switching from kernel mode to user mode?

[Jun Sun]

I am doing some measurement to see how long Linux runs in
the kernel mode.  I need to capture all the places where
Linux actually switches from kernel mode to user mode.

I am looking at ver 2.2.12 on i386.  Currently I set a checkpoint
right before RESTORE_ALL in arch/i386/kernel/entry.S (excluding
the excution paths coming from ret_from_intr).

Apparently, the results I got seem to suggest I am missing other
places where kernel can switch back to user mode.  One of 
such a place that I am suspicious of is __switch_to() called inside
schedule().  So the question is : 

Can __switch_to() actually go back directly to user mode?
Are there any other places where kernel jumps back to user mode?

Thanks.

Jun