Re: [PATCH] usb: gadget: u_ether: remove interrupt throttling

[Felipe Balbi <felipe.balbi@linux.intel.com>]

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Hi,

Peter Chen <hzpeterchen@gmail.com> writes:
>> > Peter Chen <hzpeterchen@gmail.com> writes:
>> > > On Wed, Nov 02, 2016 at 11:22:54AM -0400, David Miller wrote:
>> > >> From: Peter Chen <hzpeterchen@gmail.com>
>> > >> Date: Wed, 2 Nov 2016 14:02:02 +0800
>> > >> 
>> > >> > Felipe, it may increase cpu utilization since more interrupts will be there,
>> > >> > it may affect the SoC which has lower cpu frequency. This code existed
>> > >> > many years, why this problem has only reported at dwc3 recently?
>> > >> 
>> > >> It's a bug, and it's going to cause TCP sockets to potentially hang.
>> > >> 
>> > >
>> > > For some controllers, it is, so we need to add parameter for user
>> > > to see if interrupt migration is supported or not.
>> > 
>> > not for some controller, for ALL networking drivers.
>> > 
>> > > But just like some ethernet controllers, some USB controllers support
>> > > hardware timeout mechanism which interrupt will be triggered after
>> > > some uFrame occurs if the transaction has completed but not required
>> > > to interrupt, it is used to support interrupt migration like ethernet.
>> > 
>> > you're missing the point. What Dave Miller is saying is that it's ALWAYS
>> > a bug to delay completion of SKBs. The only thing you're doing with
>> > chipidea is delaying interrupt by up to 125us; which is still a bug from
>> > the point of view of the networking layer, but it's more difficult to
>> > perceive any problems because of the short time where interrupt is
>> > delayed.
>> > 
>> 
>> If it is ALWAYS a bug to delay completion of SKBs, how the local
>> ethernet driver designs interrupt migration?
>> 
>
> Just a quick test, I delete dev_kfree_skb_any at tx_complete, not find
> any problems by using simple "ping test", just free memory is less and
> less. David, do you really mean free tx skb buffer with limited time,
> but not return NETDEV_TX_OK by ->ndo_start_xmit with limited time?

ping *will* work just fine. One easy test to *see* the problem is to SSH
to a machine using g_ether, then run:

$ while true; do dmesg; done

you will notice it is rather laggy. Now remove throttling and the lags
are gone.

-- 
balbi

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