On 2018年06月29日 16:09, Toshiaki Makita wrote:
Under heavy load vhost busypoll may run without suppressing
notification. For example tx zerocopy callback can push tx work while
handle_tx() is running, then busyloop exits due to vhost_has_work()
condition and enables notification but immediately reenters handle_tx()
because the pushed work was tx. In this case handle_tx() tries to
disable notification again, but when using event_idx it by design
cannot. Then busyloop will run without suppressing notification.
Another example is the case where handle_tx() tries to enable
notification but avail idx is advanced so disables it again. This case
also lead to the same situation with event_idx.
Good catch.
The problem is that once we enter this situation busyloop does not work
under heavy load for considerable amount of time, because notification
is likely to happen during busyloop and handle_tx() immediately enables
notification after notification happens. Specifically busyloop detects
notification by vhost_has_work() and then handle_tx() calls
vhost_enable_notify(). Because the detected work was the tx work, it
enters handle_tx(), and enters busyloop without suppression again.
This is likely to be repeated, so with event_idx we are almost not able
to suppress notification in this case.
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 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.
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?
- 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.
Signed-off-by: Toshiaki Makita <makita.toshiaki@xxxxxxxxxxxxx>
---
drivers/vhost/net.c | 94 +++++++++++++++++++++++++++++++++++----------------
drivers/vhost/vhost.c | 1 +
drivers/vhost/vhost.h | 1 +
3 files changed, 66 insertions(+), 30 deletions(-)
diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index eeaf6739215f..0e85f628b965 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -391,13 +391,14 @@ static inline unsigned long busy_clock(void)
return local_clock() >> 10;
}
-static bool vhost_can_busy_poll(struct vhost_dev *dev,
- unsigned long endtime)
+static bool vhost_can_busy_poll(unsigned long endtime)
{
- return likely(!need_resched()) &&
- likely(!time_after(busy_clock(), endtime)) &&
- likely(!signal_pending(current)) &&
- !vhost_has_work(dev);
+ return likely(!need_resched() && !time_after(busy_clock(), endtime));
+}
+
+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.
+ } 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);
@@ -509,12 +521,16 @@ static void handle_tx(struct vhost_net *net)
break;
/* Nothing new? Wait for eventfd to tell us they refilled. */
if (head == vq->num) {
- if (unlikely(vhost_enable_notify(&net->dev, vq))) {
+ if (unlikely(vq->busyloop_endtime)) {
+ /* Busy poll is interrupted. */
+ vhost_poll_queue(&vq->poll);
+ } else if (unlikely(vhost_enable_notify(&net->dev, vq))) {
vhost_disable_notify(&net->dev, vq);
continue;
}
break;
}
+ vq->busyloop_endtime = 0;
if (in) {
vq_err(vq, "Unexpected descriptor format for TX: "
"out %d, int %d\n", out, in);
@@ -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.
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.
+ }
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;
@@ -804,6 +832,7 @@ static void handle_rx(struct vhost_net *net)
mergeable = vhost_has_feature(vq, VIRTIO_NET_F_MRG_RXBUF);
while ((sock_len = vhost_net_rx_peek_head_len(net, sock->sk))) {
+ vq->busyloop_endtime = 0;
sock_len += sock_hlen;
vhost_len = sock_len + vhost_hlen;
headcount = get_rx_bufs(vq, vq->heads + nvq->done_idx,
@@ -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
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
out:
vhost_rx_signal_used(nvq);
mutex_unlock(&vq->mutex);
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 9beefa6ed1ce..fe83578fe336 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -323,6 +323,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
vhost_reset_is_le(vq);
vhost_disable_cross_endian(vq);
vq->busyloop_timeout = 0;
+ vq->busyloop_endtime = 0;
vq->umem = NULL;
vq->iotlb = NULL;
__vhost_vq_meta_reset(vq);
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index 6c844b90a168..7e9cf80ccae9 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -144,6 +144,7 @@ struct vhost_virtqueue {
bool user_be;
#endif
u32 busyloop_timeout;
+ unsigned long busyloop_endtime;
};
struct vhost_msg_node {
