Message-ID: <CAL+tcoA=9S6USV5EufhtrNuUF=8BHOQk2duqfsUH3uhdrwSAKw@mail.gmail.com>
Date: Sat, 30 Aug 2025 18:30:23 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc: bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net, 
	andrii@...nel.org, netdev@...r.kernel.org, magnus.karlsson@...el.com, 
	stfomichev@...il.com, Eryk Kubanski <e.kubanski@...tner.samsung.com>
Subject: Re: [PATCH v7 bpf] xsk: fix immature cq descriptor production

On Sat, Aug 30, 2025 at 2:10 AM Maciej Fijalkowski
<maciej.fijalkowski@...el.com> wrote:
>
> Eryk reported an issue that I have put under Closes: tag, related to
> umem addrs being prematurely produced onto pool's completion queue.
> Let us make the skb's destructor responsible for producing all addrs
> that given skb used.
>
> Commit from fixes tag introduced the buggy behavior, it was not broken
> from day 1, but rather when XSK multi-buffer got introduced.
>
> In order to mitigate performance impact as much as possible, mimic the
> linear and frag parts within skb by storing the first address from XSK
> descriptor at sk_buff::destructor_arg. For fragments, store them at ::cb
> via list. The nodes that will go onto list will be allocated via
> kmem_cache. xsk_destruct_skb() will consume address stored at
> ::destructor_arg and optionally go through list from ::cb, if count of
> descriptors associated with this particular skb is bigger than 1.
>
> Previous approach where whole array for storing UMEM addresses from XSK
> descriptors was pre-allocated during first fragment processing yielded
> too big performance regression for 64b traffic. In current approach
> impact is much reduced on my tests and for jumbo frames I observed
> traffic being slower by at most 9%.
>
> Magnus suggested to have this way of processing special cased for
> XDP_SHARED_UMEM, so we would identify this during bind and set different
> hooks for 'backpressure mechanism' on CQ and for skb destructor, but
> given that results looked promising on my side I decided to have a
> single data path for XSK generic Tx. I suppose other auxiliary stuff
> such as helpers introduced in this patch would have to land as well in
> order to make it work, so we might have ended up with more noisy diff.
>
> Fixes: b7f72a30e9ac ("xsk: introduce wrappers and helpers for supporting multi-buffer in Tx path")
> Reported-by: Eryk Kubanski <e.kubanski@...tner.samsung.com>
> Closes: https://lore.kernel.org/netdev/20250530103456.53564-1-e.kubanski@partner.samsung.com/
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> ---
>
> Jason, please test this v7 on your setup, I would appreciate if you
> would report results from your testbed. Thanks!

Thanks for reworking!

And I see the performance only goes down by 1-2% on my VM which looks
much better than before. But I cannot tell where the decrease comes
from...

>
> v1:
> https://lore.kernel.org/bpf/20250702101648.1942562-1-maciej.fijalkowski@intel.com/
> v2:
> https://lore.kernel.org/bpf/20250705135512.1963216-1-maciej.fijalkowski@intel.com/
> v3:
> https://lore.kernel.org/bpf/20250806154127.2161434-1-maciej.fijalkowski@intel.com/
> v4:
> https://lore.kernel.org/bpf/20250813171210.2205259-1-maciej.fijalkowski@intel.com/
> v5:
> https://lore.kernel.org/bpf/aKXBHGPxjpBDKOHq@boxer/T/
> v6:
> https://lore.kernel.org/bpf/20250820154416.2248012-1-maciej.fijalkowski@intel.com/
>
> v1->v2:
> * store addrs in array carried via destructor_arg instead having them
>   stored in skb headroom; cleaner and less hacky approach;
> v2->v3:
> * use kmem_cache for xsk_addrs allocation (Stan/Olek)
> * set err when xsk_addrs allocation fails (Dan)
> * change xsk_addrs layout to avoid holes
> * free xsk_addrs on error path
> * rebase
> v3->v4:
> * have kmem_cache as percpu vars
> * don't drop unnecessary braces (unrelated) (Stan)
> * use idx + i in xskq_prod_write_addr (Stan)
> * alloc kmem_cache on bind (Stan)
> * keep num_descs as first member in xsk_addrs (Magnus)
> * add ack from Magnus
> v4->v5:
> * have a single kmem_cache per xsk subsystem (Stan)
> v5->v6:
> * free skb in xsk_build_skb_zerocopy() when xsk_addrs allocation fails
>   (Stan)
> * unregister netdev notifier if creating kmem_cache fails (Stan)
> v6->v7:
> * don't include Acks from Magnus/Stan; let them review the new
>   approach:)
> * store first desc at sk_buff::destructor_arg and rest of frags in list
>   stored at sk_buff::cb
> * keep the kmem_cache but don't use it for allocation of whole array at
>   one shot but rather alloc single nodes of list
>
> ---
>  net/xdp/xsk.c       | 99 ++++++++++++++++++++++++++++++++++++++-------
>  net/xdp/xsk_queue.h | 12 ++++++
>  2 files changed, 97 insertions(+), 14 deletions(-)
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 9c3acecc14b1..3d12d1fbda41 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -36,6 +36,20 @@
>  #define TX_BATCH_SIZE 32
>  #define MAX_PER_SOCKET_BUDGET 32
>
> +struct xsk_addr_node {
> +       u64 addr;
> +       struct list_head addr_node;
> +};
> +
> +struct xsk_addr_head {
> +       u32 num_descs;
> +       struct list_head addrs_list;
> +};
> +
> +static struct kmem_cache *xsk_tx_generic_cache;
> +
> +#define XSKCB(skb) ((struct xsk_addr_head *)((skb)->cb))
> +
>  void xsk_set_rx_need_wakeup(struct xsk_buff_pool *pool)
>  {
>         if (pool->cached_need_wakeup & XDP_WAKEUP_RX)
> @@ -532,24 +546,41 @@ static int xsk_wakeup(struct xdp_sock *xs, u8 flags)
>         return dev->netdev_ops->ndo_xsk_wakeup(dev, xs->queue_id, flags);
>  }
>
> -static int xsk_cq_reserve_addr_locked(struct xsk_buff_pool *pool, u64 addr)
> +static int xsk_cq_reserve_locked(struct xsk_buff_pool *pool)
>  {
>         unsigned long flags;
>         int ret;
>
>         spin_lock_irqsave(&pool->cq_lock, flags);
> -       ret = xskq_prod_reserve_addr(pool->cq, addr);
> +       ret = xskq_prod_reserve(pool->cq);
>         spin_unlock_irqrestore(&pool->cq_lock, flags);
>
>         return ret;
>  }
>
> -static void xsk_cq_submit_locked(struct xsk_buff_pool *pool, u32 n)
> +static void xsk_cq_submit_addr_locked(struct xsk_buff_pool *pool,
> +                                     struct sk_buff *skb)
>  {
> +       struct xsk_addr_node *pos, *tmp;
>         unsigned long flags;
> +       u32 i = 0;
> +       u32 idx;
>
>         spin_lock_irqsave(&pool->cq_lock, flags);
> -       xskq_prod_submit_n(pool->cq, n);
> +       idx = xskq_get_prod(pool->cq);
> +
> +       xskq_prod_write_addr(pool->cq, idx, (u64)skb_shinfo(skb)->destructor_arg);
> +       i++;
> +
> +       if (unlikely(XSKCB(skb)->num_descs > 1)) {

IIUC, the line you lately added is used to see if it matches the case?
But the condition is still a bit loose. How about adding a more strict
condition: testing whether the umem is shared or not?

Thanks,
Jason

> +               list_for_each_entry_safe(pos, tmp, &XSKCB(skb)->addrs_list, addr_node) {
> +                       xskq_prod_write_addr(pool->cq, idx + i, pos->addr);
> +                       i++;
> +                       list_del(&pos->addr_node);
> +                       kmem_cache_free(xsk_tx_generic_cache, pos);
> +               }
> +       }
> +       xskq_prod_submit_n(pool->cq, i);
>         spin_unlock_irqrestore(&pool->cq_lock, flags);
>  }
>
> @@ -562,9 +593,14 @@ static void xsk_cq_cancel_locked(struct xsk_buff_pool *pool, u32 n)
>         spin_unlock_irqrestore(&pool->cq_lock, flags);
>  }
>
> +static void xsk_inc_num_desc(struct sk_buff *skb)
> +{
> +       XSKCB(skb)->num_descs++;
> +}
> +
>  static u32 xsk_get_num_desc(struct sk_buff *skb)
>  {
> -       return skb ? (long)skb_shinfo(skb)->destructor_arg : 0;
> +       return XSKCB(skb)->num_descs;
>  }
>
>  static void xsk_destruct_skb(struct sk_buff *skb)
> @@ -576,23 +612,32 @@ static void xsk_destruct_skb(struct sk_buff *skb)
>                 *compl->tx_timestamp = ktime_get_tai_fast_ns();
>         }
>
> -       xsk_cq_submit_locked(xdp_sk(skb->sk)->pool, xsk_get_num_desc(skb));
> +       xsk_cq_submit_addr_locked(xdp_sk(skb->sk)->pool, skb);
>         sock_wfree(skb);
>  }
>
> -static void xsk_set_destructor_arg(struct sk_buff *skb)
> +static void xsk_set_destructor_arg(struct sk_buff *skb, u64 addr)
>  {
> -       long num = xsk_get_num_desc(xdp_sk(skb->sk)->skb) + 1;
> -
> -       skb_shinfo(skb)->destructor_arg = (void *)num;
> +       INIT_LIST_HEAD(&XSKCB(skb)->addrs_list);
> +       XSKCB(skb)->num_descs = 0;
> +       skb_shinfo(skb)->destructor_arg = (void *)addr;
>  }
>
>  static void xsk_consume_skb(struct sk_buff *skb)
>  {
>         struct xdp_sock *xs = xdp_sk(skb->sk);
> +       u32 num_descs = xsk_get_num_desc(skb);
> +       struct xsk_addr_node *pos, *tmp;
> +
> +       if (unlikely(num_descs > 1)) {
> +               list_for_each_entry_safe(pos, tmp, &XSKCB(skb)->addrs_list, addr_node) {
> +                       list_del(&pos->addr_node);
> +                       kmem_cache_free(xsk_tx_generic_cache, pos);
> +               }
> +       }
>
>         skb->destructor = sock_wfree;
> -       xsk_cq_cancel_locked(xs->pool, xsk_get_num_desc(skb));
> +       xsk_cq_cancel_locked(xs->pool, num_descs);
>         /* Free skb without triggering the perf drop trace */
>         consume_skb(skb);
>         xs->skb = NULL;
> @@ -623,6 +668,8 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
>                         return ERR_PTR(err);
>
>                 skb_reserve(skb, hr);
> +
> +               xsk_set_destructor_arg(skb, desc->addr);
>         }
>
>         addr = desc->addr;
> @@ -694,6 +741,8 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>                         err = skb_store_bits(skb, 0, buffer, len);
>                         if (unlikely(err))
>                                 goto free_err;
> +
> +                       xsk_set_destructor_arg(skb, desc->addr);
>                 } else {
>                         int nr_frags = skb_shinfo(skb)->nr_frags;
>                         struct page *page;
> @@ -759,7 +808,19 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>         skb->mark = READ_ONCE(xs->sk.sk_mark);
>         skb->destructor = xsk_destruct_skb;
>         xsk_tx_metadata_to_compl(meta, &skb_shinfo(skb)->xsk_meta);
> -       xsk_set_destructor_arg(skb);
> +
> +       xsk_inc_num_desc(skb);
> +       if (unlikely(xsk_get_num_desc(skb) > 1)) {
> +               struct xsk_addr_node *xsk_addr;
> +
> +               xsk_addr = kmem_cache_zalloc(xsk_tx_generic_cache, GFP_KERNEL);
> +               if (!xsk_addr) {
> +                       err = -ENOMEM;
> +                       goto free_err;
> +               }
> +               xsk_addr->addr = desc->addr;
> +               list_add_tail(&xsk_addr->addr_node, &XSKCB(skb)->addrs_list);
> +       }
>
>         return skb;
>
> @@ -769,7 +830,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
>
>         if (err == -EOVERFLOW) {
>                 /* Drop the packet */
> -               xsk_set_destructor_arg(xs->skb);
> +               xsk_inc_num_desc(xs->skb);
>                 xsk_drop_skb(xs->skb);
>                 xskq_cons_release(xs->tx);
>         } else {
> @@ -812,7 +873,7 @@ static int __xsk_generic_xmit(struct sock *sk)
>                  * if there is space in it. This avoids having to implement
>                  * any buffering in the Tx path.
>                  */
> -               err = xsk_cq_reserve_addr_locked(xs->pool, desc.addr);
> +               err = xsk_cq_reserve_locked(xs->pool);
>                 if (err) {
>                         err = -EAGAIN;
>                         goto out;
> @@ -1815,8 +1876,18 @@ static int __init xsk_init(void)
>         if (err)
>                 goto out_pernet;
>
> +       xsk_tx_generic_cache = kmem_cache_create("xsk_generic_xmit_cache",
> +                                                sizeof(struct xsk_addr_node),
> +                                                0, SLAB_HWCACHE_ALIGN, NULL);
> +       if (!xsk_tx_generic_cache) {
> +               err = -ENOMEM;
> +               goto out_unreg_notif;
> +       }
> +
>         return 0;
>
> +out_unreg_notif:
> +       unregister_netdevice_notifier(&xsk_netdev_notifier);
>  out_pernet:
>         unregister_pernet_subsys(&xsk_net_ops);
>  out_sk:
> diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
> index 46d87e961ad6..f16f390370dc 100644
> --- a/net/xdp/xsk_queue.h
> +++ b/net/xdp/xsk_queue.h
> @@ -344,6 +344,11 @@ static inline u32 xskq_cons_present_entries(struct xsk_queue *q)
>
>  /* Functions for producers */
>
> +static inline u32 xskq_get_prod(struct xsk_queue *q)
> +{
> +       return READ_ONCE(q->ring->producer);
> +}
> +
>  static inline u32 xskq_prod_nb_free(struct xsk_queue *q, u32 max)
>  {
>         u32 free_entries = q->nentries - (q->cached_prod - q->cached_cons);
> @@ -390,6 +395,13 @@ static inline int xskq_prod_reserve_addr(struct xsk_queue *q, u64 addr)
>         return 0;
>  }
>
> +static inline void xskq_prod_write_addr(struct xsk_queue *q, u32 idx, u64 addr)
> +{
> +       struct xdp_umem_ring *ring = (struct xdp_umem_ring *)q->ring;
> +
> +       ring->desc[idx & q->ring_mask] = addr;
> +}
> +
>  static inline void xskq_prod_write_addr_batch(struct xsk_queue *q, struct xdp_desc *descs,
>                                               u32 nb_entries)
>  {
> --
> 2.34.1
>

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