This is a note to let you know that I've just added the patch titled
cpu/hotplug: Don't offline the last non-isolated CPU
to the 6.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
cpu-hotplug-don-t-offline-the-last-non-isolated-cpu.patch
and it can be found in the queue-6.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit a8be1012743b6db4aad1518222a2c92b3f5826f5
Author: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
Date: Tue Oct 17 17:09:53 2023 +0800
cpu/hotplug: Don't offline the last non-isolated CPU
[ 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>
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 1a189da3bdac5..303cb0591b4b1 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1523,11 +1523,14 @@ 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)
