netdev - Re: [PATCH v2 net-next 4/4] vhost_net: Avoid rx vring kicks during busyloop

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Date:   Wed, 4 Jul 2018 10:21:03 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Tonghao Zhang <xiangxia.m.yue@...il.com>
Subject: Re: [PATCH v2 net-next 4/4] vhost_net: Avoid rx vring kicks during
 busyloop

On 2018/07/03 18:05, Jason Wang wrote:
> On 2018年07月03日 15:31, Toshiaki Makita wrote:
>> We may run out of avail rx ring descriptor under heavy load but busypoll
>> did not detect it so busypoll may have exited prematurely. Avoid this by
>> checking rx ring full during busypoll.
> 
> Actually, we're checking whether it was empty in fact?

My understanding is that on rx empty avail means no free descriptors
from the perspective of host so the ring is conceptually full.

>> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
>> ---
>>   drivers/vhost/net.c | 10 +++++++---
>>   1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>> index 791bc8b..b224036 100644
>> --- a/drivers/vhost/net.c
>> +++ b/drivers/vhost/net.c
>> @@ -658,6 +658,7 @@ static int vhost_net_rx_peek_head_len(struct
>> vhost_net *net, struct sock *sk,
>>   {
>>       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;
>>       unsigned long uninitialized_var(endtime);
>>       int len = peek_head_len(rnvq, sk);
>> @@ -677,7 +678,8 @@ static int vhost_net_rx_peek_head_len(struct
>> vhost_net *net, struct sock *sk,
>>                   *busyloop_intr = true;
>>                   break;
>>               }
>> -            if (sk_has_rx_data(sk) ||
>> +            if ((sk_has_rx_data(sk) &&
>> +                 !vhost_vq_avail_empty(&net->dev, rvq)) ||
>>                   !vhost_vq_avail_empty(&net->dev, tvq))
>>                   break;
>>               cpu_relax();
>> @@ -827,7 +829,6 @@ static void handle_rx(struct vhost_net *net)
>>   
> 
> I thought below codes should belong to patch 3. Or I may miss something.

That codes are added for the case that busypoll is waiting for rx vq
avail but interrupted by another work. At the point of patch 3 busypoll
does not wait for rx vq avail so I don't think we move them to patch 3.

> 
> Thanks
> 
>>       while ((sock_len = vhost_net_rx_peek_head_len(net, sock->sk,
>>                                 &busyloop_intr))) {
>> -        busyloop_intr = false;
>>           sock_len += sock_hlen;
>>           vhost_len = sock_len + vhost_hlen;
>>           headcount = get_rx_bufs(vq, vq->heads + nvq->done_idx,
>> @@ -838,7 +839,9 @@ static void handle_rx(struct vhost_net *net)
>>               goto out;
>>           /* OK, now we need to know about added descriptors. */
>>           if (!headcount) {
>> -            if (unlikely(vhost_enable_notify(&net->dev, vq))) {
>> +            if (unlikely(busyloop_intr)) {
>> +                vhost_poll_queue(&vq->poll);
>> +            } else if (unlikely(vhost_enable_notify(&net->dev, vq))) {
>>                   /* They have slipped one in as we were
>>                    * doing that: check again. */
>>                   vhost_disable_notify(&net->dev, vq);
>> @@ -848,6 +851,7 @@ static void handle_rx(struct vhost_net *net)
>>                * they refilled. */
>>               goto out;
>>           }
>> +        busyloop_intr = false;
>>           if (nvq->rx_ring)
>>               msg.msg_control = vhost_net_buf_consume(&nvq->rxq);
>>           /* On overrun, truncate and discard */

-- 
Toshiaki Makita