The typical usage in drivers is to create task queues at attach(9E). Most taskq_dispatch() invocations are from interrupt context.
This section describes two techniques that you can use to monitor the system resources that are consumed by a task queue. Task queues export statistics on the use of system time by task queue threads. Task queues also use DTrace SDT probes to determine when a task queue starts and finishes execution of a task.
Every task queue has an associated set of kstat counters. Examine the output of the following kstat(1M) command:
$ kstat -c taskq
module: unix instance: 0
name: ata_nexus_enum_tq class: taskq
crtime 53.877907833
executed 0
maxtasks 0
nactive 1
nalloc 0
priority 60
snaptime 258059.249256749
tasks 0
threads 1
totaltime 0
module: unix instance: 0
name: callout_taskq class: taskq
crtime 0
executed 13956358
maxtasks 4
nactive 4
nalloc 0
priority 99
snaptime 258059.24981709
tasks 13956358
threads 2
totaltime 120247890619The kstat output shown above includes the following information:
The name of the task queue and its instance number
The number of scheduled (tasks) and executed (executed) tasks
The number of kernel threads processing the task queue (threads) and their priority (priority)
The total time (in nanoseconds) spent processing all the tasks (totaltime)
The following example shows how you can use the kstat command to observe how a counter (number of scheduled tasks) increases over time:
$ kstat -p unix:0:callout_taskq:tasks 1 5 unix:0:callout_taskq:tasks 13994642 unix:0:callout_taskq:tasks 13994711 unix:0:callout_taskq:tasks 13994784 unix:0:callout_taskq:tasks 13994855 unix:0:callout_taskq:tasks 13994926
Task queues provide several useful SDT probes. All the probes described in this section have the following two arguments:
The task queue pointer returned by ddi_taskq_create()
The pointer to the taskq_ent_t structure. Use this pointer in your D script to extract the function and the argument.
You can use these probes to collect precise timing information about individual task queues and individual tasks being executed through them. For example, the following script prints the functions that were scheduled through task queues for every 10 seconds:
# !/usr/sbin/dtrace -qs
sdt:genunix::taskq-enqueue
{
this->tq = (taskq_t *)arg0;
this->tqe = (taskq_ent_t *) arg1;
@[this->tq->tq_name,
this->tq->tq_instance,
this->tqe->tqent_func] = count();
}
tick-10s
{
printa ("%s(%d): %a called %@d times\n", @);
trunc(@);
}On a particular machine, the above D script produced the following output:
callout_taskq(1): genunix`callout_execute called 51 times callout_taskq(0): genunix`callout_execute called 701 times kmem_taskq(0): genunix`kmem_update_timeout called 1 times kmem_taskq(0): genunix`kmem_hash_rescale called 4 times callout_taskq(1): genunix`callout_execute called 40 times USB_hid_81_pipehndl_tq_1(14): usba`hcdi_cb_thread called 256 times callout_taskq(0): genunix`callout_execute called 702 times kmem_taskq(0): genunix`kmem_update_timeout called 1 times kmem_taskq(0): genunix`kmem_hash_rescale called 4 times callout_taskq(1): genunix`callout_execute called 28 times USB_hid_81_pipehndl_tq_1(14): usba`hcdi_cb_thread called 228 times callout_taskq(0): genunix`callout_execute called 706 times callout_taskq(1): genunix`callout_execute called 24 times USB_hid_81_pipehndl_tq_1(14): usba`hcdi_cb_thread called 141 times callout_taskq(0): genunix`callout_execute called 708 times