323 lines
12 KiB
Plaintext
323 lines
12 KiB
Plaintext
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====================================
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SLOW WORK ITEM EXECUTION THREAD POOL
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====================================
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By: David Howells <dhowells@redhat.com>
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The slow work item execution thread pool is a pool of threads for performing
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things that take a relatively long time, such as making mkdir calls.
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Typically, when processing something, these items will spend a lot of time
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blocking a thread on I/O, thus making that thread unavailable for doing other
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work.
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The standard workqueue model is unsuitable for this class of work item as that
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limits the owner to a single thread or a single thread per CPU. For some
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tasks, however, more threads - or fewer - are required.
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There is just one pool per system. It contains no threads unless something
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wants to use it - and that something must register its interest first. When
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the pool is active, the number of threads it contains is dynamic, varying
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between a maximum and minimum setting, depending on the load.
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====================
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CLASSES OF WORK ITEM
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====================
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This pool support two classes of work items:
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(*) Slow work items.
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(*) Very slow work items.
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The former are expected to finish much quicker than the latter.
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An operation of the very slow class may do a batch combination of several
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lookups, mkdirs, and a create for instance.
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An operation of the ordinarily slow class may, for example, write stuff or
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expand files, provided the time taken to do so isn't too long.
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Operations of both types may sleep during execution, thus tying up the thread
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loaned to it.
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A further class of work item is available, based on the slow work item class:
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(*) Delayed slow work items.
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These are slow work items that have a timer to defer queueing of the item for
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a while.
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THREAD-TO-CLASS ALLOCATION
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--------------------------
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Not all the threads in the pool are available to work on very slow work items.
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The number will be between one and one fewer than the number of active threads.
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This is configurable (see the "Pool Configuration" section).
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All the threads are available to work on ordinarily slow work items, but a
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percentage of the threads will prefer to work on very slow work items.
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The configuration ensures that at least one thread will be available to work on
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very slow work items, and at least one thread will be available that won't work
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on very slow work items at all.
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=====================
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USING SLOW WORK ITEMS
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=====================
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Firstly, a module or subsystem wanting to make use of slow work items must
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register its interest:
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int ret = slow_work_register_user(struct module *module);
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This will return 0 if successful, or a -ve error upon failure. The module
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pointer should be the module interested in using this facility (almost
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certainly THIS_MODULE).
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Slow work items may then be set up by:
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(1) Declaring a slow_work struct type variable:
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#include <linux/slow-work.h>
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struct slow_work myitem;
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(2) Declaring the operations to be used for this item:
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struct slow_work_ops myitem_ops = {
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.get_ref = myitem_get_ref,
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.put_ref = myitem_put_ref,
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.execute = myitem_execute,
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};
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[*] For a description of the ops, see section "Item Operations".
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(3) Initialising the item:
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slow_work_init(&myitem, &myitem_ops);
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or:
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delayed_slow_work_init(&myitem, &myitem_ops);
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or:
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vslow_work_init(&myitem, &myitem_ops);
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depending on its class.
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A suitably set up work item can then be enqueued for processing:
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int ret = slow_work_enqueue(&myitem);
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This will return a -ve error if the thread pool is unable to gain a reference
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on the item, 0 otherwise, or (for delayed work):
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int ret = delayed_slow_work_enqueue(&myitem, my_jiffy_delay);
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The items are reference counted, so there ought to be no need for a flush
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operation. But as the reference counting is optional, means to cancel
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existing work items are also included:
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cancel_slow_work(&myitem);
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cancel_delayed_slow_work(&myitem);
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can be used to cancel pending work. The above cancel function waits for
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existing work to have been executed (or prevent execution of them, depending
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on timing).
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When all a module's slow work items have been processed, and the
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module has no further interest in the facility, it should unregister its
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interest:
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slow_work_unregister_user(struct module *module);
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The module pointer is used to wait for all outstanding work items for that
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module before completing the unregistration. This prevents the put_ref() code
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from being taken away before it completes. module should almost certainly be
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THIS_MODULE.
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================
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HELPER FUNCTIONS
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================
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The slow-work facility provides a function by which it can be determined
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whether or not an item is queued for later execution:
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bool queued = slow_work_is_queued(struct slow_work *work);
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If it returns false, then the item is not on the queue (it may be executing
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with a requeue pending). This can be used to work out whether an item on which
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another depends is on the queue, thus allowing a dependent item to be queued
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after it.
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If the above shows an item on which another depends not to be queued, then the
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owner of the dependent item might need to wait. However, to avoid locking up
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the threads unnecessarily be sleeping in them, it can make sense under some
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circumstances to return the work item to the queue, thus deferring it until
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some other items have had a chance to make use of the yielded thread.
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To yield a thread and defer an item, the work function should simply enqueue
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the work item again and return. However, this doesn't work if there's nothing
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actually on the queue, as the thread just vacated will jump straight back into
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the item's work function, thus busy waiting on a CPU.
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Instead, the item should use the thread to wait for the dependency to go away,
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but rather than using schedule() or schedule_timeout() to sleep, it should use
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the following function:
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bool requeue = slow_work_sleep_till_thread_needed(
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struct slow_work *work,
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signed long *_timeout);
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This will add a second wait and then sleep, such that it will be woken up if
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either something appears on the queue that could usefully make use of the
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thread - and behind which this item can be queued, or if the event the caller
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set up to wait for happens. True will be returned if something else appeared
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on the queue and this work function should perhaps return, of false if
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something else woke it up. The timeout is as for schedule_timeout().
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For example:
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wq = bit_waitqueue(&my_flags, MY_BIT);
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init_wait(&wait);
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requeue = false;
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do {
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prepare_to_wait(wq, &wait, TASK_UNINTERRUPTIBLE);
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if (!test_bit(MY_BIT, &my_flags))
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break;
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requeue = slow_work_sleep_till_thread_needed(&my_work,
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&timeout);
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} while (timeout > 0 && !requeue);
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finish_wait(wq, &wait);
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if (!test_bit(MY_BIT, &my_flags)
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goto do_my_thing;
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if (requeue)
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return; // to slow_work
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===============
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ITEM OPERATIONS
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===============
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Each work item requires a table of operations of type struct slow_work_ops.
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Only ->execute() is required; the getting and putting of a reference and the
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describing of an item are all optional.
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(*) Get a reference on an item:
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int (*get_ref)(struct slow_work *work);
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This allows the thread pool to attempt to pin an item by getting a
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reference on it. This function should return 0 if the reference was
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granted, or a -ve error otherwise. If an error is returned,
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slow_work_enqueue() will fail.
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The reference is held whilst the item is queued and whilst it is being
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executed. The item may then be requeued with the same reference held, or
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the reference will be released.
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(*) Release a reference on an item:
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void (*put_ref)(struct slow_work *work);
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This allows the thread pool to unpin an item by releasing the reference on
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it. The thread pool will not touch the item again once this has been
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called.
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(*) Execute an item:
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void (*execute)(struct slow_work *work);
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This should perform the work required of the item. It may sleep, it may
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perform disk I/O and it may wait for locks.
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(*) View an item through /proc:
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void (*desc)(struct slow_work *work, struct seq_file *m);
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If supplied, this should print to 'm' a small string describing the work
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the item is to do. This should be no more than about 40 characters, and
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shouldn't include a newline character.
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See the 'Viewing executing and queued items' section below.
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==================
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POOL CONFIGURATION
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==================
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The slow-work thread pool has a number of configurables:
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(*) /proc/sys/kernel/slow-work/min-threads
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The minimum number of threads that should be in the pool whilst it is in
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use. This may be anywhere between 2 and max-threads.
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(*) /proc/sys/kernel/slow-work/max-threads
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The maximum number of threads that should in the pool. This may be
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anywhere between min-threads and 255 or NR_CPUS * 2, whichever is greater.
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(*) /proc/sys/kernel/slow-work/vslow-percentage
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The percentage of active threads in the pool that may be used to execute
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very slow work items. This may be between 1 and 99. The resultant number
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is bounded to between 1 and one fewer than the number of active threads.
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This ensures there is always at least one thread that can process very
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slow work items, and always at least one thread that won't.
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==================================
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VIEWING EXECUTING AND QUEUED ITEMS
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==================================
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If CONFIG_SLOW_WORK_DEBUG is enabled, a debugfs file is made available:
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/sys/kernel/debug/slow_work/runqueue
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through which the list of work items being executed and the queues of items to
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be executed may be viewed. The owner of a work item is given the chance to
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add some information of its own.
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The contents look something like the following:
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THR PID ITEM ADDR FL MARK DESC
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=== ===== ================ == ===== ==========
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0 3005 ffff880023f52348 a 952ms FSC: OBJ17d3: LOOK
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1 3006 ffff880024e33668 2 160ms FSC: OBJ17e5 OP60d3b: Write1/Store fl=2
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2 3165 ffff8800296dd180 a 424ms FSC: OBJ17e4: LOOK
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3 4089 ffff8800262c8d78 a 212ms FSC: OBJ17ea: CRTN
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4 4090 ffff88002792bed8 2 388ms FSC: OBJ17e8 OP60d36: Write1/Store fl=2
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5 4092 ffff88002a0ef308 2 388ms FSC: OBJ17e7 OP60d2e: Write1/Store fl=2
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6 4094 ffff88002abaf4b8 2 132ms FSC: OBJ17e2 OP60d4e: Write1/Store fl=2
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7 4095 ffff88002bb188e0 a 388ms FSC: OBJ17e9: CRTN
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vsq - ffff880023d99668 1 308ms FSC: OBJ17e0 OP60f91: Write1/EnQ fl=2
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vsq - ffff8800295d1740 1 212ms FSC: OBJ16be OP4d4b6: Write1/EnQ fl=2
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vsq - ffff880025ba3308 1 160ms FSC: OBJ179a OP58dec: Write1/EnQ fl=2
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vsq - ffff880024ec83e0 1 160ms FSC: OBJ17ae OP599f2: Write1/EnQ fl=2
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vsq - ffff880026618e00 1 160ms FSC: OBJ17e6 OP60d33: Write1/EnQ fl=2
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vsq - ffff880025a2a4b8 1 132ms FSC: OBJ16a2 OP4d583: Write1/EnQ fl=2
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vsq - ffff880023cbe6d8 9 212ms FSC: OBJ17eb: LOOK
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vsq - ffff880024d37590 9 212ms FSC: OBJ17ec: LOOK
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vsq - ffff880027746cb0 9 212ms FSC: OBJ17ed: LOOK
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vsq - ffff880024d37ae8 9 212ms FSC: OBJ17ee: LOOK
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vsq - ffff880024d37cb0 9 212ms FSC: OBJ17ef: LOOK
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vsq - ffff880025036550 9 212ms FSC: OBJ17f0: LOOK
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vsq - ffff8800250368e0 9 212ms FSC: OBJ17f1: LOOK
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vsq - ffff880025036aa8 9 212ms FSC: OBJ17f2: LOOK
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In the 'THR' column, executing items show the thread they're occupying and
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queued threads indicate which queue they're on. 'PID' shows the process ID of
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a slow-work thread that's executing something. 'FL' shows the work item flags.
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'MARK' indicates how long since an item was queued or began executing. Lastly,
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the 'DESC' column permits the owner of an item to give some information.
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