sched/topology: Don't try to build empty sched domains

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:

    Internal error: Oops: 96000004 [#1] PREEMPT SMP
    Modules linked in:
    CPU: 0 PID: 235 Comm: kworker/5:2 Not tainted 5.4.0-rc1-00005-g8d495477d62e #23
    Hardware name: ARM Juno development board (r0) (DT)
    Workqueue: events cpuset_hotplug_workfn
    pstate: 60000005 (nZCv daif -PAN -UAO)
    pc : build_sched_domains (./include/linux/arch_topology.h:23 kernel/sched/topology.c:1898 kernel/sched/topology.c:1969)
    lr : build_sched_domains (kernel/sched/topology.c:1966)
    Call trace:
    build_sched_domains (./include/linux/arch_topology.h:23 kernel/sched/topology.c:1898 kernel/sched/topology.c:1969)
    partition_sched_domains_locked (kernel/sched/topology.c:2250)
    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)
    rebuild_sched_domains (kernel/cgroup/cpuset.c:1032)
    cpuset_hotplug_workfn (kernel/cgroup/cpuset.c:3205 (discriminator 2))
    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)
    worker_thread (./include/linux/compiler.h:199 ./include/linux/list.h:268 kernel/workqueue.c:2416)
    kthread (kernel/kthread.c:255)
    ret_from_fork (arch/arm64/kernel/entry.S:1167)
    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 the following reproducer:

  $ 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

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Dietmar.Eggemann@arm.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: hannes@cmpxchg.org
Cc: lizefan@huawei.com
Cc: morten.rasmussen@arm.com
Cc: qperret@google.com
Cc: tj@kernel.org
Cc: vincent.guittot@linaro.org
Fixes: 05484e098448 ("sched/topology: Add SD_ASYM_CPUCAPACITY flag detection")
Link: https://lkml.kernel.org/r/20191023153745.19515-2-valentin.schneider@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index c52bc91f882b29e44ea8eb56fd0ba9889c6a86df..c87ee6412b36aa8f088844c42abc2b8aa70d2749 100644 (file)
--- 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 b5667a273bf67e0718371a783b259ffb4c91cbc7..9318acf1d1fedf8241325f7b5341a113b9c955b3 100644 (file)
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1948,7 +1948,7 @@ next_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;