sched_ext: Report NMI kicks with scx_error()

The per-cpu kick lists are protected by IRQ masking which doesn't stop NMIs,
so scx_bpf_kick_cpu() from NMI silently drops the kick after a one-time
warning. A dropped kick can leave a CPU idle when the scheduler believes it
was woken, which is a correctness problem for the scheduler even if the
kernel is fine. Now that scx_error() works from NMI, abort the scheduler
instead so that the bug is surfaced deterministically. The warned_nmi_kick
tracking is no longer needed.

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

kernel/sched/ext/ext.c

@@ -9219,11 +9219,7 @@ void scx_kick_cpu(struct scx_sched *sch, s32 cpu, u64 flags)
 	 * not mask NMIs, so kicking from NMI could corrupt it and is unsupported.
 	 */
 	if (unlikely(in_nmi())) {
-		if (!sch->warned_nmi_kick) {
-			sch->warned_nmi_kick = true;
-			pr_warn("sched_ext: %s: scx_bpf_kick_cpu() from NMI ignored\n",
-				sch->ops.name);
-		}
+		scx_error(sch, "scx_bpf_kick_cpu() called from NMI");
 		return;
 	}
 

kernel/sched/ext/internal.h

@@ -1543,7 +1543,6 @@ struct scx_sched {
 	 */
 	bool			warned_zero_slice:1;
 	bool			warned_unassoc_progs:1;
-	bool			warned_nmi_kick:1;
 
 	struct list_head	all;