Exit points for returning from kernel mode to user mode

[Mitesh]

 I am trying to understand exit points from kernel mode to user mode. I believe this happens when the scheduler is invoked, there are interrupts, exceptions, or the kernel is executing system call in process context. 

Is there a single function which is called in the ppc kernel when the mode is switched back to user mode ? 

I have linux 2.6.16.21 kernel running on a 4-way p550 IBM pSeries with POWER5 processors. Everytime before a process begins executing in user mode I want to execute a special SMT thread priority assembler instruction. 

What code files should I be looking at, is there one place this can be done? Any tips are appreciated.
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