In the environment of massive software interrupts.
<idle>-0     [003] ..s1   106.421020: softirq_entry: vec=6 [action=TASKLET]
<idle>-0     [000] ..s1   106.421063: softirq_entry: vec=3 [action=NET_RX]
<idle>-0     [003] ..s1   106.421083: softirq_exit: vec=6 [action=TASKLET]

Follow the above ftrace logs, we know the correct vec-6 start timestamp is replaced with incorrect vec-3.
Because PID is idle-0. It will produce the wrong result after calculating delta.