Why does the timestamp in ftrace output jump around?

Issue

• Why does the timestamp in ftrace output jump around?

Example:

    15d.... 84936.642635: funcgraph_entry:        0.498 us   |     native_read_cr0();
    15d.... 84936.642636: funcgraph_entry:        0.279 us   |     native_write_cr0();
--> 13d.... 84936.720054: sched_wakeup:         25221:?:? +  25222:39:? java [015] Success
    13d.... 84936.721371: sched_wakeup:         25221:?:? +  25222:39:? java [015] Success
    14d.... 84936.735276: sched_wakeup:         25210:?:? +  25222:39:? java [015] Success
    13d.... 84936.749354: funcgraph_exit:         1.171 us   |                   }
    13d.... 84936.749354: funcgraph_entry:                   |                   __enqueue_rt_entity() {
    13d.... 84936.749354: funcgraph_entry:                   |                     cpupri_set() {
--> 13d.... 84936.642664: funcgraph_entry:        0.533 us   |                       _spin_lock_irqsave();
    13d.... 84936.642664: funcgraph_entry:        0.237 us   |                       _spin_unlock_irqrestore();