The BUG is as follow:
1. Suppose there are two ready tasks t1 and t2 have the same priority, and t1 is the running.
2. A new, higher priority task t0 (such as hpwork) is ready, then switch to t0.
3. After t0 is suspended, then switch to t1 and reset the t1's timeslice.
4. goto 2
The t2 will have no chance to run.
Signed-off-by: Zeng Zhaoxiu <zhaoxiu.zeng@gmail.com>