Martin Schwidefsky wrote:
Well, currently there seems to be only the pfault interrupt that can
set TIF_NEED_RESCHED and pfault interrupts can't happen for idle. So
it should work. But if there is any chance that an interrupt will ever
set TIF_NEED_RESCHED we have to disable the interrupts before doing
the resched check.
But other processors can set your TIF_NEED_RESCHED too,
so simply disabling local interrupts does not give any
synchronisation of TIF_NEED_RESCHED.
But these other processors need to send the idle cpu an IPI to make it
aware of the TIF_NEED_RESCHED bit. The order of events is important
here. While the interrupts are disabled the IPI is pending, only after
the enabled-wait psw has been loaded the IPI interrupts comes in.
Yep.
After you threw in set_bit(TIF_NEED_RESCHED) from another processor
I realized that without the local_irq_disable() the code would even be
wrong. We can loose a reschedule without it, consider the following:
cpu-a checks for reschedule, cpu-b sets TIF_NEED_RESCHED, cpu-b sends
IPI, cpu-a receives IPI as external interrupt, cpu-a continues in
default_idle() after the reschedule check. With the NO_IDLE_HZ option
the cpu-a can now wait for a very long time until the next interrupt
wakes it up again.
OK, thanks. I'll restore s390's behaviour in the next rev.
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