scx_flatcg: Use a user DSQ for fallback instead of SCX_DSQ_GLOBAL

scx_flatcg was using SCX_DSQ_GLOBAL for fallback handling. However, it is
assuming that SCX_DSQ_GLOBAL isn't automatically consumed, which was true a
while ago but is no longer the case. Also, there are further changes planned
for SCX_DSQ_GLOBAL which will disallow explicit consumption from it. Switch
to a user DSQ for fallback.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: David Vernet <void@manifault.com>

tools/sched_ext/scx_flatcg.bpf.c

@@ -49,7 +49,10 @@
 /*
  * Maximum amount of retries to find a valid cgroup.
  */
-#define CGROUP_MAX_RETRIES 1024
+enum {
+	FALLBACK_DSQ		= 0,
+	CGROUP_MAX_RETRIES	= 1024,
+};
 
 char _license[] SEC("license") = "GPL";
 
@@ -377,7 +380,7 @@ void BPF_STRUCT_OPS(fcg_enqueue, struct task_struct *p, u64 enq_flags)
 			scx_bpf_dispatch(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, enq_flags);
 		} else {
 			stat_inc(FCG_STAT_GLOBAL);
-			scx_bpf_dispatch(p, SCX_DSQ_GLOBAL, SCX_SLICE_DFL, enq_flags);
+			scx_bpf_dispatch(p, FALLBACK_DSQ, SCX_SLICE_DFL, enq_flags);
 		}
 		return;
 	}
@@ -780,7 +783,7 @@ void BPF_STRUCT_OPS(fcg_dispatch, s32 cpu, struct task_struct *prev)
 pick_next_cgroup:
 	cpuc->cur_at = now;
 
-	if (scx_bpf_consume(SCX_DSQ_GLOBAL)) {
+	if (scx_bpf_consume(FALLBACK_DSQ)) {
 		cpuc->cur_cgid = 0;
 		return;
 	}
@@ -837,7 +840,7 @@ int BPF_STRUCT_OPS_SLEEPABLE(fcg_cgroup_init, struct cgroup *cgrp,
 	int ret;
 
 	/*
-	 * Technically incorrect as cgroup ID is full 64bit while dq ID is
+	 * Technically incorrect as cgroup ID is full 64bit while dsq ID is
 	 * 63bit. Should not be a problem in practice and easy to spot in the
 	 * unlikely case that it breaks.
 	 */
@@ -925,6 +928,11 @@ void BPF_STRUCT_OPS(fcg_cgroup_move, struct task_struct *p,
 	p->scx.dsq_vtime = to_cgc->tvtime_now + vtime_delta;
 }
 
+s32 BPF_STRUCT_OPS_SLEEPABLE(fcg_init)
+{
+	return scx_bpf_create_dsq(FALLBACK_DSQ, -1);
+}
+
 void BPF_STRUCT_OPS(fcg_exit, struct scx_exit_info *ei)
 {
 	UEI_RECORD(uei, ei);
@@ -943,6 +951,7 @@ SCX_OPS_DEFINE(flatcg_ops,
 	       .cgroup_init		= (void *)fcg_cgroup_init,
 	       .cgroup_exit		= (void *)fcg_cgroup_exit,
 	       .cgroup_move		= (void *)fcg_cgroup_move,
+	       .init			= (void *)fcg_init,
 	       .exit			= (void *)fcg_exit,
 	       .flags			= SCX_OPS_HAS_CGROUP_WEIGHT | SCX_OPS_ENQ_EXITING,
 	       .name			= "flatcg");