Re: [Xen-devel] [PATCH v2 09/13] xen/pvcalls: implement recvmsg

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 07/25/2017 05:22 PM, Stefano Stabellini wrote:
> Implement recvmsg by copying data from the "in" ring. If not enough data
> is available and the recvmsg call is blocking, then wait on the
> inflight_conn_req waitqueue. Take the active socket in_mutex so that
> only one function can access the ring at any given time.
>
> If not enough data is available on the ring, rather than returning
> immediately or sleep-waiting, spin for up to 5000 cycles. This small
> optimization turns out to improve performance and latency significantly.
>
> Signed-off-by: Stefano Stabellini <stefano@xxxxxxxxxxx>
> CC: boris.ostrovsky@xxxxxxxxxx
> CC: jgross@xxxxxxxx
> ---
>  drivers/xen/pvcalls-front.c | 106 
> ++++++++++++++++++++++++++++++++++++++++++++
>  drivers/xen/pvcalls-front.h |   4 ++
>  2 files changed, 110 insertions(+)
>
> diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> index d8ed280..b4ca569 100644
> --- a/drivers/xen/pvcalls-front.c
> +++ b/drivers/xen/pvcalls-front.c
> @@ -96,6 +96,20 @@ static int pvcalls_front_write_todo(struct sock_mapping 
> *map)
>       return size - pvcalls_queued(prod, cons, size);
>  }
>  
> +static bool pvcalls_front_read_todo(struct sock_mapping *map)
> +{
> +     struct pvcalls_data_intf *intf = map->active.ring;
> +     RING_IDX cons, prod;
> +     int32_t error;
> +
> +     cons = intf->in_cons;
> +     prod = intf->in_prod;
> +     error = intf->in_error;
> +     return (error != 0 ||
> +             pvcalls_queued(prod, cons,
> +                            XEN_FLEX_RING_SIZE(intf->ring_order))) != 0;
> +}
> +
>  static irqreturn_t pvcalls_front_event_handler(int irq, void *dev_id)
>  {
>       struct xenbus_device *dev = dev_id;
> @@ -418,6 +432,98 @@ int pvcalls_front_sendmsg(struct socket *sock, struct 
> msghdr *msg,
>       return tot_sent;
>  }
>  
> +static int __read_ring(struct pvcalls_data_intf *intf,
> +                    struct pvcalls_data *data,
> +                    struct iov_iter *msg_iter,
> +                    size_t len, int flags)
> +{
> +     RING_IDX cons, prod, size, masked_prod, masked_cons;
> +     RING_IDX array_size = XEN_FLEX_RING_SIZE(intf->ring_order);
> +     int32_t error;
> +
> +     cons = intf->in_cons;
> +     prod = intf->in_prod;
> +     error = intf->in_error;
> +     /* get pointers before reading from the ring */
> +     virt_rmb();
> +     if (error < 0)
> +             return error;
> +
> +     size = pvcalls_queued(prod, cons, array_size);
> +     masked_prod = pvcalls_mask(prod, array_size);
> +     masked_cons = pvcalls_mask(cons, array_size);
> +
> +     if (size == 0)
> +             return 0;
> +
> +     if (len > size)
> +             len = size;
> +
> +     if (masked_prod > masked_cons) {
> +             copy_to_iter(data->in + masked_cons, len, msg_iter);
> +     } else {
> +             if (len > (array_size - masked_cons)) {
> +                     copy_to_iter(data->in + masked_cons,
> +                                  array_size - masked_cons, msg_iter);
> +                     copy_to_iter(data->in,
> +                                  len - (array_size - masked_cons),
> +                                  msg_iter);
> +             } else {
> +                     copy_to_iter(data->in + masked_cons, len, msg_iter);
> +             }
> +     }
> +     /* read data from the ring before increasing the index */
> +     virt_mb();
> +     if (!(flags & MSG_PEEK))
> +             intf->in_cons += len;
> +
> +     return len;
> +}
> +
> +int pvcalls_front_recvmsg(struct socket *sock, struct msghdr *msg, size_t 
> len,
> +                  int flags)
> +{
> +     struct pvcalls_bedata *bedata;
> +     int ret = -EAGAIN;
> +     struct sock_mapping *map;
> +     int count = 0;
> +
> +     if (!pvcalls_front_dev)
> +             return -ENOTCONN;
> +     bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
> +
> +     map = (struct sock_mapping *) READ_ONCE(sock->sk->sk_send_head);
> +     if (!map)
> +             return -ENOTSOCK;
> +
> +     if (flags & (MSG_CMSG_CLOEXEC|MSG_ERRQUEUE|MSG_OOB|MSG_TRUNC))
> +             return -EOPNOTSUPP;
> +
> +     mutex_lock(&map->active.in_mutex);
> +     if (len > XEN_FLEX_RING_SIZE(map->active.ring->ring_order))
> +             len = XEN_FLEX_RING_SIZE(map->active.ring->ring_order);
> +
> +     while (!(flags & MSG_DONTWAIT) && !pvcalls_front_read_todo(map)) {
> +             if (count < PVCALLS_FRONT_MAX_SPIN)
> +                     count++;
> +             else
> +                     wait_event_interruptible(map->active.inflight_conn_req,
> +                                              pvcalls_front_read_todo(map));
> +     }

Should we be using PVCALLS_FRONT_MAX_SPIN here? In sendmsg it is
counting non-sleeping iterations but here we are sleeping so
PVCALLS_FRONT_MAX_SPIN (5000) may take a while.

In fact, what shouldn't this waiting be a function of MSG_DONTWAIT
and/or socket's O_NONBLOCK?

-boris


> +     ret = __read_ring(map->active.ring, &map->active.data,
> +                       &msg->msg_iter, len, flags);
> +
> +     if (ret > 0)
> +             notify_remote_via_irq(map->active.irq);
> +     if (ret == 0)
> +             ret = -EAGAIN;
> +     if (ret == -ENOTCONN)
> +             ret = 0;
> +
> +     mutex_unlock(&map->active.in_mutex);
> +     return ret;
> +}
> +
>  int pvcalls_front_bind(struct socket *sock, struct sockaddr *addr, int 
> addr_len)
>  {
>       struct pvcalls_bedata *bedata;
> diff --git a/drivers/xen/pvcalls-front.h b/drivers/xen/pvcalls-front.h
> index d937c24..de24041 100644
> --- a/drivers/xen/pvcalls-front.h
> +++ b/drivers/xen/pvcalls-front.h
> @@ -16,5 +16,9 @@ int pvcalls_front_accept(struct socket *sock,
>  int pvcalls_front_sendmsg(struct socket *sock,
>                         struct msghdr *msg,
>                         size_t len);
> +int pvcalls_front_recvmsg(struct socket *sock,
> +                       struct msghdr *msg,
> +                       size_t len,
> +                       int flags);
>  
>  #endif


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