If I understand the semantics of the cpu_stat_off correctly, please read.
cpu_stat_off = a set of such CPU: the cpu is online && vmstat_work is off
I consider some code forget to guarantee each cpu in cpu_stat_off is online.
Thanks,
Lai
On 07/10/2014 10:04 PM, Christoph Lameter wrote:
> +
> +/*
> + * Shepherd worker thread that checks the
> + * differentials of processors that have their worker
> + * threads for vm statistics updates disabled because of
> + * inactivity.
> + */
> +static void vmstat_shepherd(struct work_struct *w);
> +
> +static DECLARE_DELAYED_WORK(shepherd, vmstat_shepherd);
> +
> +static void vmstat_shepherd(struct work_struct *w)
> +{
> + int cpu;
> +
> + /* Check processors whose vmstat worker threads have been disabled */
I think the bug is here, it re-queues the per_cpu(vmstat_work, cpu) which is offline
(after vmstat_cpuup_callback(CPU_DOWN_PREPARE). And cpu_stat_off is accessed without
proper lock.
I suggest to use get_cpu_online() or a new cpu_stat_off_mutex to protect it.
get_cpu_online(); /* mutex_lock(&cpu_stat_off_mutex); */
for_each_cpu(cpu, cpu_stat_off)
if (need_update(cpu) &&
cpumask_test_and_clear_cpu(cpu, cpu_stat_off))
schedule_delayed_work_on(cpu, &per_cpu(vmstat_work, cpu),
__round_jiffies_relative(sysctl_stat_interval, cpu));
put_cpu_online(); /* mutex_unlock(&cpu_stat_off_mutex); */
> +
> +
> + schedule_delayed_work(&shepherd,
> round_jiffies_relative(sysctl_stat_interval));
> +
> }
>
> -static void start_cpu_timer(int cpu)
> +static void __init start_shepherd_timer(void)
> {
> - struct delayed_work *work = &per_cpu(vmstat_work, cpu);
> + int cpu;
> +
> + for_each_possible_cpu(cpu)
> + INIT_DEFERRABLE_WORK(per_cpu_ptr(&vmstat_work, cpu),
> + vmstat_update);
> +
> + cpu_stat_off = kmalloc(cpumask_size(), GFP_KERNEL);
> + cpumask_copy(cpu_stat_off, cpu_online_mask);
>
> - INIT_DEFERRABLE_WORK(work, vmstat_update);
> - schedule_delayed_work_on(cpu, work, __round_jiffies_relative(HZ, cpu));
> + schedule_delayed_work(&shepherd,
> + round_jiffies_relative(sysctl_stat_interval));
> }
>
> static void vmstat_cpu_dead(int node)
> @@ -1272,17 +1373,17 @@ static int vmstat_cpuup_callback(struct
> case CPU_ONLINE:
> case CPU_ONLINE_FROZEN:
> refresh_zone_stat_thresholds();
> - start_cpu_timer(cpu);
> node_set_state(cpu_to_node(cpu), N_CPU);
> + cpumask_set_cpu(cpu, cpu_stat_off);
> break;
> case CPU_DOWN_PREPARE:
> case CPU_DOWN_PREPARE_FROZEN:
> - cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
> - per_cpu(vmstat_work, cpu).work.func = NULL;
> + if (!cpumask_test_and_set_cpu(cpu, cpu_stat_off))
> + cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
It is suggest that cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu)) should
be called unconditionally. And the cpu should be cleared from cpu_stat_off.
(you set it, it is BUG according to vmstat_shepherd() and the semantics of the
cpu_stat_off).
/* mutex_lock(&cpu_stat_off_mutex); */
/*if you use cpu_stat_off_mutex instead of get_cpu_online() */
cancel_delayed_work_sync(&per_cpu(vmstat_work, cpu));
cpumask_clear_cpu(cpu, cpu_stat_off);
/* mutex_unlock(&cpu_stat_off_mutex); */
/* don't forget to use cpu_stat_off_mutex on other place for
accessing to cpu_stat_off except the one in vmstat_update() which
is protected by cancel_delayed_work_sync() + other stuffs
please also update that comments and keep that VM_BUG_ON() */
> break;
> case CPU_DOWN_FAILED:
> case CPU_DOWN_FAILED_FROZEN:
> - start_cpu_timer(cpu);
> + cpumask_set_cpu(cpu, cpu_stat_off);
> break;
> case CPU_DEAD:
> case CPU_DEAD_FROZEN:
> @@ -1302,15 +1403,10 @@ static struct notifier_block vmstat_noti
> static int __init setup_vmstat(void)
> {
> #ifdef CONFIG_SMP
> - int cpu;
> -
> cpu_notifier_register_begin();
> __register_cpu_notifier(&vmstat_notifier);
>
> - for_each_online_cpu(cpu) {
> - start_cpu_timer(cpu);
> - node_set_state(cpu_to_node(cpu), N_CPU);
> - }
> + start_shepherd_timer();
> cpu_notifier_register_done();
> #endif
> #ifdef CONFIG_PROC_FS
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