linux-kernel - Re: [PATCH net-next v4 0/8] bpf: cpumap: enable GRO for XDP

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Message-ID: <6311be48-641b-4411-bf6d-d97a643a5806@intel.com>
Date: Fri, 14 Feb 2025 16:43:09 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: Eric Dumazet <edumazet@...gle.com>, Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>, Paolo Abeni <pabeni@...hat.com>,
	Lorenzo Bianconi <lorenzo@...nel.org>, Daniel Xu <dxu@...uu.xyz>, "Alexei
 Starovoitov" <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
	"Andrii Nakryiko" <andrii@...nel.org>, John Fastabend
	<john.fastabend@...il.com>, Toke Høiland-Jørgensen
	<toke@...nel.org>, "Jesper Dangaard Brouer" <hawk@...nel.org>, Martin KaFai
 Lau <martin.lau@...ux.dev>, <netdev@...r.kernel.org>, <bpf@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v4 0/8] bpf: cpumap: enable GRO for XDP_PASS
 frames

From: Jakub Kicinski <kuba@...nel.org>
Date: Wed, 12 Feb 2025 10:29:36 -0800

> On Wed, 12 Feb 2025 16:55:52 +0100 Alexander Lobakin wrote:
>>> You mean to cache napi_id in gro_node?
>>>
>>> Then we get +8 bytes to sizeof(napi_struct) for little reason...
> 
> Right but I think the expectation would be that we don't ever touch
> that on the fast path, right? The "real" napi_id would basically
> go down below:
> 
> 	/* control-path-only fields follow */
> 
> 8B of cold data doesn't matter at all. But I haven't checked if
> we need the napi->napi_id access anywhere hot, do we?

Hmm, if the "hot" napi_id will be in cached in gro_node, then maybe
napi_struct::napi_id could really be in the cold part, let me recheck.

> 
>>> Dunno, if you really prefer, I can do it that way.  
>>
>> Alternative to avoid +8 bytes:
>>
>> struct napi_struct {
>> 	...
>>
>> 	union {
>> 		struct gro_node	gro;
>> 		struct {
>> 			u8 pad[offsetof(struct gro_node, napi_id)];
>> 			u32 napi_id;
>> 		};
>> 	};
>>
>> This is effectively the same what struct_group() does, just more ugly.
>> But allows to declare gro_node separately.

Thanks,
Olek