Re: RFC: issues concerning the next NAPI interface

[Jan-Bernd Themann <ossthema@de.ibm.com>]

On Monday 27 August 2007 22:37, David Miller wrote:
> From: Jan-Bernd Themann <ossthema@de.ibm.com>
> Date: Mon, 27 Aug 2007 11:47:01 +0200
> 
> > So the question is simply: Do we want drivers that need (benefit
> > from) a timer based polling support to implement their own timers
> > each, or should there be a generic support?
> 
> I'm trying to figure out how an hrtimer implementation would
> even work.
> 
> Would you start the timer from the chip interrupt handler?  If so,
> that's taking two steps backwards as you've already taken all of the
> overhead of running the interrupt handler.

I'm also still trying to understand how hrtimer work exactly. 
The implementation of hrtimers for P6 has not been finished yet, so
I can't do experiments with hrtimers and eHEA now.

I will try the following scheme (once we get hrtimers):
Each device (queue) has a hrtimer.
Schedule the timer in the poll function instead of reactivating IRQs
when a high load situation has been detected and all packets have
been emptied from the receive queue.
The timer function could then just call netif_rx_schedule to register
the rx_queue for NAPI again. 

The advantages of this scheme (if it works as I understood it) would be:
- we don't have to modify NAPI
- benefit from fairness amoung rx_queues / network devices 
- The poll function can decide how long to stick to the timer based
  polling mode, and when to switch back to it's HW IRQs.
- driver can determine the time to wait based on the receive queue length and
  speed

Regards,
Jan-Bernd