netdev - Re: [PATCH net-next v2 5/9] xsk: add xsk_alloc_batch

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Message-ID: <f0c0a512-900b-4d12-9e59-5fbcd35ed495@intel.com>
Date: Thu, 28 Aug 2025 17:28:53 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Jason Xing <kerneljasonxing@...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>, <horms@...nel.org>,
	<andrew+netdev@...n.ch>, <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v2 5/9] xsk: add xsk_alloc_batch_skb() to build
 skbs in batch

From: Jason Xing <kerneljasonxing@...il.com>
Date: Thu, 28 Aug 2025 08:38:42 +0800

> On Wed, Aug 27, 2025 at 10:33 PM Alexander Lobakin
> <aleksander.lobakin@...el.com> wrote:
>>
>> From: Jason Xing <kerneljasonxing@...il.com>
>> Date: Mon, 25 Aug 2025 21:53:38 +0800
>>
>>> From: Jason Xing <kernelxing@...cent.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
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