what is the difference between itlb_misses.miss_causes_a_walk, itlb_misses.walk_completed, and itlb_misses.walk_pending

I ran perf stat -e itlb_misses.miss_causes_a_walk,itlb_misses.walk_completed,itlb_misses.walk_pending grep Hecuba /storage/nvme6/complete_shakespeare_4096_times and got Performance counter stats for 'grep Hecuba /storage/nvme6/complete_shakespeare_4096_times': 27,732 itlb_misses.miss_causes_a_walk 13,492 itlb_misses.walk_completed 1,048,087 itlb_misses.walk_pending 0.252038252 seconds time elapsed So obviously these three are all different. The actual workload I'm running perf against is not important in that whatever workload I run relative values for the counters are similar. I know why a miss would cause a walk, but the above seems to imply that not all walks complete. Why is this? Also why are the pending walks so much greater than the caused walks? And finally if I want to understand the itlb miss rate, which of the above counters should I use?