Re: [PATCH sched_ext/for-7.1] tools/sched_ext: Kick idle CPU for pinned tasks in scx_qmap

Cheng-Yang Chou
2026-04-11T14:27:43Z

Hi Tejun,

On Sat, Apr 11, 2026 at 08:57:42PM +0800, Cheng-Yang Chou wrote:
> On Sat, Apr 11, 2026 at 01:33:56AM -1000, Tejun Heo wrote:
> > scx_qmap uses global BPF queue maps for task dispatch. A task pinned to a
> > single CPU can only be dispatched by its home CPU's ops.dispatch(), but an
> > idle CPU won't call ops.dispatch() on its own. This leaves per-CPU kthreads
> > like ksoftirqd stranded, causing NOHZ tick-stop warnings from pending
> > softirqs.
> >
> > Kick the target CPU with SCX_KICK_IDLE when enqueueing a pinned task.
> >
> > Signed-off-by: Tejun Heo <tj@kernel.org>
> > ---
> > tools/sched_ext/scx_qmap.bpf.c | 8 ++++++++
> > 1 file changed, 8 insertions(+)
> >
> > diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
> > index f3587fb709c9..09d1624fb869 100644
> > --- a/tools/sched_ext/scx_qmap.bpf.c
> > +++ b/tools/sched_ext/scx_qmap.bpf.c
> > @@ -314,6 +314,14 @@ void BPF_STRUCT_OPS(qmap_enqueue, struct task_struct *p, u64 enq_flags)
> > __sync_fetch_and_add(&nr_highpri_queued, 1);
> > }
> > __sync_fetch_and_add(&nr_enqueued, 1);
> > +
> > + /*
> > + * Kick idle target CPU for pinned tasks. Without this, the CPU can
> > + * idle while ksoftirqd is pending in the BPF queue, triggering NOHZ
> > + * tick-stop warnings.
> > + */
> > + if (p->nr_cpus_allowed == 1)
> > + scx_bpf_kick_cpu(scx_bpf_task_cpu(p), SCX_KICK_IDLE);
> > }
> >
> > /*
> > --
> > 2.53.0
>
> Looks good to me! The same issue exists in scx_userland where pinned
> tasks can be dispatched to SCX_DSQ_GLOBAL without kicking the idle
> target CPU. I'll follow a patch to add the same fix there!
>
> Reviewed-by: Cheng-Yang Chou <yphbchou0911@gmail.com>

Actually, would it make more sense to fold this directly into the same
patch?

diff --git a/tools/sched_ext/scx_userland.bpf.c b/tools/sched_ext/scx_userland.bpf.c
index f29862b89386..56c53d457f45 100644
--- a/tools/sched_ext/scx_userland.bpf.c
+++ b/tools/sched_ext/scx_userland.bpf.c
@@ -195,6 +195,14 @@ static void enqueue_task_in_user_space(struct task_struct *p, u64 enq_flags)
*/
__sync_fetch_and_add(&nr_failed_enqueues, 1);
scx_bpf_dsq_insert(p, SCX_DSQ_GLOBAL, SCX_SLICE_DFL, enq_flags);
+
+ /*
+ * Kick idle target CPU for pinned tasks. Without this, the CPU can
+ * idle while ksoftirqd is pending in the BPF queue, triggering NOHZ
+ * tick-stop warnings.
+ */
+ if (p->nr_cpus_allowed == 1)
+ scx_bpf_kick_cpu(scx_bpf_task_cpu(p), SCX_KICK_IDLE);
} else {
__sync_fetch_and_add(&nr_user_enqueues, 1);
set_usersched_needed();

--
Thanks,
Cheng-Yang