1.\" -*- nroff -*- 2.\" 3.\" Copyright (c) 2000 Doug Rabson 4.\" 5.\" All rights reserved. 6.\" 7.\" This program is free software. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. 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29.\" $FreeBSD$ 30.\" 31.Dd August 18, 2009 32.Dt TASKQUEUE 9 33.Os 34.Sh NAME 35.Nm taskqueue 36.Nd asynchronous task execution 37.Sh SYNOPSIS 38.In sys/param.h 39.In sys/kernel.h 40.In sys/malloc.h 41.In sys/queue.h 42.In sys/taskqueue.h 43.Bd -literal 44typedef void (*task_fn_t)(void *context, int pending); 45 46typedef void (*taskqueue_enqueue_fn)(void *context); 47 48struct task { 49 STAILQ_ENTRY(task) ta_link; /* link for queue */ 50 u_short ta_pending; /* count times queued */ 51 u_short ta_priority; /* priority of task in queue */ 52 task_fn_t ta_func; /* task handler */ 53 void *ta_context; /* argument for handler */ 54}; 55.Ed 56.Ft struct taskqueue * 57.Fn taskqueue_create "const char *name" "int mflags" "taskqueue_enqueue_fn enqueue" "void *context" 58.Ft struct taskqueue * 59.Fn taskqueue_create_fast "const char *name" "int mflags" "taskqueue_enqueue_fn enqueue" "void *context" 60.Ft void 61.Fn taskqueue_free "struct taskqueue *queue" 62.Ft int 63.Fn taskqueue_enqueue "struct taskqueue *queue" "struct task *task" 64.Ft int 65.Fn taskqueue_enqueue_fast "struct taskqueue *queue" "struct task *task" 66.Ft void 67.Fn taskqueue_run "struct taskqueue *queue" 68.Ft void 69.Fn taskqueue_run_fast "struct taskqueue *queue" 70.Ft void 71.Fn taskqueue_drain "struct taskqueue *queue" "struct task *task" 72.Ft int 73.Fn taskqueue_member "struct taskqueue *queue" "struct thread *td" 74.Fn TASK_INIT "struct task *task" "int priority" "task_fn_t *func" "void *context" 75.Fn TASKQUEUE_DECLARE "name" 76.Fn TASKQUEUE_DEFINE "name" "taskqueue_enqueue_fn enqueue" "void *context" "init" 77.Fn TASKQUEUE_FAST_DEFINE "name" "taskqueue_enqueue_fn enqueue" "void *context" "init" 78.Fn TASKQUEUE_DEFINE_THREAD "name" 79.Fn TASKQUEUE_FAST_DEFINE_THREAD "name" 80.Sh DESCRIPTION 81These functions provide a simple interface for asynchronous execution 82of code. 83.Pp 84The function 85.Fn taskqueue_create 86is used to create new queues. 87The arguments to 88.Fn taskqueue_create 89include a name that should be unique, 90a set of 91.Xr malloc 9 92flags that specify whether the call to 93.Fn malloc 94is allowed to sleep, 95a function that is called from 96.Fn taskqueue_enqueue 97when a task is added to the queue, 98and a pointer to the memory location where the identity of the 99thread that services the queue is recorded. 100.\" XXX The rest of the sentence gets lots in relation to the first part. 101The function called from 102.Fn taskqueue_enqueue 103must arrange for the queue to be processed 104(for instance by scheduling a software interrupt or waking a kernel 105thread). 106The memory location where the thread identity is recorded is used 107to signal the service thread(s) to terminate--when this value is set to 108zero and the thread is signaled it will terminate. 109If the queue is intended for use in fast interrupt handlers 110.Fn taskqueue_create_fast 111should be used in place of 112.Fn taskqueue_create . 113.Pp 114The function 115.Fn taskqueue_free 116should be used to free the memory used by the queue. 117Any tasks that are on the queue will be executed at this time after 118which the thread servicing the queue will be signaled that it should exit. 119.Pp 120To add a task to the list of tasks queued on a taskqueue, call 121.Fn taskqueue_enqueue 122with pointers to the queue and task. 123If the task's 124.Va ta_pending 125field is non-zero, 126then it is simply incremented to reflect the number of times the task 127was enqueued. 128Otherwise, 129the task is added to the list before the first task which has a lower 130.Va ta_priority 131value or at the end of the list if no tasks have a lower priority. 132Enqueueing a task does not perform any memory allocation which makes 133it suitable for calling from an interrupt handler. 134This function will return 135.Er EPIPE 136if the queue is being freed. 137.Pp 138The function 139.Fn taskqueue_enqueue_fast 140should be used in place of 141.Fn taskqueue_enqueue 142when the enqueuing must happen from a fast interrupt handler. 143This method uses spin locks to avoid the possibility of sleeping in the fast 144interrupt context. 145.Pp 146To execute all the tasks on a queue, 147call 148.Fn taskqueue_run 149or 150.Fn taskqueue_run_fast 151depending on the flavour of the queue. 152When a task is executed, 153first it is removed from the queue, 154the value of 155.Va ta_pending 156is recorded and then the field is zeroed. 157The function 158.Va ta_func 159from the task structure is called with the value of the field 160.Va ta_context 161as its first argument 162and the value of 163.Va ta_pending 164as its second argument. 165After the function 166.Va ta_func 167returns, 168.Xr wakeup 9 169is called on the task pointer passed to 170.Fn taskqueue_enqueue . 171.Pp 172The 173.Fn taskqueue_drain 174function is used to wait for the task to finish. 175There is no guarantee that the task will not be 176enqueued after call to 177.Fn taskqueue_drain . 178.Pp 179The 180.Fn taskqueue_member 181function returns 182.No 1 183if the given thread 184.Fa td 185is part of the given taskqeueue 186.Fa queue 187and 188.No 0 189otherwise. 190.Pp 191A convenience macro, 192.Fn TASK_INIT "task" "priority" "func" "context" 193is provided to initialise a 194.Va task 195structure. 196The values of 197.Va priority , 198.Va func , 199and 200.Va context 201are simply copied into the task structure fields and the 202.Va ta_pending 203field is cleared. 204.Pp 205Five macros 206.Fn TASKQUEUE_DECLARE "name" , 207.Fn TASKQUEUE_DEFINE "name" "enqueue" "context" "init" , 208.Fn TASKQUEUE_FAST_DEFINE "name" "enqueue" "context" "init" , 209and 210.Fn TASKQUEUE_DEFINE_THREAD "name" 211.Fn TASKQUEUE_FAST_DEFINE_THREAD "name" 212are used to declare a reference to a global queue, to define the 213implementation of the queue, and declare a queue that uses its own thread. 214The 215.Fn TASKQUEUE_DEFINE 216macro arranges to call 217.Fn taskqueue_create 218with the values of its 219.Va name , 220.Va enqueue 221and 222.Va context 223arguments during system initialisation. 224After calling 225.Fn taskqueue_create , 226the 227.Va init 228argument to the macro is executed as a C statement, 229allowing any further initialisation to be performed 230(such as registering an interrupt handler etc.) 231.Pp 232The 233.Fn TASKQUEUE_DEFINE_THREAD 234macro defines a new taskqueue with its own kernel thread to serve tasks. 235The variable 236.Vt struct taskqueue *taskqueue_name 237is used to enqueue tasks onto the queue. 238.Pp 239.Fn TASKQUEUE_FAST_DEFINE 240and 241.Fn TASKQUEUE_FAST_DEFINE_THREAD 242act just like 243.Fn TASKQUEUE_DEFINE 244and 245.Fn TASKQUEUE_DEFINE_THREAD 246respectively but taskqueue is created with 247.Fn taskqueue_create_fast . 248.Ss Predefined Task Queues 249The system provides four global taskqueues, 250.Va taskqueue_fast , 251.Va taskqueue_swi , 252.Va taskqueue_swi_giant , 253and 254.Va taskqueue_thread . 255The 256.Va taskqueue_fast 257queue is for swi handlers dispatched from fast interrupt handlers, 258where sleep mutexes cannot be used. 259The swi taskqueues are run via a software interrupt mechanism. 260The 261.Va taskqueue_swi 262queue runs without the protection of the 263.Va Giant 264kernel lock, and the 265.Va taskqueue_swi_giant 266queue runs with the protection of the 267.Va Giant 268kernel lock. 269The thread taskqueue 270.Va taskqueue_thread 271runs in a kernel thread context, and tasks run from this thread do 272not run under the 273.Va Giant 274kernel lock. 275If the caller wants to run under 276.Va Giant , 277he should explicitly acquire and release 278.Va Giant 279in his taskqueue handler routine. 280.Pp 281To use these queues, 282call 283.Fn taskqueue_enqueue 284with the value of the global taskqueue variable for the queue you wish to 285use 286.Va ( taskqueue_swi , 287.Va taskqueue_swi_giant , 288or 289.Va taskqueue_thread ) . 290Use 291.Fn taskqueue_enqueue_fast 292for the global taskqueue variable 293.Va taskqueue_fast . 294.Pp 295The software interrupt queues can be used, 296for instance, for implementing interrupt handlers which must perform a 297significant amount of processing in the handler. 298The hardware interrupt handler would perform minimal processing of the 299interrupt and then enqueue a task to finish the work. 300This reduces to a minimum 301the amount of time spent with interrupts disabled. 302.Pp 303The thread queue can be used, for instance, by interrupt level routines 304that need to call kernel functions that do things that can only be done 305from a thread context. 306(e.g., call malloc with the M_WAITOK flag.) 307.Pp 308Note that tasks queued on shared taskqueues such as 309.Va taskqueue_swi 310may be delayed an indeterminate amount of time before execution. 311If queueing delays cannot be tolerated then a private taskqueue should 312be created with a dedicated processing thread. 313.Sh SEE ALSO 314.Xr ithread 9 , 315.Xr kthread 9 , 316.Xr swi 9 317.Sh HISTORY 318This interface first appeared in 319.Fx 5.0 . 320There is a similar facility called tqueue in the Linux kernel. 321.Sh AUTHORS 322This manual page was written by 323.An Doug Rabson . 324