[RFC PATCH v4 0/5] netdev: show a process of packets

[Koki Sanagi <sanagi.koki@jp.fujitsu.com>]

CHANGE-LOG since v3:
    1) change arguments of softirq tracepoint into original one.
    2) remove tracepoint of dev_kfree_skb_irq and skb_free_datagram_locked
       and add trace_kfree_skb before __kfree_skb instead of them.
    3) add tracepoint to netif_rx and display it by netdev-times script.

These patch-set adds tracepoints to show us a process of packets.
Using these tracepoints and existing points, we can get the time when
packet passes through some points in transmit or receive sequence.
For example, this is an output of perf script which is attached by patch 5/5.

106133.171439sec cpu=0
  irq_entry(+0.000msec irq=24:eth4)
         |
  softirq_entry(+0.006msec)
         |
         |---netif_receive_skb(+0.010msec skb=f2d15900 len=100)
         |            |
         |      skb_copy_datagram_iovec(+0.039msec 10291::10291)
         |
  napi_poll_exit(+0.022msec eth4)

106134.175634sec cpu=1
  irq_entry(+0.000msec irq=28:eth1)
         |
         |---netif_rx(+0.009msec skb=f3ef0a00)
         |
  softirq_entry(+0.018msec)
         |
         |---netif_receive_skb(+0.021msec skb=f3ef0a00 len=84)
         |            |
         |      skb_copy_datagram_iovec(+0.033msec 0:swapper)
         |
  napi_poll_exit(+0.035msec (no_device))

The above is a receive side(eth4 is NAPI. eth1 is non-NAPI). Like this, it can
show receive sequence frominterrupt(irq_entry) to application
(skb_copy_datagram_iovec). 
This script shows one NET_RX softirq and events related to it. All relative
time bases on first irq_entry which raise NET_RX softirq.

   dev    len      Qdisc               netdevice             free
   eth4    74 106125.030004sec        0.006msec             0.009msec
   eth4    87 106125.041020sec        0.007msec             0.023msec
   eth4    66 106125.042291sec        0.003msec             0.012msec
   eth4    66 106125.043274sec        0.006msec             0.004msec
   eth4   850 106125.044283sec        0.007msec             0.018msec

The above is a transmit side. There are three check-time-points.
Point1 is before putting a packet to Qdisc. point2 is after ndo_start_xmit in
dev_hard_start_xmit. It indicates finishing putting a packet to driver.
point3 is in consume_skb and kfree_skb. It indicates freeing a transmitted packet.
Values of this script are, from left, device name, length of a packet, a time of
point1, an interval time between point1 and point2 and an interval time between
point2 and point3.

These times are useful to analyze a performance or to detect a point where
packet delays. For example,
- NET_RX softirq calling is late.
- Application is late to take a packet.
- It takes much time to put a transmitting packet to driver
  (It may be caused by packed queue)

And also, these tracepoint help us to investigate a network driver's trouble
from memory dump because ftrace records it to memory. And ftrace is so light
even if always trace on. So, in a case investigating a problem which doesn't
reproduce, it is useful.

Thanks,
Koki Sanagi.