netdev - Re: [PATCH net-next v5 01/14] page_pool: make sure frag API fields don't span between cachelines

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Date: Sat, 25 Nov 2023 20:29:22 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>, "David S. Miller"
	<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
CC: Maciej Fijalkowski <maciej.fijalkowski@...el.com>, Michal Kubiak
	<michal.kubiak@...el.com>, Larysa Zaremba <larysa.zaremba@...el.com>,
	Alexander Duyck <alexanderduyck@...com>, David Christensen
	<drc@...ux.vnet.ibm.com>, Jesper Dangaard Brouer <hawk@...nel.org>, Ilias
 Apalodimas <ilias.apalodimas@...aro.org>, Paul Menzel
	<pmenzel@...gen.mpg.de>, <netdev@...r.kernel.org>,
	<intel-wired-lan@...ts.osuosl.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v5 01/14] page_pool: make sure frag API fields
 don't span between cachelines

On 2023/11/24 23:47, Alexander Lobakin wrote:
> After commit 5027ec19f104 ("net: page_pool: split the page_pool_params
> into fast and slow") that made &page_pool contain only "hot" params at
> the start, cacheline boundary chops frag API fields group in the middle
> again.
> To not bother with this each time fast params get expanded or shrunk,
> let's just align them to `4 * sizeof(long)`, the closest upper pow-2 to
> their actual size (2 longs + 2 ints). This ensures 16-byte alignment for
> the 32-bit architectures and 32-byte alignment for the 64-bit ones,
> excluding unnecessary false-sharing.
> 
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
> ---
>  include/net/page_pool/types.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/net/page_pool/types.h b/include/net/page_pool/types.h
> index e1bb92c192de..989d07b831fc 100644
> --- a/include/net/page_pool/types.h
> +++ b/include/net/page_pool/types.h
> @@ -127,7 +127,7 @@ struct page_pool {
>  
>  	bool has_init_callback;

It seems odd to have only a slow field between tow fast
field group, isn't it better to move it to the end of
page_pool or where is more appropriate?

>  
> -	long frag_users;
> +	long frag_users __aligned(4 * sizeof(long));

If we need that, why not just use '____cacheline_aligned_in_smp'?

>  	struct page *frag_page;
>  	unsigned int frag_offset;
>  	u32 pages_state_hold_cnt;
>