Re: [PATCH net-next v2 5/9] xsk: add xsk_alloc_batch_skb() to build skbs in batch

[Jason Xing <kerneljasonxing@gmail.com>]

On Thu, Aug 28, 2025 at 11:29 PM Alexander Lobakin
<aleksander.lobakin@intel.com> wrote:
>
> From: Jason Xing <kerneljasonxing@gmail.com>
> Date: Thu, 28 Aug 2025 08:38:42 +0800
>
> > On Wed, Aug 27, 2025 at 10:33 PM Alexander Lobakin
> > <aleksander.lobakin@intel.com> wrote:
> >>
> >> From: Jason Xing <kerneljasonxing@gmail.com>
> >> Date: Mon, 25 Aug 2025 21:53:38 +0800
> >>
> >>> From: Jason Xing <kernelxing@tencent.com>
> >>>
> >>> Support allocating and building skbs in batch.
> >>
> >> [...]
> >>
> >>> +     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 (xs->skb_count >= nb_pkts)
> >>> +             goto build;
> >>> +
> >>> +     if (xs->skb) {
> >>> +             i = 1;
> >>> +             xs->skb_count++;
> >>> +     }
> >>> +
> >>> +     xs->skb_count += kmem_cache_alloc_bulk(net_hotdata.skbuff_cache,
> >>> +                                            gfp_mask, nb_pkts - xs->skb_count,
> >>> +                                            (void **)&skbs[xs->skb_count]);
> >>
> >> Have you tried napi_skb_cache_get_bulk()? Depending on the workload, it
> >> may give better perf numbers.
> >
> > Sure, my initial try is using this interface. But later I want to see
> > a standalone cache belonging to xsk. The whole xsk_alloc_batch_skb
> > function I added is quite similar to napi_skb_cache_get_bulk(), to
> > some extent.
> >
> > And if using napi_xxx(), we need a lock to avoid the race between this
> > context and softirq context on the same core.
>
> Are you saying this particular function is not run in the softirq

No, it runs in process context. Please see this chain:
sendto->__xsk_generic_xmit-> (allocating skbs function handling).

Thanks,
Jason

> context? I thought all Tx is done in BH.
> If it's not BH, then ignore my suggestion -- napi_skb_cache_get_bulk()
> requires BH, that's true.
>
> >
> > Thanks,
> > Jason
>
> Thanks,
> Olek