Re: [PATCH] net: stmmac: xgmac2: disable RBUE in default RX interrupt mask

[Maxime Chevallier]

Hi,

>> to note that it'll
>> now make the "rx_buf_unav_irq" stat (that we can query from userspace)
>> stuck at 0 now.
>>
>> That's a user-visible behaviour change, for a stat that can be useful,
>> one could consider that a uAPI change...
>>
>> Do you have some numbers on the perf impact you see when buffer
>> starvation hits to see what we are gaining/losing here ?
>>
>> Maxime
>>
> Hi Maxime,
> 
> Thank you for the review and for pointing out the rx_buf_unav_irq concern.
> 
> We do have interrupt storm data to share. On Agilex5, under heavy RX 
> pressure we observed:
> 
> Under heavy RX pressure we observed eth0:mac firing 1,821,811,555 times 
> vs 2,618,627 actual RX completions on eth0:rx-0 — a ~695x ratio.

Ouch indeed. The stat itself has to stay, but the behaviour of that
counter is already different depending on the DWMAC core version.

I think we can get this patch in, but please state in the commit message
the performance measurements you're mentioning, as well as the fact
that the counter will now always show at 0 on XGMAC2 devices.

Maybe we'll eventually need some switch to toggle these counters, as it's
some valuable information when debugging, but has a significant impact on
speeds.

With the commit log updates,

Reviewed-by: Maxime Chevallier <maxime.chevallier@xxxxxxxxxxx>

> 
> BR,
> Nazim