Re: [PATCH v4 net-next] xsk: avoid starving the xsk further down the list

[Magnus Karlsson]

On Mon, 23 Oct 2023 at 14:57, Albert Huang
<huangjie.albert@xxxxxxxxxxxxx> wrote:
>
> In the previous implementation, when multiple xsk sockets were
> associated with a single xsk_buff_pool, a situation could arise
> where the xsk_tx_list maintained data at the front for one xsk
> socket while starving the xsk sockets at the back of the list.
> This could result in issues such as the inability to transmit packets,
> increased latency, and jitter. To address this problem, we introduce
> a new variable called tx_budget_spent, which limits each xsk to transmit
> a maximum of MAX_PER_SOCKET_BUDGET tx descriptors. This allocation ensures
> equitable opportunities for subsequent xsk sockets to send tx descriptors.
> The value of MAX_PER_SOCKET_BUDGET is set to 32.

Thank you Albert for implementing this feature!

Acked-by: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>

> Signed-off-by: Albert Huang <huangjie.albert@xxxxxxxxxxxxx>
> ---
>  include/net/xdp_sock.h |  7 +++++++
>  net/xdp/xsk.c          | 18 ++++++++++++++++++
>  2 files changed, 25 insertions(+)
>
> diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> index 69b472604b86..de6819e50d54 100644
> --- a/include/net/xdp_sock.h
> +++ b/include/net/xdp_sock.h
> @@ -63,6 +63,13 @@ struct xdp_sock {
>
>         struct xsk_queue *tx ____cacheline_aligned_in_smp;
>         struct list_head tx_list;
> +       /* record the number of tx descriptors sent by this xsk and
> +        * when it exceeds MAX_PER_SOCKET_BUDGET, an opportunity needs
> +        * to be given to other xsks for sending tx descriptors, thereby
> +        * preventing other XSKs from being starved.
> +        */
> +       u32 tx_budget_spent;
> +
>         /* Protects generic receive. */
>         spinlock_t rx_lock;
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index f5e96e0d6e01..65c32b85c326 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -33,6 +33,7 @@
>  #include "xsk.h"
>
>  #define TX_BATCH_SIZE 32
> +#define MAX_PER_SOCKET_BUDGET (TX_BATCH_SIZE)
>
>  static DEFINE_PER_CPU(struct list_head, xskmap_flush_list);
>
> @@ -413,16 +414,25 @@ EXPORT_SYMBOL(xsk_tx_release);
>
>  bool xsk_tx_peek_desc(struct xsk_buff_pool *pool, struct xdp_desc *desc)
>  {
> +       bool budget_exhausted = false;
>         struct xdp_sock *xs;
>
>         rcu_read_lock();
> +again:
>         list_for_each_entry_rcu(xs, &pool->xsk_tx_list, tx_list) {
> +               if (xs->tx_budget_spent >= MAX_PER_SOCKET_BUDGET) {
> +                       budget_exhausted = true;
> +                       continue;
> +               }
> +
>                 if (!xskq_cons_peek_desc(xs->tx, desc, pool)) {
>                         if (xskq_has_descs(xs->tx))
>                                 xskq_cons_release(xs->tx);
>                         continue;
>                 }
>
> +               xs->tx_budget_spent++;
> +
>                 /* This is the backpressure mechanism for the Tx path.
>                  * Reserve space in the completion queue and only proceed
>                  * if there is space in it. This avoids having to implement
> @@ -436,6 +446,14 @@ bool xsk_tx_peek_desc(struct xsk_buff_pool *pool, struct xdp_desc *desc)
>                 return true;
>         }
>
> +       if (budget_exhausted) {
> +               list_for_each_entry_rcu(xs, &pool->xsk_tx_list, tx_list)
> +                       xs->tx_budget_spent = 0;
> +
> +               budget_exhausted = false;
> +               goto again;
> +       }
> +
>  out:
>         rcu_read_unlock();
>         return false;
> --
> 2.20.1
>
>