[PATCH AUTOSEL 6.6 11/13] cpu/hotplug: Don't offline the last non-isolated CPU

[Sasha Levin]

From: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>

[ Upstream commit 38685e2a0476127db766f81b1c06019ddc4c9ffa ]

If a system has isolated CPUs via the "isolcpus=" command line parameter,
then an attempt to offline the last housekeeping CPU will result in a
WARN_ON() when rebuilding the scheduler domains and a subsequent panic due
to and unhandled empty CPU mas in partition_sched_domains_locked().

cpuset_hotplug_workfn()
  rebuild_sched_domains_locked()
    ndoms = generate_sched_domains(&doms, &attr);
      cpumask_and(doms[0], top_cpuset.effective_cpus, housekeeping_cpumask(HK_FLAG_DOMAIN));

Thus results in an empty CPU mask which triggers the warning and then the
subsequent crash:

WARNING: CPU: 4 PID: 80 at kernel/sched/topology.c:2366 build_sched_domains+0x120c/0x1408
Call trace:
 build_sched_domains+0x120c/0x1408
 partition_sched_domains_locked+0x234/0x880
 rebuild_sched_domains_locked+0x37c/0x798
 rebuild_sched_domains+0x30/0x58
 cpuset_hotplug_workfn+0x2a8/0x930

Unable to handle kernel paging request at virtual address fffe80027ab37080
 partition_sched_domains_locked+0x318/0x880
 rebuild_sched_domains_locked+0x37c/0x798

Aside of the resulting crash, it does not make any sense to offline the last
last housekeeping CPU.

Prevent this by masking out the non-housekeeping CPUs when selecting a
target CPU for initiating the CPU unplug operation via the work queue.

Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/202310171709530660462@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 kernel/cpu.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6de7c6bb74eee..94430ea8b4a54 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1515,11 +1515,14 @@ static int cpu_down_maps_locked(unsigned int cpu, enum cpuhp_state target)
 	/*
 	 * Ensure that the control task does not run on the to be offlined
 	 * CPU to prevent a deadlock against cfs_b->period_timer.
+	 * Also keep at least one housekeeping cpu onlined to avoid generating
+	 * an empty sched_domain span.
 	 */
-	cpu = cpumask_any_but(cpu_online_mask, cpu);
-	if (cpu >= nr_cpu_ids)
-		return -EBUSY;
-	return work_on_cpu(cpu, __cpu_down_maps_locked, &work);
+	for_each_cpu_and(cpu, cpu_online_mask, housekeeping_cpumask(HK_TYPE_DOMAIN)) {
+		if (cpu != work.cpu)
+			return work_on_cpu(cpu, __cpu_down_maps_locked, &work);
+	}
+	return -EBUSY;
 }
 
 static int cpu_down(unsigned int cpu, enum cpuhp_state target)
-- 
2.42.0