On 2017年09月01日 23:51, Michael S. Tsirkin wrote:
On Fri, Sep 01, 2017 at 05:02:50PM +0800, Jason Wang wrote:
We check tx avail through vhost_enable_notify() in the past which is
wrong since it only checks whether or not guest has filled more
available buffer since last avail idx synchronization which was just
done by vhost_vq_avail_empty() before. What we really want is checking
pending buffers in the avail ring.
These are rx buffers, right? I'm not even sure why do we need to poll
for them. Running out of rx buffers is a slow path.
Actually it polls for tx buffer here. I admit the code (or probably the
variable name) is confusing here.
Fix this by calling
vhost_vq_avail_empty() instead.
This issue could be noticed by doing netperf TCP_RR benchmark as
client from guest (but not host). With this fix, TCP_RR from guest to
localhost restores from 1375.91 trans per sec to 55235.28 trans per
sec on my laptop (Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz).
Fixes: 030881372460 ("vhost_net: basic polling support")
Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
---
- The patch is needed for -stable
---
drivers/vhost/net.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 06d0448..1b68253 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -634,7 +634,7 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk)
In fact why does it poll the ring at all? I thought this function's
job is to poll the socket, isn't it?
Tx notification is disabled to try to avoid vmexits, so we poll tx avail
buffers too.
preempt_enable();
- if (vhost_enable_notify(&net->dev, vq))
+ if (!vhost_vq_avail_empty(&net->dev, vq))
vhost_poll_queue(&vq->poll);
mutex_unlock(&vq->mutex);
Adding more contex:
mutex_lock(&vq->mutex);
vhost_disable_notify(&net->dev, vq);
preempt_disable();
endtime = busy_clock() + vq->busyloop_timeout;
while (vhost_can_busy_poll(&net->dev, endtime) &&
!sk_has_rx_data(sk) &&
vhost_vq_avail_empty(&net->dev, vq))
cpu_relax();
preempt_enable();
if (vhost_enable_notify(&net->dev, vq))
vhost_poll_queue(&vq->poll);
mutex_unlock(&vq->mutex);
len = peek_head_len(rvq, sk);
If you drop this we'll exit the function with notifications
disabled. Seems wrong to me.
Yes, will fix this in V2.
Thanks
--
2.7.4
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