Re: RFC: issues concerning the next NAPI interface

James Chapman
2007-08-28T14:57:45Z

Jan-Bernd Themann wrote:
> On Tuesday 28 August 2007 11:22, James Chapman wrote:
>>> So in this scheme what runs ->poll() to process incoming packets?
>>> The hrtimer?
>> No, the regular NAPI networking core calls ->poll() as usual; no timers
>> are involved. This scheme simply delays the napi_complete() from the
>> driver so the device stays in the poll list longer. It means that its
>> ->poll() will be called when there is no work to do for 1-2 jiffies,
>> hence the optimization at the top of ->poll() to efficiently handle that
>> case. The device's ->poll() is called by the NAPI core until it has
>> continuously done no work for 1-2 jiffies, at which point it finally
>> does the netif_rx_complete() and re-enables its interrupts.
>>
> I'm not sure if I understand your approach correctly.
> This approach may reduce the number of interrupts, but it does so
> by blocking the CPU for up to 1 jiffy (that can be quite some time
> on some platforms). So no other application / tasklet / softIRQ type
> can do anything in between.

I think I've misread the reworked NAPI net_rx_action code. I thought
that it ran each device ->poll() just once, rescheduling the NET_RX
softirq again if a device stayed in polled mode. I can see now that it
loops while one or more devices stays in the poll list for up to a
jiffy, just like it always has. So by keeping the device in the poll
list and not consuming quota, net_rx_action() spins until the next jiffy
tick unless another device consumes quota, like you say.

I can only assume that the encouraging results that I get with this
scheme are specific to my test setups (measuring packet forwarding
rates). I agree that it isn't desirable to tie up the CPU for up to a
jiffy in net_rx_action() in order to do this. I need to go away and
rework my ideas. Perhaps it is possible to get the behavior I'm looking
for by somehow special-casing the zero return from ->poll() in
net_rx_action(), but I'm not sure.

Thanks for asking questions.

--
James Chapman
Katalix Systems Ltd
http://www.katalix.com
Catalysts for your Embedded Linux software development