..43a06da 100644
--- a/qemu/hw/virtio-net.c
+++ b/qemu/hw/virtio-net.c
@@ -35,8 +35,6 @@
 #define VIRTIO_NET_F_HOST_ECN	13	/* Host can handle TSO[6] w/ ECN in. */
 #define VIRTIO_NET_F_HOST_UFO	14	/* Host can handle UFO in. */
 
-#define TX_TIMER_INTERVAL 150000 /* 150 us */
-
 /* The config defining mac address (6 bytes) */
 struct virtio_net_config
 {
@@ -68,8 +66,7 @@ typedef struct VirtIONet
     VirtQueue *rx_vq;
     VirtQueue *tx_vq;
     VLANClientState *vc;
-    QEMUTimer *tx_timer;
-    int tx_timer_active;
+    QEMUBH *tx_bh;
 } VirtIONet;
 
 /* TODO
@@ -227,13 +224,14 @@ static void virtio_net_receive(void *opaque, const uint8_t *buf, int size)
 }
 
 /* TX */
-static void virtio_net_flush_tx(VirtIONet *n, VirtQueue *vq)
+static int virtio_net_flush_tx(VirtIONet *n, VirtQueue *vq)
 {
     VirtQueueElement elem;
     int has_vnet_hdr = tap_has_vnet_hdr(n->vc->vlan->first_client);
+    int num_packets = 0;
 
     if (!(n->vdev.status & VIRTIO_CONFIG_S_DRIVER_OK))
-        return;
+        return num_packets;
 
     while (virtqueue_pop(vq, &elem)) {
 	ssize_t len = 0;
@@ -256,38 +254,38 @@ static void virtio_net_flush_tx(VirtIONet *n, VirtQueue *vq)
 
 	virtqueue_push(vq, &elem, len);
 	virtio_notify(&n->vdev, vq);
+
+	num_packets++;
     }
+
+    return num_packets;
 }
 
 static void virtio_net_handle_tx(VirtIODevice *vdev, VirtQueue *vq)
 {
     VirtIONet *n = to_virtio_net(vdev);
 
-    if (n->tx_timer_active) {
-	vq->vring.used->flags &= ~VRING_USED_F_NO_NOTIFY;
-	qemu_del_timer(n->tx_timer);
-	n->tx_timer_active = 0;
-	virtio_net_flush_tx(n, vq);
-    } else {
-	qemu_mod_timer(n->tx_timer,
-		       qemu_get_clock(vm_clock) + TX_TIMER_INTERVAL);
-	n->tx_timer_active = 1;
-	vq->vring.used->flags |= VRING_USED_F_NO_NOTIFY;
-    }
+    vq->vring.used->flags |= VRING_USED_F_NO_NOTIFY;
+    qemu_bh_schedule(n->tx_bh);
 }
 
-static void virtio_net_tx_timer(void *opaque)
+static void virtio_net_tx_bh(void *opaque)
 {
     VirtIONet *n = opaque;
+    VirtQueue *vq = n->tx_vq;
 
-    n->tx_timer_active = 0;
+    /* Re-enable notifications if empty */
+    if (!virtio_net_flush_tx(n, vq)) {
+        vq->vring.used->flags &= ~VRING_USED_F_NO_NOTIFY;
 
-    /* Just in case the driver is not ready on more */
-    if (!(n->vdev.status & VIRTIO_CONFIG_S_DRIVER_OK))
-        return;
+        /* Re-check to be sure we didn't miss something */
+        if (!virtio_net_flush_tx(n, vq))
+            return;
+    }
 
-    n->tx_vq->vring.used->flags &= ~VRING_USED_F_NO_NOTIFY;
-    virtio_net_flush_tx(n, n->tx_vq);
+    /* We flushed some packets; let's try again soon */
+    vq->vring.used->flags |= VRING_USED_F_NO_NOTIFY;
+    qemu_bh_schedule(n->tx_bh);
 }
 
 static void virtio_net_save(QEMUFile *f, void *opaque)
@@ -297,7 +295,6 @@ static void virtio_net_save(QEMUFile *f, void *opaque)
     virtio_save(&n->vdev, f);
 
     qemu_put_buffer(f, n->mac, 6);
-    qemu_put_be32(f, n->tx_timer_active);
 }
 
 static int virtio_net_load(QEMUFile *f, void *opaque, int version_id)
@@ -310,12 +307,15 @@ static int virtio_net_load(QEMUFile *f, void *opaque, int version_id)
     virtio_load(&n->vdev, f);
 
     qemu_get_buffer(f, n->mac, 6);
-    n->tx_timer_active = qemu_get_be32(f);
 
-    if (n->tx_timer_active) {
-	qemu_mod_timer(n->tx_timer,
-		       qemu_get_clock(vm_clock) + TX_TIMER_INTERVAL);
-    }
+    return 0;
+}
+
+static int virtio_net_uninit(PCIDevice *dev)
+{
+    VirtIONet *n = (VirtIONet *)dev;
+
+    qemu_bh_delete(n->tx_bh);
 
     return 0;
 }
@@ -341,8 +341,9 @@ PCIDevice *virtio_net_init(PCIBus *bus, NICInfo *nd, int devfn)
     n->vc = qemu_new_vlan_client(nd->vlan, virtio_net_receive,
                                  virtio_net_can_receive, n);
 
-    n->tx_timer = qemu_new_timer(vm_clock, virtio_net_tx_timer, n);
-    n->tx_timer_active = 0;
+    n->tx_bh = qemu_bh_new(virtio_net_tx_bh, n);
+
+    n->vdev.pci_dev.unregister = virtio_net_uninit;
 
     register_savevm("virtio-net", virtio_net_id++, 1,
 		    virtio_net_save, virtio_net_load, n);
-- 
1.6.0.1