Running the sched_preempt handler in a critical section causes us to acquire the scheduler lock twice in a row. It is not necessary since interrupts aredisabled already in this path.

[jeff]

Summary

I am working on reducing the diffs in my scheduler locking change branch. https://github.com/freebsd/freebsd/compare/master...jwroberson:schedlock

This is a small change that allows the direct switch optimization in sched_preempt() to work as intended. Otherwise we grab the sched lock, set owepreempt, drop the sched lock, critical_exit, acquire the sched lock, see owepreempt, then switch.

[kib]

sys/x86/x86/mp_x86.c

1275 ↗

(On Diff #65102)

I must note that having oldframe and td_intr_frame correct in larger region is sometimes useful. I do not mind much about intr_nesting_level.

[jeff]

sys/x86/x86/mp_x86.c

1275 ↗

(On Diff #65102)

I can move it back out. I just wanted to be precise about what the different handlers required.

[markj]

sys/x86/x86/mp_x86.c

1275 ↗

(On Diff #65102)

I've found td_intr_frame from preempted threads useful as well, FWIW.

[jeff]

sys/x86/x86/mp_x86.c

1275 ↗

(On Diff #65102)

I will move this code back in the committed patch after testing.

[jhb]

I suspect the original critical section was only for hardclock() (wanted all of hardclock() to finish before preempting to run softclock()) and that sched_preempt ended up under it due to inertia.