netdev - Re: [PATCH] net :mana : Add per-cpu stats for MANA device

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Message-ID: <20240307090145.2fc7aa2e@kernel.org>
Date: Thu, 7 Mar 2024 09:01:45 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Haiyang Zhang <haiyangz@...rosoft.com>
Cc: Shradha Gupta <shradhagupta@...ux.microsoft.com>, Shradha Gupta
 <shradhagupta@...rosoft.com>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "linux-hyperv@...r.kernel.org"
 <linux-hyperv@...r.kernel.org>, "linux-rdma@...r.kernel.org"
 <linux-rdma@...r.kernel.org>, "netdev@...r.kernel.org"
 <netdev@...r.kernel.org>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
 <pabeni@...hat.com>, Ajay Sharma <sharmaajay@...rosoft.com>, Leon
 Romanovsky <leon@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>, KY Srinivasan
 <kys@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, Dexuan Cui
 <decui@...rosoft.com>, Long Li <longli@...rosoft.com>, Michael Kelley
 <mikelley@...rosoft.com>
Subject: Re: [PATCH] net :mana : Add per-cpu stats for MANA device

On Thu, 7 Mar 2024 15:49:15 +0000 Haiyang Zhang wrote:
> > > Extend 'ethtool -S' output for mana devices to include per-CPU packet
> > > stats  
> > 
> > But why? You already have per queue stats.  
> Yes. But the q to cpu binding is dynamic, we also want the per-CPU stat 
> to analyze the CPU usage by counting the packets and bytes on each CPU.

Dynamic is a bit of an exaggeration, right? On a well-configured system
each CPU should use a single queue assigned thru XPS. And for manual
debug bpftrace should serve the purpose quite well.

Please note that you can't use num_present_cpus() to size stats in
ethtool -S , you have to use possible_cpus(), because the retrieval
of the stats is done in a multi-syscall fashion and there are no
explicit lengths in the API. So you must always report all possible
stats, not just currently active :(