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tools/sched_ext: scx_qmap: Silence task_ctx lookup miss

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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: 4f8b122848 ("sched_ext: Add basic building blocks for nested sub-scheduler dispatching")
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Zhao Mengmeng <zhaomengmeng@kylinos.cn>
This commit is contained in:
Tejun Heo 2026-04-20 21:17:11 -10:00
parent 4fe9852927
commit 05909810a9
1 changed files with 6 additions and 18 deletions
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20
tools/sched_ext/scx_qmap.bpf.c
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@ -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;
@ -540,11 +534,7 @@ 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;
}
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];