netdev - Re: [PATCH net-next] net: fec: Enable SOC specific rx-usecs coalescence default setting

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Message-ID:
 <PAXPR04MB9185762B86A4A17A7E2C716189A12@PAXPR04MB9185.eurprd04.prod.outlook.com>
Date: Mon, 15 Jul 2024 21:09:19 +0000
From: Shenwei Wang <shenwei.wang@....com>
To: Andrew Lunn <andrew@...n.ch>
CC: Wei Fang <wei.fang@....com>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Clark Wang <xiaoning.wang@....com>, Rasmus Villemoes
	<linux@...musvillemoes.dk>, "imx@...ts.linux.dev" <imx@...ts.linux.dev>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, dl-linux-imx
	<linux-imx@....com>
Subject: Re: [PATCH net-next] net: fec: Enable SOC specific rx-usecs
 coalescence default setting



> -----Original Message-----
> From: Andrew Lunn <andrew@...n.ch>
> Sent: Monday, July 15, 2024 3:47 PM
> To: Shenwei Wang <shenwei.wang@....com>
> Cc: Wei Fang <wei.fang@....com>; David S. Miller <davem@...emloft.net>;
> Eric Dumazet <edumazet@...gle.com>; Jakub Kicinski <kuba@...nel.org>;
> Paolo Abeni <pabeni@...hat.com>; Clark Wang <xiaoning.wang@....com>;
> Rasmus Villemoes <linux@...musvillemoes.dk>; imx@...ts.linux.dev;
> netdev@...r.kernel.org; dl-linux-imx <linux-imx@....com>
> Subject: [EXT] Re: [PATCH net-next] net: fec: Enable SOC specific rx-usecs
> coalescence default setting
> On Mon, Jul 15, 2024 at 02:54:49PM -0500, Shenwei Wang wrote:
> > The current FEC driver uses a single default rx-usecs coalescence
> > setting across all SoCs. This approach leads to suboptimal latency on
> > newer, high performance SoCs such as i.MX8QM and i.MX8M.
> 
> Does ethtool -C work on these devices?
> 

Yes, ethtool -C also works. This patch is to enable the proper default value so that 
a user won't have to reconfigure it via ethtool.

> Interrupt coalescence is more than latency. It is also about CPU load.
> 
> Does NAPI polling work correctly on these devices? If so, interrupts should be
> disabled quite a bit, and then interrupt latency does not matter so much.
> 
> Have you benchmarked CPU usage with this patch, for a range of traffic
> bandwidths and burst patterns. How does it differ?
> 

The NAPI polling works correctly. The traffic bandwidth and CPU usage is 
no notable impact during iperf testing.

> > 64 bytes from 192.168.0.195: icmp_seq=4 ttl=64 time=0.486 ms
> >
> > Fixes: df727d4547de ("net: fec: don't reset irq coalesce settings to
> > defaults on "ip link up"")
> 
> Fixes is not correct here. It was never broken. This is maybe an optimisation,
> maybe a deoptimisation, depending on your use case.
> 

Em, just an optimization. 

Regards,
Shenwei

> And next-next is closed at the moment anyway.
> 
>         Andrew