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Date: Wed, 12 Feb 2025 10:29:36 -0800
From: Jakub Kicinski <kuba@kernel.org>
To: Alexander Lobakin <aleksander.lobakin@intel.com>
Cc: Eric Dumazet <edumazet@google.com>, Andrew Lunn <andrew+netdev@lunn.ch>,
 "David S. Miller" <davem@davemloft.net>, Paolo Abeni <pabeni@redhat.com>,
 Lorenzo Bianconi <lorenzo@kernel.org>, Daniel Xu <dxu@dxuuu.xyz>, "Alexei
 Starovoitov" <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>,
 "Andrii Nakryiko" <andrii@kernel.org>, John Fastabend
 <john.fastabend@gmail.com>, Toke =?UTF-8?B?SMO4aWxhbmQtSsO4cmdlbnNlbg==?=
 <toke@kernel.org>, "Jesper Dangaard Brouer" <hawk@kernel.org>, Martin KaFai
 Lau <martin.lau@linux.dev>, <netdev@vger.kernel.org>,
 <bpf@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net-next v4 0/8] bpf: cpumap: enable GRO for XDP_PASS
 frames
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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?

> > 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.