[PATCH v2] sched_ext: keep running prev when prev->scx.slice != 0

["Henry Huang" <henry.hj@antgroup.com>]

In our environment, we have various types of tasks. Some tasks can keep running after their slice is exhausted,
while others need to be dispatched into the global DSQ for rescheduling. Therefore, we set %SCX_OPS_ENQ_LAST.

However, we encountered a problem:
Because put_prev_task_scx() is executed after pick_task_scx(), @prev only has the opportunity to be dispatched
into local DSQ in put_prev_task_scx(). Since pick_task_scx returns NULL, the CPU enters the idle state instead of running @prev.

Our current workaround is to set a value to @prev->scx.slice in ops.dispatch() and call scx_bpf_kick_cpu(cpu, 0) to trigger a reschedule.
Of course, this approach introduces some overhead.

Our solution:
When %SCX_OPS_ENQ_LAST is set and prev->scx.slice != 0, we only need to set %SCX_RQ_BAL_KEEP in blance_one to
ensure pick_task_scx() can pick the correct task.

For v2:
- set %SCX_RQ_BAL_KEEP in dispatch loop.
- cache prev->scx.flags & SCX_TASK_QUEUED result.

Henry Huang (1):
  sched_ext: keep running prev when prev->scx.slice != 0

 kernel/sched/ext.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

-- 
Henry