RE: [RFC net-next 2/2] net: fec: add ndo_select_queue to fix TX bandwidth fluctuations

Hi Andrew,

-----Original Message-----
From: Andrew Lunn <andrew@lunn.ch>
Sent: 2021年5月25日 21:58
To: Joakim Zhang <redacted>
Cc: davem@davemloft.net; kuba@kernel.org; frieder.schrempf@kontron.de;
netdev@vger.kernel.org; linux-kernel@vger.kernel.org; dl-linux-imx
[off-list ref]
Subject: Re: [RFC net-next 2/2] net: fec: add ndo_select_queue to fix TX
bandwidth fluctuations

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On Sun, May 23, 2021 at 06:20:19PM +0800, Joakim Zhang wrote:

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From: Fugang Duan <redacted>

As we know that AVB is enabled by default, and the ENET IP design
is queue 0 for best effort, queue 1&2 for AVB Class A&B. Bandwidth
of queue 1&2 set in driver is 50%, TX bandwidth fluctuated when
selecting tx queues randomly with FEC_QUIRK_HAS_AVB quirk available.

How is the driver currently scheduling between these queues? Given
the 802.1q priorities, i think we want queue 2 with the highest
priority for scheduling. Then queue 0 and lastly queue 1.

I think currently there is no schedule between these queues in the driver.

So queues 1 and 2 are limited to 50% the total bandwidth, but are otherwise
not prioritised over queue 0? That sounds odd.

No, when enable AVB (configured as Credit-based scheme), queue 0 is best effort, queue 1 is limited to 50% bandwidth, queue 2 is also limited to 50% bandwidth.
I may misunderstand to you, what I mean is that there is no scheduler from software level, Net core calls netdev_pick_tx() to select a queue. From the hardware level, schedule with Credit-based.

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Could you please point me where I can find mapping between priorities and

queues? You prefer to below mapping?

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static const u16 fec_enet_vlan_pri_to_queue[8] = {1, 1, 0, 0, 0, 2, 2,
2};

https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fen.wikip
edia.org%2Fwiki%2FIEEE_P802.1p&data=04%7C01%7Cqiangqing.zhang%
40nxp.com%7C30d975f09a79446030d608d91f852c39%7C686ea1d3bc2b4c6fa
92cd99c5c301635%7C0%7C0%7C637575479097053920%7CUnknown%7CTWF
pbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVC
I6Mn0%3D%7C1000&sdata=g6WxJJVXLbaX3QWBq3yB4fk1Zh4tN%2Ff8Y
w%2FAATabL%2Bg%3D&reserved=0

Err, this link isn't available at my side. It's seems 802.1q spec.

I'm not sure i actually believe the hardware does not prioritise the queues. It
seems to me, it is more likely to take frames from queues 1 and 2 if they have
not consumed their 50% share.

Hardware xmits with Credit-based scheme.

PCP value 0 is best effort. That should really be given the same priority as a
packet without a VLAN header. Which is why i suggested putting those packets
into queue 0.

Make sense.

Also, if the hardware is performing prioritisation, PCP value 1, background,
when put into queue 1 will end up as higher priority then best effort?

I don't think so, Credit-based scheduler would schedule based on credit, not the PCP filed.

Best Regards,
Joakim Zhang

     Andrew