Re: [PATCH v3 net-next] xsk: avoid starving the xsk further down the list

[Magnus Karlsson]

On Mon, 23 Oct 2023 at 13:53, 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 introduced

we introduced -> introduce

> a new variable called tx_budget_cache, 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 temporarily set to TX_BATCH_SIZE(32).

It is not temporary I hope ;-). Just say "The value of
MAX_PER_SOCKET_BUDGET is set to 32."

>
> Signed-off-by: Albert Huang <huangjie.albert@xxxxxxxxxxxxx>
> ---
>  include/net/xdp_sock.h |  5 +++++
>  net/xdp/xsk.c          | 19 +++++++++++++++++++
>  2 files changed, 24 insertions(+)
>
> diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> index 69b472604b86..08cbdf6fca85 100644
> --- a/include/net/xdp_sock.h
> +++ b/include/net/xdp_sock.h
> @@ -63,6 +63,11 @@ struct xdp_sock {
>
>         struct xsk_queue *tx ____cacheline_aligned_in_smp;
>         struct list_head tx_list;
> +       /* Record the actual number of times xsk has transmitted a tx
> +        * descriptor, with a maximum limit not exceeding MAX_PER_SOCKET_BUDGET
> +        */
> +       u32 tx_budget_cache;

Since this is not a cache, I would prefer a name like tx_budget_spent.
Reflects more what the purpose is of this variable. Do not forget to
change this in the commit message too.

> +
>         /* Protects generic receive. */
>         spinlock_t rx_lock;
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index f5e96e0d6e01..fd0d54b7c046 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 xsk_cache_full = false;

Same comment here, it is not a cache. How about just budget_exhausted?
No need for the xsk_ since this is a local variable.

>         struct xdp_sock *xs;
>
>         rcu_read_lock();
> +again:
>         list_for_each_entry_rcu(xs, &pool->xsk_tx_list, tx_list) {
> +               if (xs->tx_budget_cache >= MAX_PER_SOCKET_BUDGET) {
> +                       xsk_cache_full = 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_cache++;
> +
>                 /* 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 (xsk_cache_full) {
> +               list_for_each_entry_rcu(xs, &pool->xsk_tx_list, tx_list) {
> +                       xs->tx_budget_cache = 0;
> +               }

No need for the {} here since there is only a single line in the loop.
Please remove them.

> +               xsk_cache_full = false;
> +               goto again;
> +       }
> +
>  out:
>         rcu_read_unlock();
>         return false;
> @@ -1230,6 +1248,7 @@ static int xsk_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
>         xs->zc = xs->umem->zc;
>         xs->sg = !!(xs->umem->flags & XDP_UMEM_SG_FLAG);
>         xs->queue_id = qid;
> +       xs->tx_budget_cache = 0;

This is not needed. The struct is zeroed at allocation.

>         xp_add_xsk(xs->pool, xs);
>
>  out_unlock:
> --
> 2.20.1
>
>