As for ice bug fixed by commit b7306b42 ("ice: manage interrupts
during poll exit") followed by commit 23be7075 ("ice: fix software
generating extra interrupts") I'm seeing the similar issue also with
i40e driver.

In certain situation when busy-loop is enabled together with adaptive
coalescing, the driver occasionally misses that there are outstanding
descriptors to clean when exiting busy poll.

Try to catch the remaining work by triggering a software interrupt
when exiting busy poll. No extra interrupts will be generated when
busy polling is not used.

The issue was found when running sockperf ping-pong tcp test with
adaptive coalescing and busy poll enabled (50 as value busy_pool
and busy_read sysctl knobs) and results in huge latency spikes
with more than 100000us.

The fix is inspired from the ice driver and do the following:
1) During napi poll exit in case of busy-poll (napo_complete_done()
   returns false) this is recorded to q_vector that we were in busy
   loop.
2) Extends i40e_buildreg_itr() to be able to add an enforced software
   interrupt into built value
2) In i40e_update_enable_itr() enforces a software interrupt trigger
   if we are exiting busy poll to catch any pending clean-ups
3) Reuses unused 3rd ITR (interrupt throttle) index and set it to
   20K interrupts per second to limit the number of these sw interrupts.

Test results
============
Prior:
[root@dell-per640-07 net]# sockperf ping-pong -i 10.9.9.1 --tcp -m 1000 --mps=max -t 120
sockperf: == version #3.10-no.git ==
sockperf[CLIENT] send on:sockperf: using recvfrom() to block on socket(s)

[ 0] IP = 10.9.9.1        PORT = 11111 # TCP
sockperf: Warmup stage (sending a few dummy messages)...
sockperf: Starting test...
sockperf: Test end (interrupted by timer)
sockperf: Test ended
sockperf: [Total Run] RunTime=119.999 sec; Warm up time=400 msec; SentMessages=2438563; ReceivedMessages=2438562
sockperf: ========= Printing statistics for Server No: 0
sockperf: [Valid Duration] RunTime=119.549 sec; SentMessages=2429473; ReceivedMessages=2429473
sockperf: ====> avg-latency=24.571 (std-dev=93.297, mean-ad=4.904, median-ad=1.510, siqr=1.063, cv=3.797, std-error=0.060, 99.0% ci=[24.417, 24.725])
sockperf: # dropped messages = 0; # duplicated messages = 0; # out-of-order messages = 0
sockperf: Summary: Latency is 24.571 usec
sockperf: Total 2429473 observations; each percentile contains 24294.73 observations
sockperf: ---> <MAX> observation = 103294.331
sockperf: ---> percentile 99.999 =   45.633
sockperf: ---> percentile 99.990 =   37.013
sockperf: ---> percentile 99.900 =   35.910
sockperf: ---> percentile 99.000 =   33.390
sockperf: ---> percentile 90.000 =   28.626
sockperf: ---> percentile 75.000 =   27.741
sockperf: ---> percentile 50.000 =   26.743
sockperf: ---> percentile 25.000 =   25.614
sockperf: ---> <MIN> observation =   12.220

After:
[root@dell-per640-07 net]# sockperf ping-pong -i 10.9.9.1 --tcp -m 1000 --mps=max -t 120
sockperf: == version #3.10-no.git ==
sockperf[CLIENT] send on:sockperf: using recvfrom() to block on socket(s)

[ 0] IP = 10.9.9.1        PORT = 11111 # TCP
sockperf: Warmup stage (sending a few dummy messages)...
sockperf: Starting test...
sockperf: Test end (interrupted by timer)
sockperf: Test ended
sockperf: [Total Run] RunTime=119.999 sec; Warm up time=400 msec; SentMessages=2400055; ReceivedMessages=2400054
sockperf: ========= Printing statistics for Server No: 0
sockperf: [Valid Duration] RunTime=119.549 sec; SentMessages=2391186; ReceivedMessages=2391186
sockperf: ====> avg-latency=24.965 (std-dev=5.934, mean-ad=4.642, median-ad=1.485, siqr=1.067, cv=0.238, std-error=0.004, 99.0% ci=[24.955, 24.975])
sockperf: # dropped messages = 0; # duplicated messages = 0; # out-of-order messages = 0
sockperf: Summary: Latency is 24.965 usec
sockperf: Total 2391186 observations; each percentile contains 23911.86 observations
sockperf: ---> <MAX> observation =  195.841
sockperf: ---> percentile 99.999 =   45.026
sockperf: ---> percentile 99.990 =   39.009
sockperf: ---> percentile 99.900 =   35.922
sockperf: ---> percentile 99.000 =   33.482
sockperf: ---> percentile 90.000 =   28.902
sockperf: ---> percentile 75.000 =   27.821
sockperf: ---> percentile 50.000 =   26.860
sockperf: ---> percentile 25.000 =   25.685
sockperf: ---> <MIN> observation =   12.277

Fixes: 0bcd952f ("ethernet/intel: consolidate NAPI and NAPI exit")
Reported-by: Hugo Ferreira <hferreir@redhat.com>
Reviewed-by: Michal Schmidt <mschmidt@redhat.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>