Martin Schwidefsky wrote:
Hi Nick,
s390 - local irq disable before checking need_resched doesn't gain
anything (removed, OK?)
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.
What it can give is a guarantee that some interrupt will
get queued and not processed. This can be used to sleep
the processor until the next interrupt (because when the
other CPU sets TIF_NEED_RESCHED it will also send an IPI).
Now it could well be that this is what you need, in which
case I'm wrong.
However:
local_irq_disable();
if (need_resched()) {
local_irq_enable();
return;
}
/* need_resched() can become true here */
[... do stuff ...]
Basically you just cut your losses and pick up the
need_resched() when you get around to testing it again.
As I implemented this I wanted to make absolutly
sure that we don't miss any reschedule. It works the way it is and
the additional cycles in idle() for local_irq_disable/local_irq_enable
won't hurt. I don't see the benefit of the change. Only chances that
it might break something we haven't thought of.
No, it doesn't hurt (well, it may slightly slow down your
interrupt processing rate, but who's counting!).
However, I would like to try to tighten things up in this
area a bit if possible now that we're looking at it.
Thanks very much,
Nick
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