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

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Message-ID: <20240308053056.GA16944@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date: Thu, 7 Mar 2024 21:30:56 -0800
From: Shradha Gupta <shradhagupta@...ux.microsoft.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Haiyang Zhang <haiyangz@...rosoft.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

Thanks for the comments, I am sending out a newer version with these fixes.

On Thu, Mar 07, 2024 at 09:01:45AM -0800, Jakub Kicinski wrote:
> 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 :(