x86: Restore the critical section around whole ipi_bitmap_handler() if hardclock IPI is delivered.

[kib]

Summary

In the current code after r355311, critical section is taken only around hardclockintr() call, and sched_preempt() is called after the section is exited. If we reschedule after exit, as we typically would due to conditions that caused IPI, in ULE the runq tdq_ipipending is not cleared, which blocks generation of further preempt IPIs.

Since all relatively modern (10 years) hardware has per-cpu event timers, restoring the critical section conditionally does not affect it.

Diagnosed and reviewed by: jeff
Reported and tested by: cy

[jeff]

thank you.

sys/x86/x86/mp_x86.c

1267 ↗

(On Diff #65343)

Comment something like:

sched_preempt() must be called to clear the pending IPI. However, the critical section will cause extra scheduler lock thrashing when used unconditionally. Only critical_enter() if hardclock must also run.