Subject: Re: [PATCH v7 net-next 1/5] Documentations: Analyze heavily used Networking related structs

From: Simon Horman <[email-masked]>
Date: Mon, 20 Nov 2023 17:19:59 +0000

On Mon, Nov 13, 2023 at 11:32:57PM +0000, Coco Li wrote:
> Analyzed a few structs in the networking stack by looking at variables
> within them that are used in the TCP/IP fast path.
> 
> Fast path is defined as TCP path where data is transferred from sender to
> receiver unidirectionally. It doesn't include phases other than
> TCP_ESTABLISHED, nor does it look at error paths.
> 
> We hope to re-organizing variables that span many cachelines whose fast
> path variables are also spread out, and this document can help future
> developers keep networking fast path cachelines small.
> 
> Optimized_cacheline field is computed as
> (Fastpath_Bytes/L3_cacheline_size_x86), and not the actual organized
> results (see patches to come for these).
> 
> Investigation is done on 6.5
> 
> Name	                Struct_Cachelines  Cur_fastpath_cache Fastpath_Bytes Optimized_cacheline
> tcp_sock	        42 (2664 Bytes)	   12   		396		8
> net_device	        39 (2240 bytes)	   12			234		4
> inet_sock	        15 (960 bytes)	   14			922		14
> Inet_connection_sock	22 (1368 bytes)	   18			1166		18
> Netns_ipv4 (sysctls)	12 (768 bytes)     4			77		2
> linux_mib	        16 (1060)	   6			104		2
> 
> Note how there isn't much improvement space for inet_sock and
> Inet_connection_sock because sk and icsk_inet respectively takes up so
> much of the struct that rest of the variables become a small portion of
> the struct size.
> 
> So, we decided to reorganize tcp_sock, net_device, Netns_ipv4, linux_mib
> 
> Signed-off-by: Coco Li <[email-masked]>
> Suggested-by: Eric Dumazet <[email-masked]>

...

> diff --git a/Documentation/networking/net_cachelines/index.rst b/Documentation/networking/net_cachelines/index.rst
> new file mode 100644
> index 0000000000000..92a6fbe93af35
> --- /dev/null
> +++ b/Documentation/networking/net_cachelines/index.rst
> @@ -0,0 +1,13 @@

Hi Coco,

A minor nit from my side: an SPDX header is probably appropriate at the top
of each .rst file

> +===================================
> +Common Networking Struct Cachelines
> +===================================
> +
> +.. toctree::
> +   :maxdepth: 1
> +
> +   inet_connection_sock
> +   inet_sock
> +   net_device
> +   netns_ipv4_sysctl
> +   snmp
> +   tcp_sock

...