[PATCH v2 sched_ext/for-7.1-fixes] tools/sched_ext: scx_qmap: Silence task_ctx lookup miss

[Tejun Heo]

scx_fork() dispatches ops.init_task to exactly one scheduler - the one
owning the forking task's cgroup. A task forked inside a sub-scheduler's
cgroup is init'd into the sub only; the root scheduler has no task_ctx
entry for it. When that task later appears as @prev in the root's
qmap_dispatch() (or flows through core-sched comparison via task_qdist),
the bpf_task_storage_get() legitimately misses.

qmap treated those misses as fatal via scx_bpf_error("task_ctx lookup
failed") and aborted the scheduler as soon as the first cross-sched
task hit the root. Drop the error in the sites where the miss is
legitimate: lookup_task_ctx() (helper; callers already check for NULL),
qmap_dispatch()'s @prev branch (bookkeeping-only), task_qdist()
(returns 0 which makes the comparison a no-op), and qmap_select_cpu()
(returns prev_cpu as a no-op fallback instead of -ESRCH). The existing
scx_error was a paranoid guard from the pre-sub-sched world where every
task was owned by the one and only scheduler.

v2: qmap_select_cpu() returns prev_cpu on NULL instead of -ESRCH, so
    the root scheduler doesn't error on cross-sched tasks that pass
    through it (Andrea Righi).

Fixes: 4f8b122848db ("sched_ext: Add basic building blocks for nested sub-scheduler dispatching")
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>
---
 tools/sched_ext/scx_qmap.bpf.c | 24 ++++++------------------
 1 file changed, 6 insertions(+), 18 deletions(-)

diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
index b68abb9e760b..aad698fe294b 100644
--- a/tools/sched_ext/scx_qmap.bpf.c
+++ b/tools/sched_ext/scx_qmap.bpf.c
@@ -159,13 +159,7 @@ static s32 pick_direct_dispatch_cpu(struct task_struct *p, s32 prev_cpu)
 
 static struct task_ctx *lookup_task_ctx(struct task_struct *p)
 {
-	struct task_ctx *tctx;
-
-	if (!(tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0))) {
-		scx_bpf_error("task_ctx lookup failed");
-		return NULL;
-	}
-	return tctx;
+	return bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
 }
 
 s32 BPF_STRUCT_OPS(qmap_select_cpu, struct task_struct *p,
@@ -175,7 +169,7 @@ s32 BPF_STRUCT_OPS(qmap_select_cpu, struct task_struct *p,
 	s32 cpu;
 
 	if (!(tctx = lookup_task_ctx(p)))
-		return -ESRCH;
+		return prev_cpu;
 
 	if (p->scx.weight < 2 && !(p->flags & PF_KTHREAD))
 		return prev_cpu;
@@ -540,13 +534,9 @@ void BPF_STRUCT_OPS(qmap_dispatch, s32 cpu, struct task_struct *prev)
 	 */
 	if (prev) {
 		tctx = bpf_task_storage_get(&task_ctx_stor, prev, 0, 0);
-		if (!tctx) {
-			scx_bpf_error("task_ctx lookup failed");
-			return;
-		}
-
-		tctx->core_sched_seq =
-			core_sched_tail_seqs[weight_to_idx(prev->scx.weight)]++;
+		if (tctx)
+			tctx->core_sched_seq =
+				core_sched_tail_seqs[weight_to_idx(prev->scx.weight)]++;
 	}
 }
 
@@ -584,10 +574,8 @@ static s64 task_qdist(struct task_struct *p)
 	s64 qdist;
 
 	tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
-	if (!tctx) {
-		scx_bpf_error("task_ctx lookup failed");
+	if (!tctx)
 		return 0;
-	}
 
 	qdist = tctx->core_sched_seq - core_sched_head_seqs[idx];
 
-- 
2.53.0