[PATCH v3 1/2] sched/topology: Don't try to build empty sched domains

[Valentin Schneider <valentin.schneider@arm.com>]

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@vger.kernel.org>
Fixes: 05484e098448 ("sched/topology: Add SD_ASYM_CPUCAPACITY flag detection")
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/cgroup/cpuset.c  | 8 ++++++++
 kernel/sched/topology.c | 5 ++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index c52bc91f882b..a859e5539440 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -817,6 +817,11 @@ static int generate_sched_domains(cpumask_var_t **domains,
 		struct cpuset *a = csa[i];
 		int apn = a->pn;
 
+		if (cpumask_empty(a->effective_cpus)) {
+			ndoms--;
+			continue;
+		}
+
 		for (j = 0; j < csn; j++) {
 			struct cpuset *b = csa[j];
 			int bpn = b->pn;
@@ -859,6 +864,9 @@ static int generate_sched_domains(cpumask_var_t **domains,
 			continue;
 		}
 
+		if (cpumask_empty(a->effective_cpus))
+			continue;
+
 		dp = doms[nslot];
 
 		if (nslot == ndoms) {
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index b5667a273bf6..9318acf1d1fe 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1948,7 +1948,7 @@ 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;
@@ -1956,6 +1956,9 @@ 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