netdev - Re: [PATCH] net: stmmac: protect statistics updates with a spinlock

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Message-ID: <20240105192148.0accd8f2@meshulam.tesarici.cz>
Date: Fri, 5 Jan 2024 19:21:48 +0100
From: Petr Tesařík <petr@...arici.cz>
To: Andrew Lunn <andrew@...n.ch>
Cc: Eric Dumazet <edumazet@...gle.com>, Alexandre Torgue
 <alexandre.torgue@...s.st.com>, Jose Abreu <joabreu@...opsys.com>, "David
 S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo
 Abeni <pabeni@...hat.com>, Maxime Coquelin <mcoquelin.stm32@...il.com>,
 Chen-Yu Tsai <wens@...e.org>, Jernej Skrabec <jernej.skrabec@...il.com>,
 Samuel Holland <samuel@...lland.org>, "open list:STMMAC ETHERNET DRIVER"
 <netdev@...r.kernel.org>, "moderated list:ARM/STM32 ARCHITECTURE"
 <linux-stm32@...md-mailman.stormreply.com>, "moderated list:ARM/STM32
 ARCHITECTURE" <linux-arm-kernel@...ts.infradead.org>, open list
 <linux-kernel@...r.kernel.org>, "open list:ARM/Allwinner sunXi SoC support"
 <linux-sunxi@...ts.linux.dev>, Jiri Pirko <jiri@...nulli.us>
Subject: Re: [PATCH] net: stmmac: protect statistics updates with a spinlock

On Fri, 5 Jan 2024 18:36:45 +0100
Andrew Lunn <andrew@...n.ch> wrote:

> > This only leaves an atomic_t in hard irq context. I have tried to find
> > something that could relax the requirement, but AFAICS at least some
> > setups use several interrupts that can be delivered to different CPUs
> > simultaneously, and all of them will walk over all channels. So we're
> > left with an atomic_t here.  
> 
> You might want to consider per CPU statistics. Since each CPU has its
> own structure of statistics, you don't need atomic.
> 
> The code actually using the statistics then needs to sum up the per
> CPU statistics, and using syncp should be sufficient for that.

Thanks for the tip.

Honestly, the more I'm looking into this, the less I am convinced that
it is worth the effort, but yeah, if we touch this code now, let's do
things properly. ;-)

> Maybe look at mvneta.c for inspiration.

I think I know what you mean, but I'll have a look.

Petr T