[PATCH 14/17] tools/sched_ext: scx_qmap: Restart on hotplug instead of cpu_online/offline

Tejun Heo
2026-04-28T20:36:01Z

The cid mapping is built from the online cpu set at scheduler enable
and stays valid for that set; routine hotplug invalidates it. The
default cid behavior is to restart the scheduler so the mapping gets
rebuilt against the new online set, and that requires not implementing
cpu_online / cpu_offline (which suppress the kernel's ACT_RESTART).

Drop the two ops along with their print_cpus() helper - the cluster
view was only useful as a hotplug demo and is meaningless over the
dense cid space the scheduler will move to. Wire main() to handle the
ACT_RESTART exit by reopening the skel and reattaching, matching the
pattern in scx_simple / scx_central / scx_flatcg etc. Reset optind so
getopt re-parses argv into the fresh skel rodata each iteration.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
---
tools/sched_ext/scx_qmap.bpf.c | 62 ----------------------------------
tools/sched_ext/scx_qmap.c | 13 +++----
2 files changed, 7 insertions(+), 68 deletions(-)

diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
index ba4879031dac..78a1dd118c7e 100644
--- a/tools/sched_ext/scx_qmap.bpf.c
+++ b/tools/sched_ext/scx_qmap.bpf.c
@@ -843,63 +843,6 @@ void BPF_STRUCT_OPS(qmap_cgroup_set_bandwidth, struct cgroup *cgrp,
cgrp->kn->id, period_us, quota_us, burst_us);
}

-/*
- * Print out the online and possible CPU map using bpf_printk() as a
- * demonstration of using the cpumask kfuncs and ops.cpu_on/offline().
- */
-static void print_cpus(void)
-{
- const struct cpumask *possible, *online;
- s32 cpu;
- char buf[128] = "", *p;
- int idx;
-
- possible = scx_bpf_get_possible_cpumask();
- online = scx_bpf_get_online_cpumask();
-
- idx = 0;
- bpf_for(cpu, 0, scx_bpf_nr_cpu_ids()) {
- if (!(p = MEMBER_VPTR(buf, [idx++])))
- break;
- if (bpf_cpumask_test_cpu(cpu, online))
- *p++ = 'O';
- else if (bpf_cpumask_test_cpu(cpu, possible))
- *p++ = 'X';
- else
- *p++ = ' ';
-
- if ((cpu & 7) == 7) {
- if (!(p = MEMBER_VPTR(buf, [idx++])))
- break;
- *p++ = '|';
- }
- }
- buf[sizeof(buf) - 1] = '\0';
-
- scx_bpf_put_cpumask(online);
- scx_bpf_put_cpumask(possible);
-
- bpf_printk("CPUS: |%s", buf);
-}
-
-void BPF_STRUCT_OPS(qmap_cpu_online, s32 cpu)
-{
- if (print_msgs) {
- bpf_printk("CPU %d coming online", cpu);
- /* @cpu is already online at this point */
- print_cpus();
- }
-}
-
-void BPF_STRUCT_OPS(qmap_cpu_offline, s32 cpu)
-{
- if (print_msgs) {
- bpf_printk("CPU %d going offline", cpu);
- /* @cpu is still online at this point */
- print_cpus();
- }
-}
-
struct monitor_timer {
struct bpf_timer timer;
};
@@ -1078,9 +1021,6 @@ s32 BPF_STRUCT_OPS_SLEEPABLE(qmap_init)
slab[i].next_free = (i + 1 < max_tasks) ? &slab[i + 1] : NULL;
qa.task_free_head = &slab[0];

- if (print_msgs && !sub_cgroup_id)
- print_cpus();
-
ret = scx_bpf_create_dsq(SHARED_DSQ, -1);
if (ret) {
scx_bpf_error("failed to create DSQ %d (%d)", SHARED_DSQ, ret);
@@ -1174,8 +1114,6 @@ SCX_OPS_DEFINE(qmap_ops,
.cgroup_set_bandwidth = (void *)qmap_cgroup_set_bandwidth,
.sub_attach = (void *)qmap_sub_attach,
.sub_detach = (void *)qmap_sub_detach,
- .cpu_online = (void *)qmap_cpu_online,
- .cpu_offline = (void *)qmap_cpu_offline,
.init = (void *)qmap_init,
.exit = (void *)qmap_exit,
.timeout_ms = 5000U,
diff --git a/tools/sched_ext/scx_qmap.c b/tools/sched_ext/scx_qmap.c
index 725c4880058d..99408b1bb1ec 100644
--- a/tools/sched_ext/scx_qmap.c
+++ b/tools/sched_ext/scx_qmap.c
@@ -67,12 +67,14 @@ int main(int argc, char **argv)
struct bpf_link *link;
struct qmap_arena *qa;
__u32 test_error_cnt = 0;
+ __u64 ecode;
int opt;

libbpf_set_print(libbpf_print_fn);
signal(SIGINT, sigint_handler);
signal(SIGTERM, sigint_handler);
-
+restart:
+ optind = 1;
skel = SCX_OPS_OPEN(qmap_ops, scx_qmap);

skel->rodata->slice_ns = __COMPAT_ENUM_OR_ZERO("scx_public_consts", "SCX_SLICE_DFL");
@@ -184,11 +186,10 @@ int main(int argc, char **argv)
}

bpf_link__destroy(link);
- UEI_REPORT(skel, uei);
+ ecode = UEI_REPORT(skel, uei);
scx_qmap__destroy(skel);
- /*
- * scx_qmap implements ops.cpu_on/offline() and doesn't need to restart
- * on CPU hotplug events.
- */
+
+ if (UEI_ECODE_RESTART(ecode))
+ goto restart;
return 0;
}
--
2.54.0