linux workqueue的名字长度小问题

在排查一个nvme的的workqueue的问题的时候，发现nvme的queue的进程名被截断了，

[root@localhost caq]# ps -ef |grep -i nvme
root       1699      2  0 10:05 ?        00:00:00 [nvme]
root      29705 106743  0 18:44 pts/31   00:00:00 grep --color=auto -i nvme
root     142222      2  0 17:42 ?        00:00:00 [xfs-buf/nvme0n1]
root     142223      2  0 17:42 ?        00:00:00 [xfs-data/nvme0n]
root     142224      2  0 17:42 ?        00:00:00 [xfs-conv/nvme0n]
root     142225      2  0 17:42 ?        00:00:00 [xfs-cil/nvme0n1]
root     142227      2  0 17:42 ?        00:00:00 [xfs-log/nvme0n1]
root     142229      2  0 17:42 ?        00:00:00 [xfsaild/nvme0n1]
root     142231      2  0 17:42 ?        00:00:00 [xfs-buf/nvme1n1]
root     142232      2  0 17:42 ?        00:00:00 [xfs-data/nvme1n]
root     142233      2  0 17:42 ?        00:00:00 [xfs-conv/nvme1n]
root     142234      2  0 17:42 ?        00:00:00 [xfs-cil/nvme1n1]
root     142236      2  0 17:42 ?        00:00:00 [xfs-log/nvme1n1]
root     142238      2  0 17:42 ?        00:00:00 [xfsaild/nvme1n1]
root     142240      2  0 17:42 ?        00:00:00 [xfs-buf/nvme2n1]
root     142241      2  0 17:42 ?        00:00:00 [xfs-data/nvme2n]
root     142242      2  0 17:42 ?        00:00:00 [xfs-conv/nvme2n]
root     142243      2  0 17:42 ?        00:00:00 [xfs-cil/nvme2n1]
root     142245      2  0 17:42 ?        00:00:00 [xfs-log/nvme2n1]
root     142247      2  0 17:42 ?        00:00:00 [xfsaild/nvme2n1]
root     142391      2  0 17:42 ?        00:00:00 [xfs-buf/nvme3n1]
root     142392      2  0 17:42 ?        00:00:00 [xfs-data/nvme3n]
root     142393      2  0 17:42 ?        00:00:00 [xfs-conv/nvme3n]
root     142394      2  0 17:42 ?        00:00:00 [xfs-cil/nvme3n1]
root     142396      2  0 17:42 ?        00:00:00 [xfs-log/nvme3n1]
root     142398      2  0 17:42 ?        00:00:00 [xfsaild/nvme3n1]
root     143771      2  0 17:43 ?        00:00:00 [dio/nvme0n1]
root     144510      2  0 17:43 ?        00:00:00 [dio/nvme2n1]
root     144533      2  0 17:43 ?        00:00:00 [dio/nvme1n1]
root     146048      2  0 17:44 ?        00:00:00 [dio/nvme3n1]

一开始怀疑是ps命令截断了，然后看一下crash中对应的长度，发现也是截断的。

crash> ps |grep xfs-reclaim/sda
  29844      2  64  ffff885f5eab3f40  IN   0.0       0      0  [xfs-reclaim/sda]
  29881      2  64  ffff8857066f8fd0  IN   0.0       0      0  [xfs-reclaim/sda]
  29996      2  64  ffff885705eb8000  IN   0.0       0      0  [xfs-reclaim/sda]
  43802      2  66  ffff8855553b8000  IN   0.0       0      0  [xfs-reclaim/sda]
  43816      2  66  ffff88291574eeb0  IN   0.0       0      0  [xfs-reclaim/sda]
  43859      2  41  ffff885555e5eeb0  IN   0.0       0      0  [xfs-reclaim/sda]
  43886      2  41  ffff8855556adee0  IN   0.0       0      0  [xfs-reclaim/sda]

一个工作队列的创建流程如下，比如使用alloc_workqueue--->__alloc_workqueue_key--->

    rescuer->rescue_wq = wq;
        rescuer->task = kthread_create(rescuer_thread, rescuer, "%s",
                           wq->name);

char name[WQ_NAME_LEN]; /* I: workqueue name */
WQ_NAME_LEN = 24,

所以按道理应该是23字节的长度，再继续看

#define kthread_create(threadfn, data, namefmt, arg...) 
    kthread_create_on_node(threadfn, data, -1, namefmt, ##arg)

struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
                       void *data, int node,
                       const char namefmt[],
                       ...)
{
    struct kthread_create_info create;

    create.threadfn = threadfn;
    create.data = data;
    create.node = node;
    init_completion(&create.done);

    spin_lock(&kthread_create_lock);
    list_add_tail(&create.list, &kthread_create_list);
    spin_unlock(&kthread_create_lock);

    wake_up_process(kthreadd_task);
    wait_for_completion(&create.done);

    if (!IS_ERR(create.result)) {
        static const struct sched_param param = { .sched_priority = 0 };
        va_list args;

        va_start(args, namefmt);
        vsnprintf(create.result->comm, sizeof(create.result->comm),
              namefmt, args);-----------被截断了
        va_end(args);

char comm[TASK_COMM_LEN]; /* executable name excluding path
                           - access with [gs]et_task_comm (which lock
                           it with task_lock())
                           - initialized normally by setup_new_exec */

/* Task command name length */
#define TASK_COMM_LEN 16

所以又被截断成了15字节。所以只能展示这么多。