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 11:26:02 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        "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年07月04日 09:21, Toshiaki Makita wrote:
> 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.

Ok.

>
>>> 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.

I get this.

Thanks

>
>> 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 */