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Message-ID: <2d2f76b5-6af6-b6f0-5c05-cc24cb355fe8@iogearbox.net> Date: Tue, 17 Oct 2023 11:06:06 +0200 From: Daniel Borkmann <daniel@...earbox.net> To: Florian Fainelli <f.fainelli@...il.com>, Coco Li <lixiaoyan@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Eric Dumazet <edumazet@...gle.com>, Neal Cardwell <ncardwell@...gle.com>, Mubashir Adnan Qureshi <mubashirq@...gle.com>, Paolo Abeni <pabeni@...hat.com> Cc: 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 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); + [...] + if (dev_proc_init()) goto out;
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