Hi Jason,

On 2018/06/29 18:30, Jason Wang wrote:

On 2018年06月29日 16:09, Toshiaki Makita wrote:

...

quoted

To fix this, poll the work instead of enabling notification when
busypoll is interrupted by something. IMHO signal_pending() and
vhost_has_work() are kind of interruptions rather than signals to
completely cancel the busypoll, so let's run busypoll after the
necessary work is done. In order to avoid too long busyloop due to
interruption, save the endtime in vq field and use it when reentering
the work function.

I think we don't care long busyloop unless e.g tx can starve rx?

I just want to keep it user-controllable. Unless memorizing it busypoll
can run unexpectedly long.

quoted

I observed this problem makes tx performance poor but does not rx. I
guess it is because rx notification from socket is not that heavy so
did not impact the performance even when we failed to mask the
notification.

I think the reason is:

For tx, we may only process used ring under handle_tx(). Busy polling
code can not recognize this case.
For rx, we call peek_head_len() after exiting busy loop, so if the work
is for rx, it can be processed in handle_rx() immediately.

Sorry but I don't see the difference. Tx busypoll calls
vhost_get_vq_desc() immediately after busypoll exits in
vhost_net_tx_get_vq_desc().

The scenario I described above (in 2nd paragraph) is a bit simplified.
To be clearer what I think is happening is:

1. handle_tx() runs busypoll with notification enabled (due to zerocopy
   callback or something).
2. Guest produces a packet in vring.
3. handle_tx() busypoll detects the produced packet and get it.
4. While vhost is processing the packet, guest kicks vring because it
   produced the packet. Vring notification is disabled automatically by
   event_idx and tx work is queued.
5. After processing the packet vhost enters busyloop again, but detects
   tx work and exits busyloop immediately. Then handle_tx() exits after
   enabling the notification.
6. Because the queued work was tx, handle_tx() is called immediately
   again. handle_tx() runs busypoll with notification enabled.
7. Repeat 2-6.

quoted

Anyway for consistency I fixed rx routine as well as tx.

Performance numbers:

- Bulk transfer from guest to external physical server.
     [Guest]->vhost_net->tap--(XDP_REDIRECT)-->i40e --(wire)--> [Server]

Just to confirm in this case since zerocopy is enabled, we are in fact
use the generic XDP datapath?

For some reason zerocopy was not applied for most packets, so in most
cases driver XDP was used. I was going to dig into it but not yet.

quoted

- Set 10us busypoll.
- Guest disables checksum and TSO because of host XDP.
- Measured single flow Mbps by netperf, and kicks by perf kvm stat
   (EPT_MISCONFIG event).

                             Before              After
                           Mbps  kicks/s      Mbps  kicks/s
UDP_STREAM 1472byte              247758                 27
                 Send   3645.37            6958.10
                 Recv   3588.56            6958.10
               1byte                9865                 37
                 Send      4.34               5.43
                 Recv      4.17               5.26
TCP_STREAM             8801.03    45794   9592.77     2884

Impressive numbers.

quoted

Signed-off-by: Toshiaki Makita <redacted>
---

...

quoted

+static bool vhost_busy_poll_interrupted(struct vhost_dev *dev)
+{
+    return unlikely(signal_pending(current)) || vhost_has_work(dev);
  }
    static void vhost_net_disable_vq(struct vhost_net *n,

@@ -437,10 +438,21 @@ static int vhost_net_tx_get_vq_desc(struct

vhost_net *net,
        if (r == vq->num && vq->busyloop_timeout) {
          preempt_disable();
-        endtime = busy_clock() + vq->busyloop_timeout;
-        while (vhost_can_busy_poll(vq->dev, endtime) &&
-               vhost_vq_avail_empty(vq->dev, vq))
+        if (vq->busyloop_endtime) {
+            endtime = vq->busyloop_endtime;
+            vq->busyloop_endtime = 0;

Looks like endtime may be before current time. So we skip busy loop in
this case.

Sure, I'll add a condition.

quoted

+        } else {
+            endtime = busy_clock() + vq->busyloop_timeout;
+        }
+        while (vhost_can_busy_poll(endtime)) {
+            if (vhost_busy_poll_interrupted(vq->dev)) {
+                vq->busyloop_endtime = endtime;
+                break;
+            }
+            if (!vhost_vq_avail_empty(vq->dev, vq))
+                break;
              cpu_relax();
+        }
          preempt_enable();
          r = vhost_get_vq_desc(vq, vq->iov, ARRAY_SIZE(vq->iov),
                        out_num, in_num, NULL, NULL);

...

quoted

@@ -642,39 +658,51 @@ static void vhost_rx_signal_used(struct

vhost_net_virtqueue *nvq)
    static int vhost_net_rx_peek_head_len(struct vhost_net *net,
struct sock *sk)
  {
-    struct vhost_net_virtqueue *rvq = &net->vqs[VHOST_NET_VQ_RX];
-    struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
-    struct vhost_virtqueue *vq = &nvq->vq;
+    struct vhost_net_virtqueue *rnvq = &net->vqs[VHOST_NET_VQ_RX];
+    struct vhost_net_virtqueue *tnvq = &net->vqs[VHOST_NET_VQ_TX];
+    struct vhost_virtqueue *rvq = &rnvq->vq;
+    struct vhost_virtqueue *tvq = &tnvq->vq;

NIt: I admit the variable name is confusing. It would be better to do
the renaming in a separate patch to ease review.

OK.

quoted

      unsigned long uninitialized_var(endtime);
-    int len = peek_head_len(rvq, sk);
+    int len = peek_head_len(rnvq, sk);
  -    if (!len && vq->busyloop_timeout) {
+    if (!len && tvq->busyloop_timeout) {
          /* Flush batched heads first */
-        vhost_rx_signal_used(rvq);
+        vhost_rx_signal_used(rnvq);
          /* Both tx vq and rx socket were polled here */
-        mutex_lock_nested(&vq->mutex, 1);
-        vhost_disable_notify(&net->dev, vq);
+        mutex_lock_nested(&tvq->mutex, 1);
+        vhost_disable_notify(&net->dev, tvq);
            preempt_disable();
-        endtime = busy_clock() + vq->busyloop_timeout;
+        if (rvq->busyloop_endtime) {
+            endtime = rvq->busyloop_endtime;
+            rvq->busyloop_endtime = 0;
+        } else {
+            endtime = busy_clock() + tvq->busyloop_timeout;
+        }
  -        while (vhost_can_busy_poll(&net->dev, endtime) &&
-               !sk_has_rx_data(sk) &&
-               vhost_vq_avail_empty(&net->dev, vq))
+        while (vhost_can_busy_poll(endtime)) {
+            if (vhost_busy_poll_interrupted(&net->dev)) {
+                rvq->busyloop_endtime = endtime;
+                break;
+            }
+            if (sk_has_rx_data(sk) ||
+                !vhost_vq_avail_empty(&net->dev, tvq))
+                break;
              cpu_relax();

Actually, I plan to poll guest rx refilling as well here to avoid rx
kicks. Want to draft another patch to do it as well? It's as simple as
add a vhost_vq_avail_empty for rvq above.

OK. will try it.

quoted

+        }
            preempt_enable();
  -        if (!vhost_vq_avail_empty(&net->dev, vq))
-            vhost_poll_queue(&vq->poll);
-        else if (unlikely(vhost_enable_notify(&net->dev, vq))) {
-            vhost_disable_notify(&net->dev, vq);
-            vhost_poll_queue(&vq->poll);
+        if (!vhost_vq_avail_empty(&net->dev, tvq)) {
+            vhost_poll_queue(&tvq->poll);
+        } else if (unlikely(vhost_enable_notify(&net->dev, tvq))) {
+            vhost_disable_notify(&net->dev, tvq);
+            vhost_poll_queue(&tvq->poll);
          }
  -        mutex_unlock(&vq->mutex);
+        mutex_unlock(&tvq->mutex);
  -        len = peek_head_len(rvq, sk);
+        len = peek_head_len(rnvq, sk);
      }
        return len;

...

quoted

@@ -889,7 +918,12 @@ static void handle_rx(struct vhost_net *net)

              goto out;
          }
      }
-    vhost_net_enable_vq(net, vq);
+    if (unlikely(vq->busyloop_endtime)) {
+        /* Busy poll is interrupted. */
+        vhost_poll_queue(&vq->poll);
+    } else {
+        vhost_net_enable_vq(net, vq);
+    }

This is to reduce the rx wake ups. Better with another patch.

So I suggest to split the patches into:

1 better naming of variable of vhost_net_rx_peek_head_len().
2 avoid tx kicks (most of what this patch did)
3 avoid rx wakeups (exactly the above 6 lines did)
4 avoid rx kicks

OK, I'll reorganize the patch.
Thank you for your feedback!

Btw, tonghao is doing some refactoring of busy polling as well. Depends
on the order of being merged, one of you may need rebasing.

Thanks for sharing. I'll take a look.

-- 
Toshiaki Makita

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