matt-nb5-mips64 cpu_idle issue

[Manuel Bouyer]

Hi,
while working on a SMP platform using code based on the matt-nb5-mips64
branch, I ran in the following issue:
a lwp has been put to sleep on cpux, switching cpux to the idle lwp.
Another CPU wakes up the lwp while cpux is in mi_switch().
Now cpux is stuck in the idle loop, never noticing that another lwp is
runnable.

The reason is that cpu_idle() loops testing (ci->ci_want_resched),
but this is not enough to detect if another thread is ready to run
(several places do cpu_need_resched(ci, 0)). cpu_idle() is itself called
in a loop, which does more thing and especially call sched_curcpu_runnable_p()
to see if the idle lwp needs to switch to another lwp.
So cpu_idle() should do whatever is needed to put the CPU into sleep
waiting for an interrupt (if possible), but should not do so in a loop.
The attached patch still check for ci->ci_want_resched as x86 does, but
I'm not sure this is needed at all.

--
Manuel Bouyer <bou...@antioche.eu.org>
NetBSD: 26 ans d'experience feront toujours la difference
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