Detect oldest blocked tasks by checking "task_struct.last_switch_time"

[xmkg]

Quoting Jay:

"In the ideal world, we could check struct task_struct.last_switch_time, this is where check_hung_task() records the last time the task had a context switch, which in turn is used to trigger the hung task timeout"

We can leverage this to find the tasks which spent the most time in D state, which will be much more valuable information than "longest running, but now blocked tasks".