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-g8d495477 #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: NValentin Schneider <valentin.schneider@arm.com>
Signed-off-by: NPeter 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: 05484e09 ("sched/topology: Add SD_ASYM_CPUCAPACITY flag detection")
Link: https://lkml.kernel.org/r/20191023153745.19515-2-valentin.schneider@arm.comSigned-off-by: NIngo Molnar <mingo@kernel.org>