netdev - Re: [PATCH net-next v3 3/9] xsk: add xsk_alloc_batch

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Message-ID: <CAL+tcoBNeghhUv=EG=NwGP+dYtXv-Ms489YBFh3rctxfRagUAQ@mail.gmail.com>
Date: Sat, 25 Oct 2025 17:11:23 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, bjorn@...nel.org, magnus.karlsson@...el.com, 
	maciej.fijalkowski@...el.com, jonathan.lemon@...il.com, sdf@...ichev.me, 
	ast@...nel.org, daniel@...earbox.net, hawk@...nel.org, 
	john.fastabend@...il.com, joe@...a.to, willemdebruijn.kernel@...il.com, 
	bpf@...r.kernel.org, netdev@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v3 3/9] xsk: add xsk_alloc_batch_skb() to build
 skbs in batch

On Sat, Oct 25, 2025 at 2:49 AM Stanislav Fomichev <stfomichev@...il.com> wrote:
>
> On 10/21, Jason Xing wrote:
> > From: Jason Xing <kernelxing@...cent.com>
> >
> > Support allocating and building skbs in batch.
> >
> > This patch uses kmem_cache_alloc_bulk() to complete the batch allocation
> > which relies on the global common cache 'net_hotdata.skbuff_cache'. Use
> > a xsk standalone skb cache (namely, xs->skb_cache) to store allocated
> > skbs instead of resorting to napi_alloc_cache that was designed for
> > softirq condition.
> >
> > After allocating memory for each of skbs, in a 'for' loop, the patch
> > borrows part of __allocate_skb() to initialize skb and then calls
> > xsk_build_skb() to complete the rest of initialization process, like
> > copying data and stuff.
> >
> > Add batch.send_queue and use the skb->list to make skbs into one chain
> > so that they can be easily sent which is shown in the subsequent patches.
> >
> > In terms of freeing skbs process, napi_consume_skb() in the tx completion
> > would put the skb into global cache 'net_hotdata.skbuff_cache' that
> > implements the deferred freeing skb feature to avoid freeing skb one
> > by one to improve the performance.
> >
> > Signed-off-by: Jason Xing <kernelxing@...cent.com>
> > ---
> >  include/net/xdp_sock.h |   3 ++
> >  net/core/skbuff.c      | 101 +++++++++++++++++++++++++++++++++++++++++
> >  net/xdp/xsk.c          |   1 +
> >  3 files changed, 105 insertions(+)
> >
> > diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> > index 8944f4782eb6..cb5aa8a314fe 100644
> > --- a/include/net/xdp_sock.h
> > +++ b/include/net/xdp_sock.h
> > @@ -47,8 +47,10 @@ struct xsk_map {
> >
> >  struct xsk_batch {
> >       u32 generic_xmit_batch;
> > +     unsigned int skb_count;
> >       struct sk_buff **skb_cache;
> >       struct xdp_desc *desc_cache;
> > +     struct sk_buff_head send_queue;
> >  };
> >
> >  struct xdp_sock {
> > @@ -130,6 +132,7 @@ struct xsk_tx_metadata_ops {
> >  struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> >                             struct sk_buff *allocated_skb,
> >                             struct xdp_desc *desc);
> > +int xsk_alloc_batch_skb(struct xdp_sock *xs, u32 nb_pkts, u32 nb_descs, int *err);
> >  #ifdef CONFIG_XDP_SOCKETS
> >
> >  int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp);
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index bc12790017b0..5b6d3b4fa895 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -81,6 +81,8 @@
> >  #include <net/page_pool/helpers.h>
> >  #include <net/psp/types.h>
> >  #include <net/dropreason.h>
> > +#include <net/xdp_sock.h>
> > +#include <net/xsk_buff_pool.h>
> >
> >  #include <linux/uaccess.h>
> >  #include <trace/events/skb.h>
> > @@ -615,6 +617,105 @@ static void *kmalloc_reserve(unsigned int *size, gfp_t flags, int node,
> >       return obj;
> >  }
> >
> > +int xsk_alloc_batch_skb(struct xdp_sock *xs, u32 nb_pkts, u32 nb_descs, int *err)
> > +{
> > +     struct xsk_batch *batch = &xs->batch;
> > +     struct xdp_desc *descs = batch->desc_cache;
> > +     struct sk_buff **skbs = batch->skb_cache;
> > +     gfp_t gfp_mask = xs->sk.sk_allocation;
> > +     struct net_device *dev = xs->dev;
> > +     int node = NUMA_NO_NODE;
> > +     struct sk_buff *skb;
> > +     u32 i = 0, j = 0;
> > +     bool pfmemalloc;
> > +     u32 base_len;
> > +     u8 *data;
> > +
> > +     base_len = max(NET_SKB_PAD, L1_CACHE_ALIGN(dev->needed_headroom));
> > +     if (!(dev->priv_flags & IFF_TX_SKB_NO_LINEAR))
> > +             base_len += dev->needed_tailroom;
> > +
> > +     if (batch->skb_count >= nb_pkts)
> > +             goto build;
> > +
> > +     if (xs->skb) {
> > +             i = 1;
>
> What is the point of setting i to 1 here? You always start the loop from
> i=0.

Oh, right, I should've removed it!

Thanks,
Jason