rt scheduler may calculate wrong rt_time

[Thomas Giesel <skoe@directbox.com>]

Friends of the scheduler,

I found that the current (well, at least 2.6.38) scheduler calculates a
wrong rt_time for realtime tasks in certain situations.

Example scenario:
- HZ = 1000, rt_runtime = 95 ms, rt_period = 100 ms (similar with other
  setups, but that's what I did)
- a high priority rt task (A) gets packets from Ethernet about every 10
  ms
- a low priority rt task (B) unfortunately runs for a longer time
  (here: endlessly :)
- no other tasks running (i.e. about 5 ms idle left per period)

When the runtime of the realtime tasks is exceeded (e.g. by (B)), they
are throttled. During this time idle is scheduled. When in idle,
tick_nohz_stop_sched_tick() will stop the scheduler tick, which causes
update_rq_clock() _not_ to be called for a while. When a realtime task
is woken up during this time (e.g. (A) by network traffic),
update_rq_clock() is called from enqueue_task(). The task is not picked
yet, because it is still throttled. After a while
sched_rt_period_timer() unthrottles the realtime tasks and cpu_idle
will call schedule().

schedule() picks (A) which has been woken up a while ago.
_pick_next_task_rt() sets exec_start to rq->clock_task. But this has
been updated last time when the task was woken up, which could have
been up to 5 ms ago in my example. So exec_start contains a time
_before_ the task was actually started. As a result of this, rt_time is
calculated too large which makes the rt tasks being throttled even
earlier in the next period. This error may even increase from interval
to interval, because the throttle-window (initially 5 ms) also
increases.

IMHO the best place to update clock_task would be to call a function
from tick_nohz_restart_sched_tick(). But currently I don't see a
suitable interface to the scheduler to do this. Currently I call
update_rq_clock(rq) just before put_prev_task() in schedule(). This
solves the issue and causes rt_runtime to be kept quite accurately.
(Well, same result would be to remove "if (...)" in put_prev_task())

What do you think is the best way to solve this issue?

Thomas