netdev - Re: [PATCH v4 net-next 2/6] cache: enforce cache groups

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Date: Thu, 26 Oct 2023 16:39:14 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <kuba@...nel.org>
CC: <andrew@...n.ch>, <corbet@....net>, <daniel@...earbox.net>,
	<dsahern@...nel.org>, <edumazet@...gle.com>, <lixiaoyan@...gle.com>,
	<mubashirq@...gle.com>, <ncardwell@...gle.com>, <netdev@...r.kernel.org>,
	<pabeni@...hat.com>, <pnemavat@...gle.com>, <weiwan@...gle.com>,
	<wwchao@...gle.com>, <kuniyu@...zon.com>
Subject: Re: [PATCH v4 net-next 2/6] cache: enforce cache groups

From: Jakub Kicinski <kuba@...nel.org>
Date: Thu, 26 Oct 2023 07:17:01 -0700
> On Thu, 26 Oct 2023 08:19:55 +0000 Coco Li wrote:
> > Set up build time warnings to safegaurd against future header changes
> > of organized structs.
> 
> TBH I had some doubts about the value of these asserts, I thought
> it was just me but I was talking to Vadim F and he brought up 
> the same question.
> 
> IIUC these markings will protect us from people moving the members
> out of the cache lines. Does that actually happen?
> 
> It'd be less typing to assert the _size_ of each group, which protects
> from both moving out, and adding stuff haphazardly, which I'd guess is
> more common. Perhaps we should do that in addition?

Also, we could assert the size of the struct itself and further
add ____cacheline_aligned_in_smp to __cacheline_group_begin() ?

If someone adds/removes a member before __cacheline_group_begin(),
two groups could share the same cacheline.