If you have pinned an interrupt to that CPU then you need to process
the softirq for it as well. If that's the device your very single user
space thread is talking to then you better want that, if you are not
interested then simply pin that device irq to some other CPU: no irq
-> no softirq.
If there is just one user space thread why is the NOOP call to the
scheduler interesting ? If you go into the kernel you have some
overhead anyway, so why would the few instructions to call schedule()
and return with the same task (as it is the only runnable) matter ?
Thanks,
tglx
--