[PATCH v4 1/2] sched/topology: Don't try to build empty sched domains

[Valentin Schneider]

Turns out hotplugging CPUs that are in exclusive cpusets can lead to the
cpuset code feeding empty cpumasks to the sched domain rebuild machinery.
This leads to the following splat:

[   30.618174] Internal error: Oops: 96000004 [#1] PREEMPT SMP
[   30.623697] Modules linked in:
[   30.626731] CPU: 0 PID: 235 Comm: kworker/5:2 Not tainted 5.4.0-rc1-00005-g8d495477d62e #23
[   30.635003] Hardware name: ARM Juno development board (r0) (DT)
[   30.640877] Workqueue: events cpuset_hotplug_workfn
[   30.645713] pstate: 60000005 (nZCv daif -PAN -UAO)
[   30.650464] pc : build_sched_domains (./include/linux/arch_topology.h:23 kernel/sched/topology.c:1898 kernel/sched/topology.c:1969)
[   30.655126] lr : build_sched_domains (kernel/sched/topology.c:1966)
[...]
[   30.742047] Call trace:
[   30.744474] build_sched_domains (./include/linux/arch_topology.h:23 kernel/sched/topology.c:1898 kernel/sched/topology.c:1969)
[   30.748793] partition_sched_domains_locked (kernel/sched/topology.c:2250)
[   30.753971] rebuild_sched_domains_locked (./include/linux/bitmap.h:370 ./include/linux/cpumask.h:538 kernel/cgroup/cpuset.c:955 kernel/cgroup/cpuset.c:978 kernel/cgroup/cpuset.c:1019)
[   30.758977] rebuild_sched_domains (kernel/cgroup/cpuset.c:1032)
[   30.763209] cpuset_hotplug_workfn (kernel/cgroup/cpuset.c:3205 (discriminator 2))
[   30.767613] process_one_work (./arch/arm64/include/asm/jump_label.h:21 ./include/linux/jump_label.h:200 ./include/trace/events/workqueue.h:114 kernel/workqueue.c:2274)
[   30.771586] worker_thread (./include/linux/compiler.h:199 ./include/linux/list.h:268 kernel/workqueue.c:2416)
[   30.775217] kthread (kernel/kthread.c:255)
[   30.778418] ret_from_fork (arch/arm64/kernel/entry.S:1167)
[ 30.781965] Code: f860dae2 912802d6 aa1603e1 12800000 (f8616853)

The faulty line in question is

  cap = arch_scale_cpu_capacity(cpumask_first(cpu_map));

and we're not checking the return value against nr_cpu_ids (we shouldn't
have to!), which leads to the above.

Prevent generate_sched_domains() from returning empty cpumasks, and add
some assertion in build_sched_domains() to scream bloody murder if it
happens again.

The above splat was obtained on my Juno r0 with:

  cgcreate -g cpuset:asym
  cgset -r cpuset.cpus=0-3 asym
  cgset -r cpuset.mems=0 asym
  cgset -r cpuset.cpu_exclusive=1 asym

  cgcreate -g cpuset:smp
  cgset -r cpuset.cpus=4-5 smp
  cgset -r cpuset.mems=0 smp
  cgset -r cpuset.cpu_exclusive=1 smp

  cgset -r cpuset.sched_load_balance=0 .

  echo 0 > /sys/devices/system/cpu/cpu4/online
  echo 0 > /sys/devices/system/cpu/cpu5/online

Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: 05484e098448 ("sched/topology: Add SD_ASYM_CPUCAPACITY flag detection")
Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
---
 kernel/cgroup/cpuset.c  | 3 ++-
 kernel/sched/topology.c | 5 ++++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index c52bc91f882b..c87ee6412b36 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -798,7 +798,8 @@ static int generate_sched_domains(cpumask_var_t **domains,
 		    cpumask_subset(cp->cpus_allowed, top_cpuset.effective_cpus))
 			continue;
 
-		if (is_sched_load_balance(cp))
+		if (is_sched_load_balance(cp) &&
+		    !cpumask_empty(cp->effective_cpus))
 			csa[csn++] = cp;
 
 		/* skip @cp's subtree if not a partition root */
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 3623ffe85d18..2e7af755e17a 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1945,7 +1945,7 @@ static struct sched_domain_topology_level
 static int
 build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *attr)
 {
-	enum s_alloc alloc_state;
+	enum s_alloc alloc_state = sa_none;
 	struct sched_domain *sd;
 	struct s_data d;
 	struct rq *rq = NULL;
@@ -1953,6 +1953,9 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
 	struct sched_domain_topology_level *tl_asym;
 	bool has_asym = false;
 
+	if (WARN_ON(cpumask_empty(cpu_map)))
+		goto error;
+
 	alloc_state = __visit_domain_allocation_hell(&d, cpu_map);
 	if (alloc_state != sa_rootdomain)
 		goto error;
-- 
2.22.0