From 05909810a946222aca5d0611d37be82d18f95228 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Mon, 20 Apr 2026 21:17:11 -1000 Subject: [PATCH] tools/sched_ext: scx_qmap: Silence task_ctx lookup miss 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 Reviewed-by: Andrea Righi Reviewed-by: Zhao Mengmeng --- 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];