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Diffstat (limited to 'Documentation/slow-work.txt')
| -rw-r--r-- | Documentation/slow-work.txt | 160 |
1 files changed, 154 insertions, 6 deletions
diff --git a/Documentation/slow-work.txt b/Documentation/slow-work.txt index ebc50f808ea..52bc3143372 100644 --- a/Documentation/slow-work.txt +++ b/Documentation/slow-work.txt @@ -41,6 +41,13 @@ expand files, provided the time taken to do so isn't too long. Operations of both types may sleep during execution, thus tying up the thread loaned to it. +A further class of work item is available, based on the slow work item class: + + (*) Delayed slow work items. + +These are slow work items that have a timer to defer queueing of the item for +a while. + THREAD-TO-CLASS ALLOCATION -------------------------- @@ -64,9 +71,11 @@ USING SLOW WORK ITEMS Firstly, a module or subsystem wanting to make use of slow work items must register its interest: - int ret = slow_work_register_user(); + int ret = slow_work_register_user(struct module *module); -This will return 0 if successful, or a -ve error upon failure. +This will return 0 if successful, or a -ve error upon failure. The module +pointer should be the module interested in using this facility (almost +certainly THIS_MODULE). Slow work items may then be set up by: @@ -93,6 +102,10 @@ Slow work items may then be set up by: or: + delayed_slow_work_init(&myitem, &myitem_ops); + + or: + vslow_work_init(&myitem, &myitem_ops); depending on its class. @@ -102,15 +115,92 @@ A suitably set up work item can then be enqueued for processing: int ret = slow_work_enqueue(&myitem); This will return a -ve error if the thread pool is unable to gain a reference -on the item, 0 otherwise. +on the item, 0 otherwise, or (for delayed work): + + int ret = delayed_slow_work_enqueue(&myitem, my_jiffy_delay); The items are reference counted, so there ought to be no need for a flush -operation. When all a module's slow work items have been processed, and the +operation. But as the reference counting is optional, means to cancel +existing work items are also included: + + cancel_slow_work(&myitem); + cancel_delayed_slow_work(&myitem); + +can be used to cancel pending work. The above cancel function waits for +existing work to have been executed (or prevent execution of them, depending +on timing). + + +When all a module's slow work items have been processed, and the module has no further interest in the facility, it should unregister its interest: - slow_work_unregister_user(); + slow_work_unregister_user(struct module *module); + +The module pointer is used to wait for all outstanding work items for that +module before completing the unregistration. This prevents the put_ref() code +from being taken away before it completes. module should almost certainly be +THIS_MODULE. + + +================ +HELPER FUNCTIONS +================ + +The slow-work facility provides a function by which it can be determined +whether or not an item is queued for later execution: + + bool queued = slow_work_is_queued(struct slow_work *work); + +If it returns false, then the item is not on the queue (it may be executing +with a requeue pending). This can be used to work out whether an item on which +another depends is on the queue, thus allowing a dependent item to be queued +after it. + +If the above shows an item on which another depends not to be queued, then the +owner of the dependent item might need to wait. However, to avoid locking up +the threads unnecessarily be sleeping in them, it can make sense under some +circumstances to return the work item to the queue, thus deferring it until +some other items have had a chance to make use of the yielded thread. + +To yield a thread and defer an item, the work function should simply enqueue +the work item again and return. However, this doesn't work if there's nothing +actually on the queue, as the thread just vacated will jump straight back into +the item's work function, thus busy waiting on a CPU. + +Instead, the item should use the thread to wait for the dependency to go away, +but rather than using schedule() or schedule_timeout() to sleep, it should use +the following function: + + bool requeue = slow_work_sleep_till_thread_needed( + struct slow_work *work, + signed long *_timeout); + +This will add a second wait and then sleep, such that it will be woken up if +either something appears on the queue that could usefully make use of the +thread - and behind which this item can be queued, or if the event the caller +set up to wait for happens. True will be returned if something else appeared +on the queue and this work function should perhaps return, of false if +something else woke it up. The timeout is as for schedule_timeout(). + +For example: + + wq = bit_waitqueue(&my_flags, MY_BIT); + init_wait(&wait); + requeue = false; + do { + prepare_to_wait(wq, &wait, TASK_UNINTERRUPTIBLE); + if (!test_bit(MY_BIT, &my_flags)) + break; + requeue = slow_work_sleep_till_thread_needed(&my_work, + &timeout); + } while (timeout > 0 && !requeue); + finish_wait(wq, &wait); + if (!test_bit(MY_BIT, &my_flags) + goto do_my_thing; + if (requeue) + return; // to slow_work =============== @@ -118,7 +208,8 @@ ITEM OPERATIONS =============== Each work item requires a table of operations of type struct slow_work_ops. -All members are required: +Only ->execute() is required; the getting and putting of a reference and the +describing of an item are all optional. (*) Get a reference on an item: @@ -148,6 +239,16 @@ All members are required: This should perform the work required of the item. It may sleep, it may perform disk I/O and it may wait for locks. + (*) View an item through /proc: + + void (*desc)(struct slow_work *work, struct seq_file *m); + + If supplied, this should print to 'm' a small string describing the work + the item is to do. This should be no more than about 40 characters, and + shouldn't include a newline character. + + See the 'Viewing executing and queued items' section below. + ================== POOL CONFIGURATION @@ -172,3 +273,50 @@ The slow-work thread pool has a number of configurables: is bounded to between 1 and one fewer than the number of active threads. This ensures there is always at least one thread that can process very slow work items, and always at least one thread that won't. + + +================================== +VIEWING EXECUTING AND QUEUED ITEMS +================================== + +If CONFIG_SLOW_WORK_PROC is enabled, a proc file is made available: + + /proc/slow_work_rq + +through which the list of work items being executed and the queues of items to +be executed may be viewed. The owner of a work item is given the chance to +add some information of its own. + +The contents look something like the following: + + THR PID ITEM ADDR FL MARK DESC + === ===== ================ == ===== ========== + 0 3005 ffff880023f52348 a 952ms FSC: OBJ17d3: LOOK + 1 3006 ffff880024e33668 2 160ms FSC: OBJ17e5 OP60d3b: Write1/Store fl=2 + 2 3165 ffff8800296dd180 a 424ms FSC: OBJ17e4: LOOK + 3 4089 ffff8800262c8d78 a 212ms FSC: OBJ17ea: CRTN + 4 4090 ffff88002792bed8 2 388ms FSC: OBJ17e8 OP60d36: Write1/Store fl=2 + 5 4092 ffff88002a0ef308 2 388ms FSC: OBJ17e7 OP60d2e: Write1/Store fl=2 + 6 4094 ffff88002abaf4b8 2 132ms FSC: OBJ17e2 OP60d4e: Write1/Store fl=2 + 7 4095 ffff88002bb188e0 a 388ms FSC: OBJ17e9: CRTN + vsq - ffff880023d99668 1 308ms FSC: OBJ17e0 OP60f91: Write1/EnQ fl=2 + vsq - ffff8800295d1740 1 212ms FSC: OBJ16be OP4d4b6: Write1/EnQ fl=2 + vsq - ffff880025ba3308 1 160ms FSC: OBJ179a OP58dec: Write1/EnQ fl=2 + vsq - ffff880024ec83e0 1 160ms FSC: OBJ17ae OP599f2: Write1/EnQ fl=2 + vsq - ffff880026618e00 1 160ms FSC: OBJ17e6 OP60d33: Write1/EnQ fl=2 + vsq - ffff880025a2a4b8 1 132ms FSC: OBJ16a2 OP4d583: Write1/EnQ fl=2 + vsq - ffff880023cbe6d8 9 212ms FSC: OBJ17eb: LOOK + vsq - ffff880024d37590 9 212ms FSC: OBJ17ec: LOOK + vsq - ffff880027746cb0 9 212ms FSC: OBJ17ed: LOOK + vsq - ffff880024d37ae8 9 212ms FSC: OBJ17ee: LOOK + vsq - ffff880024d37cb0 9 212ms FSC: OBJ17ef: LOOK + vsq - ffff880025036550 9 212ms FSC: OBJ17f0: LOOK + vsq - ffff8800250368e0 9 212ms FSC: OBJ17f1: LOOK + vsq - ffff880025036aa8 9 212ms FSC: OBJ17f2: LOOK + +In the 'THR' column, executing items show the thread they're occupying and +queued threads indicate which queue they're on. 'PID' shows the process ID of +a slow-work thread that's executing something. 'FL' shows the work item flags. +'MARK' indicates how long since an item was queued or began executing. Lastly, +the 'DESC' column permits the owner of an item to give some information. + |