netdev - Re: [PATCH bpf-next 4/6] xsk: carry a copy of xdp_zc_max_segs within xsk_buff

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Message-ID: <4aa21712-2643-42e5-a995-d53cf0a53158@linux.dev>
Date: Wed, 2 Oct 2024 20:41:33 +0100
From: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>, bpf@...r.kernel.org,
 ast@...nel.org, daniel@...earbox.net, andrii@...nel.org
Cc: netdev@...r.kernel.org, magnus.karlsson@...el.com, bjorn@...nel.org
Subject: Re: [PATCH bpf-next 4/6] xsk: carry a copy of xdp_zc_max_segs within
 xsk_buff_pool

On 02/10/2024 16:54, Maciej Fijalkowski wrote:
> This so we avoid dereferencing struct net_device within hot path.
> 
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> ---
>   include/net/xsk_buff_pool.h | 1 +
>   net/xdp/xsk_buff_pool.c     | 1 +
>   net/xdp/xsk_queue.h         | 2 +-
>   3 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/xsk_buff_pool.h b/include/net/xsk_buff_pool.h
> index 468a23b1b4c5..8223581d95f8 100644
> --- a/include/net/xsk_buff_pool.h
> +++ b/include/net/xsk_buff_pool.h
> @@ -77,6 +77,7 @@ struct xsk_buff_pool {
>   	u32 chunk_shift;
>   	u32 frame_len;
>   	u8 tx_metadata_len; /* inherited from umem */
> +	u32 xdp_zc_max_segs;

It's better not to make holes in the struct. And looks like it's better
to move it closer to free_list_cnt to put it on the same cache line with
tx_descs which is accessed earlier in xskq_cons_read_desc_batch()
(though the last point is not strict because both cache lines should be
hot at the moment)

>   	u8 cached_need_wakeup;
>   	bool uses_need_wakeup;
>   	bool unaligned;