/* Worker thread pool for slow items, such as filesystem lookups or mkdirs
*
* Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
* Written by David Howells (dhowells@redhat.com)
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public Licence
* as published by the Free Software Foundation; either version
* 2 of the Licence, or (at your option) any later version.
*/
#include <linux/module.h>
#include <linux/slow-work.h>
#include <linux/kthread.h>
#include <linux/freezer.h>
#include <linux/wait.h>
#include <asm/system.h>
/*
* The pool of threads has at least min threads in it as long as someone is
* using the facility, and may have as many as max.
*
* A portion of the pool may be processing very slow operations.
*/
static unsigned slow_work_min_threads = 2;
static unsigned slow_work_max_threads = 4;
static unsigned vslow_work_proportion = 50; /* % of threads that may process
* very slow work */
static atomic_t slow_work_thread_count;
static atomic_t vslow_work_executing_count;
/*
* The queues of work items and the lock governing access to them. These are
* shared between all the CPUs. It doesn't make sense to have per-CPU queues
* as the number of threads bears no relation to the number of CPUs.
*
* There are two queues of work items: one for slow work items, and one for
* very slow work items.
*/
static LIST_HEAD(slow_work_queue);
static LIST_HEAD(vslow_work_queue);
static DEFINE_SPINLOCK(slow_work_queue_lock);
/*
* The thread controls. A variable used to signal to the threads that they
* should exit when the queue is empty, a waitqueue used by the threads to wait
* for signals, and a completion set by the last thread to exit.
*/
static bool slow_work_threads_should_exit;
static DECLARE_WAIT_QUEUE_HEAD(slow_work_thread_wq);
static DECLARE_COMPLETION(slow_work_last_thread_exited);
/*
* The number of users of the thread pool and its lock. Whilst this is zero we
* have no threads hanging around, and when this reaches zero, we wait for all
* active or queued work items to complete and kill all the threads we do have.
*/
static int slow_work_user_count;
static DEFINE_MUTEX(slow_work_user_lock);
/*
* Calculate the maximum number of active threads in the pool that are
* permitted to process very slow work items.
*
* The answer is rounded up to at least 1, but may not equal or exceed the
* maximum number of the threads in the pool. This means we always have at
* least one thread that can process slow work items, and we always have at
* least one thread that won't get tied up doing so.
*/
static unsigned slow_work_calc_vsmax(void)
{
unsigned vsmax;
vsmax = atomic_read(&slow_work_thread_count) * vslow_work_proportion;
vsmax /= 100;
vsmax = max(vsmax, 1U);
return min(vsmax, slow_work_max_threads - 1);
}
/*
* Attempt to execute stuff queued on a slow thread. Return true if we managed
* it, false if there was nothing to do.
*/
static bool slow_work_execute(void)
{
struct slow_work *work = NULL;
unsigned vsmax;
bool very_slow;
vsmax = slow_work_calc_vsmax();
/* find something to execute */
spin_lock_irq(&slow_work_queue_lock);
if (!list_empty(&vslow_work_queue) &&
atomic_read(&vslow_work_executing_count) < vsmax) {
work = list_entry(vslow_work_queue.next,
struct slow_work, link);
if (test_and_set_bit_lock(SLOW_WORK_EXECUTING, &work->flags))
BUG();
list_del_init(&work->link);
atomic_inc(&vslow_work_executing_count);
very_slow = true;
} else if (!list_empty(&slow_work_queue)) {
work = list_entry(slow_work_queue.next,
struct slow_work, link);
if (test_and_set_bit_lock(SLOW_WORK_EXECUTING, &work->flags))
BUG();
list_del_init(&work->link);
very_slow = false;
} else {
very_slow = false; /* avoid the compiler warning */
}
spin_unlock_irq(&slow_work_queue_lock);
if (!work)
return false;
if (!test_and_clear_bit(SLOW_WORK_PENDING, &work->flags))
BUG();
work->ops->execute(work);
if (very_slow)
atomic_dec(&vslow_work_executing_count);
clear_bit_unlock(SLOW_WORK_EXECUTING, &work->flags);
/* if someone tried to enqueue the item whilst we were executing it,
* then it'll be left unenqueued to avoid multiple threads trying to
* execute it simultaneously
*
* there is, however, a race between us testing the pending flag and
* getting the spinlock, and between the enqueuer setting the pending
* flag and getting the spinlock, so we use a deferral bit to tell us
* if the enqueuer got there first
*/
if (test_bit(SLOW_WORK_PENDING, &work->flags)) {
spin_lock_irq(&slow_work_queue_lock);
if (!test_bit(SLOW_WORK_EXECUTING, &work->flags) &&
test_and_clear_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags))
goto auto_requeue;
spin_unlock_irq(&slow_work_queue_lock);
}
work->ops->put_ref(work);
return true;
auto_requeue:
/* we must complete the enqueue operation
* - we transfer our ref on the item back to the appropriate queue
* - don't wake another thread up as we're awake already
*/
if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags))
list_add_tail(&work->link, &vslow_work_queue);
else
list_add_tail(&work->link, &slow_work_queue);
spin_unlock_irq(&slow_work_queue_lock);
return true;
}
/**
* slow_work_enqueue - Schedule a slow work item for processing
* @work: The work item to queue
*
* Schedule a slow work item for processing. If the item is already undergoing
* execution, this guarantees not to re-enter the execution routine until the
* first execution finishes.
*
* The item is pinned by this function as it retains a reference to it, managed
* through the item operations. The item is unpinned once it has been
* executed.
*
* An item may hog the thread that is running it for a relatively large amount
* of time, sufficient, for example, to perform several lookup, mkdir, create
* and setxattr operations. It may sleep on I/O and may sleep to obtain locks.
*
* Conversely, if a number of items are awaiting processing, it may take some
* time before any given item is given attention. The number of threads in the
* pool may be increased to deal with demand, but only up to a limit.
*
* If SLOW_WORK_VERY_SLOW is set on the work item, then it will be placed in
* the very slow queue, from which only a portion of the threads will be
* allowed to pick items to execute. This ensures that very slow items won't
* overly block ones that are just ordinarily slow.
*
* Returns 0 if successful, -EAGAIN if not.
*/
int slow_work_enqueue(struct slow_work *work)
{
unsigned long flags;
BUG_ON(slow_work_user_count <= 0);
BUG_ON(!work);
BUG_ON(!work->ops);
BUG_ON(!work->ops->get_ref);
/* when honouring an enqueue request, we only promise that we will run
* the work function in the future; we do not promise to run it once
* per enqueue request
*
* we use the PENDING bit to merge together repeat requests without
* having to disable IRQs and take the spinlock, whilst still
* maintaining our promise
*/
if (!test_and_set_bit_lock(SLOW_WORK_PENDING, &work->flags)) {
spin_lock_irqsave(&slow_work_queue_lock, flags);
/* we promise that we will not attempt to execute the work
* function in more than one thread simultaneously
*
* this, however, leaves us with a problem if we're asked to
* enqueue the work whilst someone is executing the work
* function as simply queueing the work immediately means that
* another thread may try executing it whilst it is already
* under execution
*
* to deal with this, we set the ENQ_DEFERRED bit instead of
* enqueueing, and the thread currently executing the work
* function will enqueue the work item when the work function
* returns and it has cleared the EXECUTING bit
*/
if (test_bit(SLOW_WORK_EXECUTING, &work->flags)) {
set_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags);
} else {
if (work->ops->get_ref(work) < 0)
goto cant_get_ref;
if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags))
list_add_tail(&work->link, &vslow_work_queue);
else
list_add_tail(&work->link, &slow_work_queue);
wake_up(&slow_work_thread_wq);
}
spin_unlock_irqrestore(&slow_work_queue_lock, flags);
}
return 0;
cant_get_ref:
spin_unlock_irqrestore(&slow_work_queue_lock, flags);
return -EAGAIN;
}
EXPORT_SYMBOL(slow_work_enqueue);
/*
* Determine if there is slow work available for dispatch
*/
static inline bool slow_work_available(int vsmax)
{
return !list_empty(&slow_work_queue) ||
(!list_empty(&vslow_work_queue) &&
atomic_read(&vslow_work_executing_count) < vsmax);
}
/*
* Worker thread dispatcher
*/
static int slow_work_thread(void *_data)
{
int vsmax;
DEFINE_WAIT(wait);
set_freezable();
set_user_nice(current, -5);
for (;;) {
vsmax = vslow_work_proportion;
vsmax *= atomic_read(&slow_work_thread_count);
vsmax /= 100;
prepare_to_wait(&slow_work_thread_wq, &wait,
TASK_INTERRUPTIBLE);
if (!freezing(current) &&
!slow_work_threads_should_exit &&
!slow_work_available(vsmax))
schedule();
finish_wait(&slow_work_thread_wq, &wait);
try_to_freeze();
vsmax = vslow_work_proportion;
vsmax *= atomic_read(&slow_work_thread_count);
vsmax /= 100;
if (slow_work_available(vsmax) && slow_work_execute()) {
cond_resched();
continue;
}
if (slow_work_threads_should_exit)
break;
}
if (atomic_dec_and_test(&slow_work_thread_count))
complete_and_exit(&slow_work_last_thread_exited, 0);
return 0;
}
/**
* slow_work_register_user - Register a user of the facility
*
* Register a user of the facility, starting up the initial threads if there
* aren't any other users at this point. This will return 0 if successful, or
* an error if not.
*/
int slow_work_register_user(void)
{
struct task_struct *p;
int loop;
mutex_lock(&slow_work_user_lock);
if (slow_work_user_count == 0) {
printk(KERN_NOTICE "Slow work thread pool: Starting up\n");
init_completion(&slow_work_last_thread_exited);
slow_work_threads_should_exit = false;
/* start the minimum number of threads */
for (loop = 0; loop < slow_work_min_threads; loop++) {
atomic_inc(&slow_work_thread_count);
p = kthread_run(slow_work_thread, NULL, "kslowd");
if (IS_ERR(p))
goto error;
}
printk(KERN_NOTICE "Slow work thread pool: Ready\n");
}
slow_work_user_count++;
mutex_unlock(&slow_work_user_lock);
return 0;
error:
if (atomic_dec_and_test(&slow_work_thread_count))
complete(&slow_work_last_thread_exited);
if (loop > 0) {
printk(KERN_ERR "Slow work thread pool:"
" Aborting startup on ENOMEM\n");
slow_work_threads_should_exit = true;
wake_up_all(&slow_work_thread_wq);
wait_for_completion(&slow_work_last_thread_exited);
printk(KERN_ERR "Slow work thread pool: Aborted\n");
}
mutex_unlock(&slow_work_user_lock);
return PTR_ERR(p);
}
EXPORT_SYMBOL(slow_work_register_user);
/**
* slow_work_unregister_user - Unregister a user of the facility
*
* Unregister a user of the facility, killing all the threads if this was the
* last one.
*/
void slow_work_unregister_user(void)
{
mutex_lock(&slow_work_user_lock);
BUG_ON(slow_work_user_count <= 0);
slow_work_user_count--;
if (slow_work_user_count == 0) {
printk(KERN_NOTICE "Slow work thread pool: Shutting down\n");
slow_work_threads_should_exit = true;
wake_up_all(&slow_work_thread_wq);
wait_for_completion(&slow_work_last_thread_exited);
printk(KERN_NOTICE "Slow work thread pool:"
" Shut down complete\n");
}
mutex_unlock(&slow_work_user_lock);
}
EXPORT_SYMBOL(slow_work_unregister_user);
/*
* Initialise the slow work facility
*/
static int __init init_slow_work(void)
{
unsigned nr_cpus = num_possible_cpus();
if (nr_cpus > slow_work_max_threads)
slow_work_max_threads = nr_cpus;
return 0;
}
subsys_initcall(init_slow_work);
