sched_ext: Add SCX_TASK_PROTECTED

A BPF scheduler can displace any of its tasks at will - cut a running one's
slice with an SCX_ENQ_PREEMPT dispatch, an SCX_KICK_PREEMPT kick or a direct
shortening, and jump a queued one with HEAD insertions. Sometimes the kernel
needs a slice and a DSQ position to stick regardless.

Add SCX_TASK_PROTECTED, guarding both:

- The slice becomes immutable. Every scheduler-reachable write is refused
  and counted as SCX_EV_SLICE_DENIED. Higher scheduling classes are
  unaffected. PREEMPT|IMMED can't preempt a running protected task and gets
  reenqueued.

- A protected task that reached the head of its DSQ keeps it - HEAD
  insertions land behind the leading run of protected tasks and reenqueue
  sweeps skip them. Only rq-owned DSQs can hold protected tasks, so the walk
  runs only for them.

The bit lives in p->scx.flags so that both the refusal and the head walk
read it under the rq lock that protects it.

Protection ends when the slice is consumed, when the task leaves the rq
except for a save/restore on the running task, on a yield, when the
scheduler enters bypass, and when the task leaves scx. The flag is
kernel-internal and not used yet.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 3166a0c3d892..b519fbc88e17 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -102,6 +102,7 @@ enum scx_ent_flags {
 	SCX_TASK_DEQD_FOR_SLEEP	= 1 << 3, /* last dequeue was for SLEEP */
 	SCX_TASK_SUB_INIT	= 1 << 4, /* task being initialized for a sub sched */
 	SCX_TASK_IMMED		= 1 << 5, /* task is on local DSQ with %SCX_ENQ_IMMED */
+	SCX_TASK_PROTECTED	= 1 << 6, /* slice and DSQ head position protected */
 
 	/*
 	 * Bits 8 to 10 are used to carry task state: