netdev - Re: [PATCH v2 net-next 0/5] Analyze and Reorganize core Networking Structs to optimize cacheline consumption

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Date: Tue, 17 Oct 2023 18:50:42 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: Florian Fainelli <f.fainelli@...il.com>, Coco Li <lixiaoyan@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Neal Cardwell <ncardwell@...gle.com>, 
	Mubashir Adnan Qureshi <mubashirq@...gle.com>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	Chao Wu <wwchao@...gle.com>, Wei Wang <weiwan@...gle.com>
Subject: Re: [PATCH v2 net-next 0/5] Analyze and Reorganize core Networking
 Structs to optimize cacheline consumption

On Tue, Oct 17, 2023 at 11:06 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 10/17/23 5:46 AM, Florian Fainelli wrote:
> > On 10/16/2023 6:47 PM, Coco Li wrote:
> >> Currently, variable-heavy structs in the networking stack is organized
> >> chronologically, logically and sometimes by cache line access.
> >>
> >> This patch series attempts to reorganize the core networking stack
> >> variables to minimize cacheline consumption during the phase of data
> >> transfer. Specifically, we looked at the TCP/IP stack and the fast
> >> path definition in TCP.
> >>
> >> For documentation purposes, we also added new files for each core data
> >> structure we considered, although not all ended up being modified due
> >> to the amount of existing cache line they span in the fast path. In
> >> the documentation, we recorded all variables we identified on the
> >> fast path and the reasons. We also hope that in the future when
> >> variables are added/modified, the document can be referred to and
> >> updated accordingly to reflect the latest variable organization.
> >
> > This is great stuff, while Eric mentioned this work during Netconf'23 one concern that came up however is how can we make sure that a future change which adds/removes/shuffles members in those structures is not going to be detrimental to the work you just did? Is there a way to "lock" the structure layout to avoid causing performance drops?
> >
> > I suppose we could use pahole before/after for these structures and ensure that the layout on a cacheline basis remains preserved, but that means adding custom scripts to CI.
>
> It should be possible without extra CI. We could probably have zero-sized markers
> as we have in sk_buff e.g. __cloned_offset[0], and use some macros to force grouping.
>
> ASSERT_CACHELINE_GROUP() could then throw a build error for example if the member is
> not within __begin_cacheline_group and __end_cacheline_group :
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 9ea3ec906b57..c664e0594da4 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -2059,6 +2059,7 @@ struct net_device {
>           */
>
>          /* TX read-mostly hotpath */
> +       __begin_cacheline_group(tx_read_mostly);
>          unsigned long long      priv_flags;
>          const struct net_device_ops *netdev_ops;
>          const struct header_ops *header_ops;
> @@ -2085,6 +2086,7 @@ struct net_device {
>   #ifdef CONFIG_NET_XGRESS
>          struct bpf_mprog_entry __rcu *tcx_egress;
>   #endif
> +       __end_cacheline_group(tx_read_mostly);
>
>          /* TXRX read-mostly hotpath */
>          unsigned int            flags;
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 97e7b9833db9..2a91bd4077ad 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -11523,6 +11523,9 @@ static int __init net_dev_init(void)
>
>          BUG_ON(!dev_boot_phase);
>
> +       ASSERT_CACHELINE_GROUP(tx_read_mostly, priv_flags);
> +       ASSERT_CACHELINE_GROUP(tx_read_mostly, netdev_ops);

Great idea, we only need to generate these automatically from the file
describing the fields (currently in Documentation/ )

I think the initial intent was to find a way to generate the layout of
the structure itself, but this looked a bit tricky.