Root/Documentation/slow-work.txt

1             ====================================
2             SLOW WORK ITEM EXECUTION THREAD POOL
3             ====================================
4
5By: David Howells <dhowells@redhat.com>
6
7The slow work item execution thread pool is a pool of threads for performing
8things that take a relatively long time, such as making mkdir calls.
9Typically, when processing something, these items will spend a lot of time
10blocking a thread on I/O, thus making that thread unavailable for doing other
11work.
12
13The standard workqueue model is unsuitable for this class of work item as that
14limits the owner to a single thread or a single thread per CPU. For some
15tasks, however, more threads - or fewer - are required.
16
17There is just one pool per system. It contains no threads unless something
18wants to use it - and that something must register its interest first. When
19the pool is active, the number of threads it contains is dynamic, varying
20between a maximum and minimum setting, depending on the load.
21
22
23====================
24CLASSES OF WORK ITEM
25====================
26
27This pool support two classes of work items:
28
29 (*) Slow work items.
30
31 (*) Very slow work items.
32
33The former are expected to finish much quicker than the latter.
34
35An operation of the very slow class may do a batch combination of several
36lookups, mkdirs, and a create for instance.
37
38An operation of the ordinarily slow class may, for example, write stuff or
39expand files, provided the time taken to do so isn't too long.
40
41Operations of both types may sleep during execution, thus tying up the thread
42loaned to it.
43
44A further class of work item is available, based on the slow work item class:
45
46 (*) Delayed slow work items.
47
48These are slow work items that have a timer to defer queueing of the item for
49a while.
50
51
52THREAD-TO-CLASS ALLOCATION
53--------------------------
54
55Not all the threads in the pool are available to work on very slow work items.
56The number will be between one and one fewer than the number of active threads.
57This is configurable (see the "Pool Configuration" section).
58
59All the threads are available to work on ordinarily slow work items, but a
60percentage of the threads will prefer to work on very slow work items.
61
62The configuration ensures that at least one thread will be available to work on
63very slow work items, and at least one thread will be available that won't work
64on very slow work items at all.
65
66
67=====================
68USING SLOW WORK ITEMS
69=====================
70
71Firstly, a module or subsystem wanting to make use of slow work items must
72register its interest:
73
74     int ret = slow_work_register_user(struct module *module);
75
76This will return 0 if successful, or a -ve error upon failure. The module
77pointer should be the module interested in using this facility (almost
78certainly THIS_MODULE).
79
80
81Slow work items may then be set up by:
82
83 (1) Declaring a slow_work struct type variable:
84
85    #include <linux/slow-work.h>
86
87    struct slow_work myitem;
88
89 (2) Declaring the operations to be used for this item:
90
91    struct slow_work_ops myitem_ops = {
92        .get_ref = myitem_get_ref,
93        .put_ref = myitem_put_ref,
94        .execute = myitem_execute,
95    };
96
97     [*] For a description of the ops, see section "Item Operations".
98
99 (3) Initialising the item:
100
101    slow_work_init(&myitem, &myitem_ops);
102
103     or:
104
105    delayed_slow_work_init(&myitem, &myitem_ops);
106
107     or:
108
109    vslow_work_init(&myitem, &myitem_ops);
110
111     depending on its class.
112
113A suitably set up work item can then be enqueued for processing:
114
115    int ret = slow_work_enqueue(&myitem);
116
117This will return a -ve error if the thread pool is unable to gain a reference
118on the item, 0 otherwise, or (for delayed work):
119
120    int ret = delayed_slow_work_enqueue(&myitem, my_jiffy_delay);
121
122
123The items are reference counted, so there ought to be no need for a flush
124operation. But as the reference counting is optional, means to cancel
125existing work items are also included:
126
127    cancel_slow_work(&myitem);
128    cancel_delayed_slow_work(&myitem);
129
130can be used to cancel pending work. The above cancel function waits for
131existing work to have been executed (or prevent execution of them, depending
132on timing).
133
134
135When all a module's slow work items have been processed, and the
136module has no further interest in the facility, it should unregister its
137interest:
138
139    slow_work_unregister_user(struct module *module);
140
141The module pointer is used to wait for all outstanding work items for that
142module before completing the unregistration. This prevents the put_ref() code
143from being taken away before it completes. module should almost certainly be
144THIS_MODULE.
145
146
147================
148HELPER FUNCTIONS
149================
150
151The slow-work facility provides a function by which it can be determined
152whether or not an item is queued for later execution:
153
154    bool queued = slow_work_is_queued(struct slow_work *work);
155
156If it returns false, then the item is not on the queue (it may be executing
157with a requeue pending). This can be used to work out whether an item on which
158another depends is on the queue, thus allowing a dependent item to be queued
159after it.
160
161If the above shows an item on which another depends not to be queued, then the
162owner of the dependent item might need to wait. However, to avoid locking up
163the threads unnecessarily be sleeping in them, it can make sense under some
164circumstances to return the work item to the queue, thus deferring it until
165some other items have had a chance to make use of the yielded thread.
166
167To yield a thread and defer an item, the work function should simply enqueue
168the work item again and return. However, this doesn't work if there's nothing
169actually on the queue, as the thread just vacated will jump straight back into
170the item's work function, thus busy waiting on a CPU.
171
172Instead, the item should use the thread to wait for the dependency to go away,
173but rather than using schedule() or schedule_timeout() to sleep, it should use
174the following function:
175
176    bool requeue = slow_work_sleep_till_thread_needed(
177            struct slow_work *work,
178            signed long *_timeout);
179
180This will add a second wait and then sleep, such that it will be woken up if
181either something appears on the queue that could usefully make use of the
182thread - and behind which this item can be queued, or if the event the caller
183set up to wait for happens. True will be returned if something else appeared
184on the queue and this work function should perhaps return, of false if
185something else woke it up. The timeout is as for schedule_timeout().
186
187For example:
188
189    wq = bit_waitqueue(&my_flags, MY_BIT);
190    init_wait(&wait);
191    requeue = false;
192    do {
193        prepare_to_wait(wq, &wait, TASK_UNINTERRUPTIBLE);
194        if (!test_bit(MY_BIT, &my_flags))
195            break;
196        requeue = slow_work_sleep_till_thread_needed(&my_work,
197                                 &timeout);
198    } while (timeout > 0 && !requeue);
199    finish_wait(wq, &wait);
200    if (!test_bit(MY_BIT, &my_flags)
201        goto do_my_thing;
202    if (requeue)
203        return; // to slow_work
204
205
206===============
207ITEM OPERATIONS
208===============
209
210Each work item requires a table of operations of type struct slow_work_ops.
211Only ->execute() is required; the getting and putting of a reference and the
212describing of an item are all optional.
213
214 (*) Get a reference on an item:
215
216    int (*get_ref)(struct slow_work *work);
217
218     This allows the thread pool to attempt to pin an item by getting a
219     reference on it. This function should return 0 if the reference was
220     granted, or a -ve error otherwise. If an error is returned,
221     slow_work_enqueue() will fail.
222
223     The reference is held whilst the item is queued and whilst it is being
224     executed. The item may then be requeued with the same reference held, or
225     the reference will be released.
226
227 (*) Release a reference on an item:
228
229    void (*put_ref)(struct slow_work *work);
230
231     This allows the thread pool to unpin an item by releasing the reference on
232     it. The thread pool will not touch the item again once this has been
233     called.
234
235 (*) Execute an item:
236
237    void (*execute)(struct slow_work *work);
238
239     This should perform the work required of the item. It may sleep, it may
240     perform disk I/O and it may wait for locks.
241
242 (*) View an item through /proc:
243
244    void (*desc)(struct slow_work *work, struct seq_file *m);
245
246     If supplied, this should print to 'm' a small string describing the work
247     the item is to do. This should be no more than about 40 characters, and
248     shouldn't include a newline character.
249
250     See the 'Viewing executing and queued items' section below.
251
252
253==================
254POOL CONFIGURATION
255==================
256
257The slow-work thread pool has a number of configurables:
258
259 (*) /proc/sys/kernel/slow-work/min-threads
260
261     The minimum number of threads that should be in the pool whilst it is in
262     use. This may be anywhere between 2 and max-threads.
263
264 (*) /proc/sys/kernel/slow-work/max-threads
265
266     The maximum number of threads that should in the pool. This may be
267     anywhere between min-threads and 255 or NR_CPUS * 2, whichever is greater.
268
269 (*) /proc/sys/kernel/slow-work/vslow-percentage
270
271     The percentage of active threads in the pool that may be used to execute
272     very slow work items. This may be between 1 and 99. The resultant number
273     is bounded to between 1 and one fewer than the number of active threads.
274     This ensures there is always at least one thread that can process very
275     slow work items, and always at least one thread that won't.
276
277
278==================================
279VIEWING EXECUTING AND QUEUED ITEMS
280==================================
281
282If CONFIG_SLOW_WORK_DEBUG is enabled, a debugfs file is made available:
283
284    /sys/kernel/debug/slow_work/runqueue
285
286through which the list of work items being executed and the queues of items to
287be executed may be viewed. The owner of a work item is given the chance to
288add some information of its own.
289
290The contents look something like the following:
291
292    THR PID ITEM ADDR FL MARK DESC
293    === ===== ================ == ===== ==========
294      0 3005 ffff880023f52348 a 952ms FSC: OBJ17d3: LOOK
295      1 3006 ffff880024e33668 2 160ms FSC: OBJ17e5 OP60d3b: Write1/Store fl=2
296      2 3165 ffff8800296dd180 a 424ms FSC: OBJ17e4: LOOK
297      3 4089 ffff8800262c8d78 a 212ms FSC: OBJ17ea: CRTN
298      4 4090 ffff88002792bed8 2 388ms FSC: OBJ17e8 OP60d36: Write1/Store fl=2
299      5 4092 ffff88002a0ef308 2 388ms FSC: OBJ17e7 OP60d2e: Write1/Store fl=2
300      6 4094 ffff88002abaf4b8 2 132ms FSC: OBJ17e2 OP60d4e: Write1/Store fl=2
301      7 4095 ffff88002bb188e0 a 388ms FSC: OBJ17e9: CRTN
302    vsq - ffff880023d99668 1 308ms FSC: OBJ17e0 OP60f91: Write1/EnQ fl=2
303    vsq - ffff8800295d1740 1 212ms FSC: OBJ16be OP4d4b6: Write1/EnQ fl=2
304    vsq - ffff880025ba3308 1 160ms FSC: OBJ179a OP58dec: Write1/EnQ fl=2
305    vsq - ffff880024ec83e0 1 160ms FSC: OBJ17ae OP599f2: Write1/EnQ fl=2
306    vsq - ffff880026618e00 1 160ms FSC: OBJ17e6 OP60d33: Write1/EnQ fl=2
307    vsq - ffff880025a2a4b8 1 132ms FSC: OBJ16a2 OP4d583: Write1/EnQ fl=2
308    vsq - ffff880023cbe6d8 9 212ms FSC: OBJ17eb: LOOK
309    vsq - ffff880024d37590 9 212ms FSC: OBJ17ec: LOOK
310    vsq - ffff880027746cb0 9 212ms FSC: OBJ17ed: LOOK
311    vsq - ffff880024d37ae8 9 212ms FSC: OBJ17ee: LOOK
312    vsq - ffff880024d37cb0 9 212ms FSC: OBJ17ef: LOOK
313    vsq - ffff880025036550 9 212ms FSC: OBJ17f0: LOOK
314    vsq - ffff8800250368e0 9 212ms FSC: OBJ17f1: LOOK
315    vsq - ffff880025036aa8 9 212ms FSC: OBJ17f2: LOOK
316
317In the 'THR' column, executing items show the thread they're occupying and
318queued threads indicate which queue they're on. 'PID' shows the process ID of
319a slow-work thread that's executing something. 'FL' shows the work item flags.
320'MARK' indicates how long since an item was queued or began executing. Lastly,
321the 'DESC' column permits the owner of an item to give some information.
322
323

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