[PATCH 11/12] sched_ext: scx_qmap - Idle-check pinned tasks before direct dispatch

[Tejun Heo <tj@kernel.org>]

pick_direct_dispatch_cid() sent a pinned task straight to its only cpu
without an idle check. An insert onto a time-shared cid is forced IMMED,
which may not wait on a busy cpu - the kernel bounces the task back to
ops.enqueue(), the shortcut re-inserts it, and the loop runs into the
reenqueue repeat limit.

Run pinned tasks through the same idle test as everyone else and queue them
when the cpu is busy. always_enq_immed forces IMMED on every local insert
and already skipped the shortcut for the same reason. The check was the last
reader of the always_enq_immed rodata mirror, so drop the variable. The -I
option still sets SCX_OPS_ALWAYS_ENQ_IMMED.

v2: Drop the now-unread always_enq_immed rodata mirror. (sashiko AI)

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 tools/sched_ext/scx_qmap.bpf.c | 4 ----
 tools/sched_ext/scx_qmap.c     | 1 -
 2 files changed, 5 deletions(-)

diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
index 8822ed11c0d8..87612fa7e00d 100644
--- a/tools/sched_ext/scx_qmap.bpf.c
+++ b/tools/sched_ext/scx_qmap.bpf.c
@@ -66,7 +66,6 @@ const volatile bool print_msgs;
 const volatile u64 sub_cgroup_id;
 const volatile s32 disallow_tgid;
 const volatile bool suppress_dump;
-const volatile bool always_enq_immed;
 const volatile u32 immed_stress_nth;
 const volatile u32 max_tasks;
 
@@ -230,9 +229,6 @@ static s32 pick_direct_dispatch_cid(struct task_struct *p, s32 prev_cid,
 	s32 cid;
 	u32 i;
 
-	if (!always_enq_immed && p->nr_cpus_allowed == 1)
-		return prev_cid;
-
 	if (cmask_test(prev_cid, &qa.self_cids.mask) &&
 	    cmask_test_and_clear(prev_cid, &qa.idle_cids.mask))
 		return prev_cid;
diff --git a/tools/sched_ext/scx_qmap.c b/tools/sched_ext/scx_qmap.c
index 3f54796e48be..c4c4cee11f8e 100644
--- a/tools/sched_ext/scx_qmap.c
+++ b/tools/sched_ext/scx_qmap.c
@@ -315,7 +315,6 @@ int main(int argc, char **argv)
 			skel->struct_ops.qmap_ops->flags |= SCX_OPS_SWITCH_PARTIAL;
 			break;
 		case 'I':
-			skel->rodata->always_enq_immed = true;
 			skel->struct_ops.qmap_ops->flags |= SCX_OPS_ALWAYS_ENQ_IMMED;
 			break;
 		case 'F':
-- 
2.55.0