sched_ext: Use runnable_at for the default core-sched task ordering

The default core-sched ordering runs the longest waiting task first by comparing p->scx.core_sched_at stamps. The stamp is maintained under two rules. touch_core_sched() stamps when a task starts waiting for a CPU and when its slice runs out. If the scheduler implements ops.core_sched_before(), touch_core_sched_dispatch() re-stamps on every dispatch. A comparison can see one stamp taken under each rule, which isn't a meaningful ordering. The dispatch rule also buys little - it only aligns bypass-mode comparisons with the local DSQ order. Multiple schedulers make the mixed comparisons more common. Wait time is what p->scx.runnable_at already tracks for the stall watchdog. Delete core_sched_at with both touch functions and compare runnable_at in the scx_prio_less() fallback. runnable_at is refreshed only on enqueue and goes stale while a task keeps occupying its CPU. Instead of re-stamping, order a running task after every waiting task as it is the most recently serviced.

THTejun Heo committed 3 months ago