Part I Designing Device Drivers for the Solaris Platform
1. Overview of Solaris Device Drivers
2. Solaris Kernel and Device Tree
5. Managing Events and Queueing Tasks
Using ddi_log_sysevent() to Log Events
Sample Code for Logging Events
7. Device Access: Programmed I/O
10. Mapping Device and Kernel Memory
14. Layered Driver Interface (LDI)
Part II Designing Specific Kinds of Device Drivers
15. Drivers for Character Devices
18. SCSI Host Bus Adapter Drivers
19. Drivers for Network Devices
Part III Building a Device Driver
21. Compiling, Loading, Packaging, and Testing Drivers
22. Debugging, Testing, and Tuning Device Drivers
23. Recommended Coding Practices
B. Summary of Solaris DDI/DKI Services
C. Making a Device Driver 64-Bit Ready
This section discusses how to use task queues to postpone processing of some tasks and delegate their execution to another kernel thread.
A common operation in kernel programming is to schedule a task to be performed at a later time, by a different thread. The following examples give some reasons that you might want a different thread to perform a task at a later time:
Your current code path is time critical. The additional task you want to perform is not time critical.
The additional task might require grabbing a lock that another thread is currently holding.
You cannot block in your current context. The additional task might need to block, for example to wait for memory.
A condition is preventing your code path from completing, but your current code path cannot sleep or fail. You need to queue the current task to execute after the condition disappears.
You need to launch multiple tasks in parallel.
In each of these cases, a task is executed in a different context. A different context is usually a different kernel thread with a different set of locks held and possibly a different priority. Task queues provide a generic kernel API for scheduling asynchronous tasks.
A task queue is a list of tasks with one or more threads to service the list. If a task queue has a single service thread, all tasks are guaranteed to execute in the order in which they are added to the list. If a task queue has more than one service thread, the order in which the tasks will execute is not known.
Note - If the task queue has more than one service thread, make sure that the execution of one task does not depend on the execution of any other task. Dependencies between tasks can cause a deadlock to occur.
The following DDI interfaces manage task queues. These interfaces are defined in the sys/sunddi.h header file. See the taskq(9F) man page for more information about these interfaces.
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The typical usage in drivers is to create task queues at attach(9E). Most taskq_dispatch() invocations are from interrupt context.
To study task queues used in Solaris drivers, go to http://hub.opensolaris.org/bin/view/Main/. In the upper right corner, click Source Browser. In the Symbol field of the search area, enter ddi_taskq_create. In the File Path field, enter amr. In the Project list, select onnv. Click the Search button. In your search results you should see the SCSI HBA driver for Dell PERC 3DC/4SC/4DC/4Di RAID devices (amr.c).
Click the file name amr.c. The ddi_taskq_create() function is called in the amr_attach() entry point. The ddi_taskq_destroy() function is called in the amr_detach() entry point and also in the error handling section of the amr_attach() entry point. The ddi_taskq_dispatch() function is called in the amr_done() function, which is called in the amr_intr() function. The amr_intr() function is an interrupt-handling function that is an argument to the ddi_add_intr(9F) function in the amr_attach() entry point.
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 120247890619
The 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