xref: /src/sys/kern/kern_threadpool.c

/*	$NetBSD: kern_threadpool.c,v 1.13 2018/12/28 00:15:57 thorpej Exp $	*/

/*-
 * Copyright (c) 2014, 2018 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Taylor R. Campbell and Jason R. Thorpe.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * Thread pools.
 *
 * A thread pool is a collection of worker threads idle or running
 * jobs, together with an overseer thread that does not run jobs but
 * can be given jobs to assign to a worker thread.  Scheduling a job in
 * a thread pool does not allocate or even sleep at all, except perhaps
 * on an adaptive lock, unlike kthread_create.  Jobs reuse threads, so
 * they do not incur the expense of creating and destroying kthreads
 * unless there is not much work to be done.
 *
 * A per-CPU thread pool (threadpool_percpu) is a collection of thread
 * pools, one per CPU bound to that CPU.  For each priority level in
 * use, there is one shared unbound thread pool (i.e., pool of threads
 * not bound to any CPU) and one shared per-CPU thread pool.
 *
 * To use the unbound thread pool at priority pri, call
 * threadpool_get(&pool, pri).  When you're done, call
 * threadpool_put(pool, pri).
 *
 * To use the per-CPU thread pools at priority pri, call
 * threadpool_percpu_get(&pool_percpu, pri), and then use the thread
 * pool returned by threadpool_percpu_ref(pool_percpu) for the current
 * CPU, or by threadpool_percpu_ref_remote(pool_percpu, ci) for another
 * CPU.  When you're done, call threadpool_percpu_put(pool_percpu,
 * pri).
 *
 * +--MACHINE-----------------------------------------------+
 * | +--CPU 0-------+ +--CPU 1-------+     +--CPU n-------+ |
 * | | <overseer 0> | | <overseer 1> | ... | <overseer n> | |
 * | | <idle 0a>    | | <running 1a> | ... | <idle na>    | |
 * | | <running 0b> | | <running 1b> | ... | <idle nb>    | |
 * | | .            | | .            | ... | .            | |
 * | | .            | | .            | ... | .            | |
 * | | .            | | .            | ... | .            | |
 * | +--------------+ +--------------+     +--------------+ |
 * |            +--unbound---------+                        |
 * |            | <overseer n+1>   |                        |
 * |            | <idle (n+1)a>    |                        |
 * |            | <running (n+1)b> |                        |
 * |            +------------------+                        |
 * +--------------------------------------------------------+
 *
 * XXX Why one overseer per CPU?  I did that originally to avoid
 * touching remote CPUs' memory when scheduling a job, but that still
 * requires interprocessor synchronization.  Perhaps we could get by
 * with a single overseer thread, at the expense of another pointer in
 * struct threadpool_job to identify the CPU on which it must run
 * in order for the overseer to schedule it correctly.
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: kern_threadpool.c,v 1.13 2018/12/28 00:15:57 thorpej Exp $");

#include <sys/types.h>
#include <sys/param.h>
#include <sys/atomic.h>
#include <sys/condvar.h>
#include <sys/cpu.h>
#include <sys/kernel.h>
#include <sys/kmem.h>
#include <sys/kthread.h>
#include <sys/mutex.h>
#include <sys/once.h>
#include <sys/percpu.h>
#include <sys/pool.h>
#include <sys/proc.h>
#include <sys/queue.h>
#include <sys/systm.h>
#include <sys/threadpool.h>

/* Data structures */

TAILQ_HEAD(job_head, threadpool_job);
TAILQ_HEAD(thread_head, threadpool_thread);

struct threadpool_thread {
	struct lwp			*tpt_lwp;
	char				*tpt_lwp_savedname;
	struct threadpool		*tpt_pool;
	struct threadpool_job		*tpt_job;
	kcondvar_t			tpt_cv;
	TAILQ_ENTRY(threadpool_thread)	tpt_entry;
};

struct threadpool {
	kmutex_t			tp_lock;
	struct threadpool_thread	tp_overseer;
	struct job_head			tp_jobs;
	struct thread_head		tp_idle_threads;
	uint64_t			tp_refcnt;
	int				tp_flags;
#define	THREADPOOL_DYING	0x01
	struct cpu_info			*tp_cpu;
	pri_t				tp_pri;
};

static void	threadpool_hold(struct threadpool *);
static void	threadpool_rele(struct threadpool *);

static int	threadpool_percpu_create(struct threadpool_percpu **, pri_t);
static void	threadpool_percpu_destroy(struct threadpool_percpu *);

static threadpool_job_fn_t threadpool_job_dead;

static void	threadpool_job_hold(struct threadpool_job *);
static void	threadpool_job_rele(struct threadpool_job *);

static void	threadpool_overseer_thread(void *) __dead;
static void	threadpool_thread(void *) __dead;

static pool_cache_t	threadpool_thread_pc __read_mostly;

static kmutex_t		threadpools_lock __cacheline_aligned;

	/* Idle out threads after 30 seconds */
#define	THREADPOOL_IDLE_TICKS	mstohz(30 * 1000)

struct threadpool_unbound {
	struct threadpool		tpu_pool;

	/* protected by threadpools_lock */
	LIST_ENTRY(threadpool_unbound)	tpu_link;
	uint64_t			tpu_refcnt;
};

static LIST_HEAD(, threadpool_unbound) unbound_threadpools;

static struct threadpool_unbound *
threadpool_lookup_unbound(pri_t pri)
{
	struct threadpool_unbound *tpu;

	LIST_FOREACH(tpu, &unbound_threadpools, tpu_link) {
		if (tpu->tpu_pool.tp_pri == pri)
			return tpu;
	}
	return NULL;
}

static void
threadpool_insert_unbound(struct threadpool_unbound *tpu)
{
	KASSERT(threadpool_lookup_unbound(tpu->tpu_pool.tp_pri) == NULL);
	LIST_INSERT_HEAD(&unbound_threadpools, tpu, tpu_link);
}

static void
threadpool_remove_unbound(struct threadpool_unbound *tpu)
{
	KASSERT(threadpool_lookup_unbound(tpu->tpu_pool.tp_pri) == tpu);
	LIST_REMOVE(tpu, tpu_link);
}

struct threadpool_percpu {
	percpu_t *			tpp_percpu;
	pri_t				tpp_pri;

	/* protected by threadpools_lock */
	LIST_ENTRY(threadpool_percpu)	tpp_link;
	uint64_t			tpp_refcnt;
};

static LIST_HEAD(, threadpool_percpu) percpu_threadpools;

static struct threadpool_percpu *
threadpool_lookup_percpu(pri_t pri)
{
	struct threadpool_percpu *tpp;

	LIST_FOREACH(tpp, &percpu_threadpools, tpp_link) {
		if (tpp->tpp_pri == pri)
			return tpp;
	}
	return NULL;
}

static void
threadpool_insert_percpu(struct threadpool_percpu *tpp)
{
	KASSERT(threadpool_lookup_percpu(tpp->tpp_pri) == NULL);
	LIST_INSERT_HEAD(&percpu_threadpools, tpp, tpp_link);
}

static void
threadpool_remove_percpu(struct threadpool_percpu *tpp)
{
	KASSERT(threadpool_lookup_percpu(tpp->tpp_pri) == tpp);
	LIST_REMOVE(tpp, tpp_link);
}

#ifdef THREADPOOL_VERBOSE
#define	TP_LOG(x)		printf x
#else
#define	TP_LOG(x)		/* nothing */
#endif /* THREADPOOL_VERBOSE */

void
threadpools_init(void)
{

	threadpool_thread_pc =
	    pool_cache_init(sizeof(struct threadpool_thread), 0, 0, 0,
		"thplthrd", NULL, IPL_NONE, NULL, NULL, NULL);

	LIST_INIT(&unbound_threadpools);
	LIST_INIT(&percpu_threadpools);
	mutex_init(&threadpools_lock, MUTEX_DEFAULT, IPL_NONE);
}

/* Thread pool creation */

static bool
threadpool_pri_is_valid(pri_t pri)
{
	return (pri == PRI_NONE || (pri >= PRI_USER && pri < PRI_COUNT));
}

static int
threadpool_create(struct threadpool *const pool, struct cpu_info *ci,
    pri_t pri)
{
	struct lwp *lwp;
	int ktflags;
	int error;

	KASSERT(threadpool_pri_is_valid(pri));

	mutex_init(&pool->tp_lock, MUTEX_DEFAULT, IPL_VM);
	/* XXX overseer */
	TAILQ_INIT(&pool->tp_jobs);
	TAILQ_INIT(&pool->tp_idle_threads);
	pool->tp_refcnt = 1;		/* overseer's reference */
	pool->tp_flags = 0;
	pool->tp_cpu = ci;
	pool->tp_pri = pri;

	pool->tp_overseer.tpt_lwp = NULL;
	pool->tp_overseer.tpt_pool = pool;
	pool->tp_overseer.tpt_job = NULL;
	cv_init(&pool->tp_overseer.tpt_cv, "poolover");

	ktflags = 0;
	ktflags |= KTHREAD_MPSAFE;
	if (pri < PRI_KERNEL)
		ktflags |= KTHREAD_TS;
	error = kthread_create(pri, ktflags, ci, &threadpool_overseer_thread,
	    &pool->tp_overseer, &lwp,
	    "pooloverseer/%d@%d", (ci ? cpu_index(ci) : -1), (int)pri);
	if (error)
		goto fail0;

	mutex_spin_enter(&pool->tp_lock);
	pool->tp_overseer.tpt_lwp = lwp;
	cv_broadcast(&pool->tp_overseer.tpt_cv);
	mutex_spin_exit(&pool->tp_lock);

	return 0;

fail0:	KASSERT(error);
	KASSERT(pool->tp_overseer.tpt_job == NULL);
	KASSERT(pool->tp_overseer.tpt_pool == pool);
	KASSERT(pool->tp_flags == 0);
	KASSERT(pool->tp_refcnt == 0);
	KASSERT(TAILQ_EMPTY(&pool->tp_idle_threads));
	KASSERT(TAILQ_EMPTY(&pool->tp_jobs));
	KASSERT(!cv_has_waiters(&pool->tp_overseer.tpt_cv));
	cv_destroy(&pool->tp_overseer.tpt_cv);
	mutex_destroy(&pool->tp_lock);
	return error;
}

/* Thread pool destruction */

static void
threadpool_destroy(struct threadpool *pool)
{
	struct threadpool_thread *thread;

	/* Mark the pool dying and wait for threads to commit suicide.  */
	mutex_spin_enter(&pool->tp_lock);
	KASSERT(TAILQ_EMPTY(&pool->tp_jobs));
	pool->tp_flags |= THREADPOOL_DYING;
	cv_broadcast(&pool->tp_overseer.tpt_cv);
	TAILQ_FOREACH(thread, &pool->tp_idle_threads, tpt_entry)
		cv_broadcast(&thread->tpt_cv);
	while (0 < pool->tp_refcnt) {
		TP_LOG(("%s: draining %u references...\n", __func__,
		    pool->tp_refcnt));
		cv_wait(&pool->tp_overseer.tpt_cv, &pool->tp_lock);
	}
	mutex_spin_exit(&pool->tp_lock);

	KASSERT(pool->tp_overseer.tpt_job == NULL);
	KASSERT(pool->tp_overseer.tpt_pool == pool);
	KASSERT(pool->tp_flags == THREADPOOL_DYING);
	KASSERT(pool->tp_refcnt == 0);
	KASSERT(TAILQ_EMPTY(&pool->tp_idle_threads));
	KASSERT(TAILQ_EMPTY(&pool->tp_jobs));
	KASSERT(!cv_has_waiters(&pool->tp_overseer.tpt_cv));
	cv_destroy(&pool->tp_overseer.tpt_cv);
	mutex_destroy(&pool->tp_lock);
}

static void
threadpool_hold(struct threadpool *pool)
{

	KASSERT(mutex_owned(&pool->tp_lock));
	pool->tp_refcnt++;
	KASSERT(pool->tp_refcnt != 0);
}

static void
threadpool_rele(struct threadpool *pool)
{

	KASSERT(mutex_owned(&pool->tp_lock));
	KASSERT(0 < pool->tp_refcnt);
	if (--pool->tp_refcnt == 0)
		cv_broadcast(&pool->tp_overseer.tpt_cv);
}

/* Unbound thread pools */

int
threadpool_get(struct threadpool **poolp, pri_t pri)
{
	struct threadpool_unbound *tpu, *tmp = NULL;
	int error;

	ASSERT_SLEEPABLE();

	if (! threadpool_pri_is_valid(pri))
		return EINVAL;

	mutex_enter(&threadpools_lock);
	tpu = threadpool_lookup_unbound(pri);
	if (tpu == NULL) {
		mutex_exit(&threadpools_lock);
		TP_LOG(("%s: No pool for pri=%d, creating one.\n",
		    __func__, (int)pri));
		tmp = kmem_zalloc(sizeof(*tmp), KM_SLEEP);
		error = threadpool_create(&tmp->tpu_pool, NULL, pri);
		if (error) {
			kmem_free(tmp, sizeof(*tmp));
			return error;
		}
		mutex_enter(&threadpools_lock);
		tpu = threadpool_lookup_unbound(pri);
		if (tpu == NULL) {
			TP_LOG(("%s: Won the creation race for pri=%d.\n",
			    __func__, (int)pri));
			tpu = tmp;
			tmp = NULL;
			threadpool_insert_unbound(tpu);
		}
	}
	KASSERT(tpu != NULL);
	tpu->tpu_refcnt++;
	KASSERT(tpu->tpu_refcnt != 0);
	mutex_exit(&threadpools_lock);

	if (tmp != NULL) {
		threadpool_destroy(&tmp->tpu_pool);
		kmem_free(tmp, sizeof(*tmp));
	}
	KASSERT(tpu != NULL);
	*poolp = &tpu->tpu_pool;
	return 0;
}

void
threadpool_put(struct threadpool *pool, pri_t pri)
{
	struct threadpool_unbound *tpu =
	    container_of(pool, struct threadpool_unbound, tpu_pool);

	ASSERT_SLEEPABLE();

	KASSERT(threadpool_pri_is_valid(pri));

	mutex_enter(&threadpools_lock);
	KASSERT(tpu == threadpool_lookup_unbound(pri));
	KASSERT(0 < tpu->tpu_refcnt);
	if (--tpu->tpu_refcnt == 0) {
		TP_LOG(("%s: Last reference for pri=%d, destroying pool.\n",
		    __func__, (int)pri));
		threadpool_remove_unbound(tpu);
	} else {
		tpu = NULL;
	}
	mutex_exit(&threadpools_lock);

	if (tpu) {
		threadpool_destroy(&tpu->tpu_pool);
		kmem_free(tpu, sizeof(*tpu));
	}
}

/* Per-CPU thread pools */

int
threadpool_percpu_get(struct threadpool_percpu **pool_percpup, pri_t pri)
{
	struct threadpool_percpu *pool_percpu, *tmp = NULL;
	int error;

	ASSERT_SLEEPABLE();

	if (! threadpool_pri_is_valid(pri))
		return EINVAL;

	mutex_enter(&threadpools_lock);
	pool_percpu = threadpool_lookup_percpu(pri);
	if (pool_percpu == NULL) {
		mutex_exit(&threadpools_lock);
		TP_LOG(("%s: No pool for pri=%d, creating one.\n",
		    __func__, (int)pri));
		error = threadpool_percpu_create(&tmp, pri);
		if (error)
			return error;
		KASSERT(tmp != NULL);
		mutex_enter(&threadpools_lock);
		pool_percpu = threadpool_lookup_percpu(pri);
		if (pool_percpu == NULL) {
			TP_LOG(("%s: Won the creation race for pri=%d.\n",
			    __func__, (int)pri));
			pool_percpu = tmp;
			tmp = NULL;
			threadpool_insert_percpu(pool_percpu);
		}
	}
	KASSERT(pool_percpu != NULL);
	pool_percpu->tpp_refcnt++;
	KASSERT(pool_percpu->tpp_refcnt != 0);
	mutex_exit(&threadpools_lock);

	if (tmp != NULL)
		threadpool_percpu_destroy(tmp);
	KASSERT(pool_percpu != NULL);
	*pool_percpup = pool_percpu;
	return 0;
}

void
threadpool_percpu_put(struct threadpool_percpu *pool_percpu, pri_t pri)
{

	ASSERT_SLEEPABLE();

	KASSERT(threadpool_pri_is_valid(pri));

	mutex_enter(&threadpools_lock);
	KASSERT(pool_percpu == threadpool_lookup_percpu(pri));
	KASSERT(0 < pool_percpu->tpp_refcnt);
	if (--pool_percpu->tpp_refcnt == 0) {
		TP_LOG(("%s: Last reference for pri=%d, destroying pool.\n",
		    __func__, (int)pri));
		threadpool_remove_percpu(pool_percpu);
	} else {
		pool_percpu = NULL;
	}
	mutex_exit(&threadpools_lock);

	if (pool_percpu)
		threadpool_percpu_destroy(pool_percpu);
}

struct threadpool *
threadpool_percpu_ref(struct threadpool_percpu *pool_percpu)
{
	struct threadpool **poolp, *pool;

	poolp = percpu_getref(pool_percpu->tpp_percpu);
	pool = *poolp;
	percpu_putref(pool_percpu->tpp_percpu);

	return pool;
}

struct threadpool *
threadpool_percpu_ref_remote(struct threadpool_percpu *pool_percpu,
    struct cpu_info *ci)
{
	struct threadpool **poolp, *pool;

	percpu_traverse_enter();
	poolp = percpu_getptr_remote(pool_percpu->tpp_percpu, ci);
	pool = *poolp;
	percpu_traverse_exit();

	return pool;
}

static int
threadpool_percpu_create(struct threadpool_percpu **pool_percpup, pri_t pri)
{
	struct threadpool_percpu *pool_percpu;
	struct cpu_info *ci;
	CPU_INFO_ITERATOR cii;
	unsigned int i, j;
	int error;

	pool_percpu = kmem_zalloc(sizeof(*pool_percpu), KM_SLEEP);
	if (pool_percpu == NULL) {
		error = ENOMEM;
		goto fail0;
	}
	pool_percpu->tpp_pri = pri;

	pool_percpu->tpp_percpu = percpu_alloc(sizeof(struct threadpool *));
	if (pool_percpu->tpp_percpu == NULL) {
		error = ENOMEM;
		goto fail1;
	}

	for (i = 0, CPU_INFO_FOREACH(cii, ci), i++) {
		struct threadpool *pool;

		pool = kmem_zalloc(sizeof(*pool), KM_SLEEP);
		error = threadpool_create(pool, ci, pri);
		if (error) {
			kmem_free(pool, sizeof(*pool));
			goto fail2;
		}
		percpu_traverse_enter();
		struct threadpool **const poolp =
		    percpu_getptr_remote(pool_percpu->tpp_percpu, ci);
		*poolp = pool;
		percpu_traverse_exit();
	}

	/* Success!  */
	*pool_percpup = (struct threadpool_percpu *)pool_percpu;
	return 0;

fail2:	for (j = 0, CPU_INFO_FOREACH(cii, ci), j++) {
		if (i <= j)
			break;
		percpu_traverse_enter();
		struct threadpool **const poolp =
		    percpu_getptr_remote(pool_percpu->tpp_percpu, ci);
		struct threadpool *const pool = *poolp;
		percpu_traverse_exit();
		threadpool_destroy(pool);
		kmem_free(pool, sizeof(*pool));
	}
	percpu_free(pool_percpu->tpp_percpu, sizeof(struct taskthread_pool *));
fail1:	kmem_free(pool_percpu, sizeof(*pool_percpu));
fail0:	return error;
}

static void
threadpool_percpu_destroy(struct threadpool_percpu *pool_percpu)
{
	struct cpu_info *ci;
	CPU_INFO_ITERATOR cii;

	for (CPU_INFO_FOREACH(cii, ci)) {
		percpu_traverse_enter();
		struct threadpool **const poolp =
		    percpu_getptr_remote(pool_percpu->tpp_percpu, ci);
		struct threadpool *const pool = *poolp;
		percpu_traverse_exit();
		threadpool_destroy(pool);
		kmem_free(pool, sizeof(*pool));
	}

	percpu_free(pool_percpu->tpp_percpu, sizeof(struct threadpool *));
	kmem_free(pool_percpu, sizeof(*pool_percpu));
}

/* Thread pool jobs */

void __printflike(4,5)
threadpool_job_init(struct threadpool_job *job, threadpool_job_fn_t fn,
    kmutex_t *lock, const char *fmt, ...)
{
	va_list ap;

	va_start(ap, fmt);
	(void)vsnprintf(job->job_name, sizeof(job->job_name), fmt, ap);
	va_end(ap);

	job->job_lock = lock;
	job->job_thread = NULL;
	job->job_refcnt = 0;
	cv_init(&job->job_cv, job->job_name);
	job->job_fn = fn;
}

static void
threadpool_job_dead(struct threadpool_job *job)
{

	panic("threadpool job %p ran after destruction", job);
}

void
threadpool_job_destroy(struct threadpool_job *job)
{

	ASSERT_SLEEPABLE();

	KASSERTMSG((job->job_thread == NULL), "job %p still running", job);

	mutex_enter(job->job_lock);
	while (0 < job->job_refcnt)
		cv_wait(&job->job_cv, job->job_lock);
	mutex_exit(job->job_lock);

	job->job_lock = NULL;
	KASSERT(job->job_thread == NULL);
	KASSERT(job->job_refcnt == 0);
	KASSERT(!cv_has_waiters(&job->job_cv));
	cv_destroy(&job->job_cv);
	job->job_fn = threadpool_job_dead;
	(void)strlcpy(job->job_name, "deadjob", sizeof(job->job_name));
}

static void
threadpool_job_hold(struct threadpool_job *job)
{
	unsigned int refcnt;

	do {
		refcnt = job->job_refcnt;
		KASSERT(refcnt != UINT_MAX);
	} while (atomic_cas_uint(&job->job_refcnt, refcnt, (refcnt + 1))
	    != refcnt);
}

static void
threadpool_job_rele(struct threadpool_job *job)
{
	unsigned int refcnt;

	KASSERT(mutex_owned(job->job_lock));

	do {
		refcnt = job->job_refcnt;
		KASSERT(0 < refcnt);
		if (refcnt == 1) {
			refcnt = atomic_dec_uint_nv(&job->job_refcnt);
			KASSERT(refcnt != UINT_MAX);
			if (refcnt == 0)
				cv_broadcast(&job->job_cv);
			return;
		}
	} while (atomic_cas_uint(&job->job_refcnt, refcnt, (refcnt - 1))
	    != refcnt);
}

void
threadpool_job_done(struct threadpool_job *job)
{

	KASSERT(mutex_owned(job->job_lock));
	KASSERT(job->job_thread != NULL);
	KASSERT(job->job_thread->tpt_lwp == curlwp);

	/*
	 * We can safely read this field; it's only modified right before
	 * we call the job work function, and we are only preserving it
	 * to use here; no one cares if it contains junk afterward.
	 */
	lwp_lock(curlwp);
	curlwp->l_name = job->job_thread->tpt_lwp_savedname;
	lwp_unlock(curlwp);

	/*
	 * Inline the work of threadpool_job_rele(); the job is already
	 * locked, the most likely scenario (XXXJRT only scenario?) is
	 * that we're dropping the last reference (the one taken in
	 * threadpool_schedule_job()), and we always do the cv_broadcast()
	 * anyway.
	 */
	KASSERT(0 < job->job_refcnt);
	unsigned int refcnt __diagused = atomic_dec_uint_nv(&job->job_refcnt);
	KASSERT(refcnt != UINT_MAX);
	cv_broadcast(&job->job_cv);
	job->job_thread = NULL;
}

void
threadpool_schedule_job(struct threadpool *pool, struct threadpool_job *job)
{

	KASSERT(mutex_owned(job->job_lock));

	/*
	 * If the job's already running, let it keep running.  The job
	 * is guaranteed by the interlock not to end early -- if it had
	 * ended early, threadpool_job_done would have set job_thread
	 * to NULL under the interlock.
	 */
	if (__predict_true(job->job_thread != NULL)) {
		TP_LOG(("%s: job '%s' already runnining.\n",
		    __func__, job->job_name));
		return;
	}

	threadpool_job_hold(job);

	/* Otherwise, try to assign a thread to the job.  */
	mutex_spin_enter(&pool->tp_lock);
	if (__predict_false(TAILQ_EMPTY(&pool->tp_idle_threads))) {
		/* Nobody's idle.  Give it to the overseer.  */
		TP_LOG(("%s: giving job '%s' to overseer.\n",
		    __func__, job->job_name));
		job->job_thread = &pool->tp_overseer;
		TAILQ_INSERT_TAIL(&pool->tp_jobs, job, job_entry);
	} else {
		/* Assign it to the first idle thread.  */
		job->job_thread = TAILQ_FIRST(&pool->tp_idle_threads);
		TP_LOG(("%s: giving job '%s' to idle thread %p.\n",
		    __func__, job->job_name, job->job_thread));
		TAILQ_REMOVE(&pool->tp_idle_threads, job->job_thread,
		    tpt_entry);
		job->job_thread->tpt_job = job;
	}

	/* Notify whomever we gave it to, overseer or idle thread.  */
	KASSERT(job->job_thread != NULL);
	cv_broadcast(&job->job_thread->tpt_cv);
	mutex_spin_exit(&pool->tp_lock);
}

bool
threadpool_cancel_job_async(struct threadpool *pool, struct threadpool_job *job)
{

	KASSERT(mutex_owned(job->job_lock));

	/*
	 * XXXJRT This fails (albeit safely) when all of the following
	 * are true:
	 *
	 *	=> "pool" is something other than what the job was
	 *	   scheduled on.  This can legitimately occur if,
	 *	   for example, a job is percpu-scheduled on CPU0
	 *	   and then CPU1 attempts to cancel it without taking
	 *	   a remote pool reference.  (this might happen by
	 *	   "luck of the draw").
	 *
	 *	=> "job" is not yet running, but is assigned to the
	 *	   overseer.
	 *
	 * When this happens, this code makes the determination that
	 * the job is already running.  The failure mode is that the
	 * caller is told the job is running, and thus has to wait.
	 * The overseer will eventually get to it and the job will
	 * proceed as if it had been already running.
	 */

	if (job->job_thread == NULL) {
		/* Nothing to do.  Guaranteed not running.  */
		return true;
	} else if (job->job_thread == &pool->tp_overseer) {
		/* Take it off the list to guarantee it won't run.  */
		job->job_thread = NULL;
		mutex_spin_enter(&pool->tp_lock);
		TAILQ_REMOVE(&pool->tp_jobs, job, job_entry);
		mutex_spin_exit(&pool->tp_lock);
		threadpool_job_rele(job);
		return true;
	} else {
		/* Too late -- already running.  */
		return false;
	}
}

void
threadpool_cancel_job(struct threadpool *pool, struct threadpool_job *job)
{

	ASSERT_SLEEPABLE();

	KASSERT(mutex_owned(job->job_lock));

	if (threadpool_cancel_job_async(pool, job))
		return;

	/* Already running.  Wait for it to complete.  */
	while (job->job_thread != NULL)
		cv_wait(&job->job_cv, job->job_lock);
}

/* Thread pool overseer thread */

static void __dead
threadpool_overseer_thread(void *arg)
{
	struct threadpool_thread *const overseer = arg;
	struct threadpool *const pool = overseer->tpt_pool;
	struct lwp *lwp = NULL;
	int ktflags;
	int error;

	KASSERT((pool->tp_cpu == NULL) || (pool->tp_cpu == curcpu()));

	/* Wait until we're initialized.  */
	mutex_spin_enter(&pool->tp_lock);
	while (overseer->tpt_lwp == NULL)
		cv_wait(&overseer->tpt_cv, &pool->tp_lock);

	TP_LOG(("%s: starting.\n", __func__));

	for (;;) {
		/* Wait until there's a job.  */
		while (TAILQ_EMPTY(&pool->tp_jobs)) {
			if (ISSET(pool->tp_flags, THREADPOOL_DYING)) {
				TP_LOG(("%s: THREADPOOL_DYING\n",
				    __func__));
				break;
			}
			cv_wait(&overseer->tpt_cv, &pool->tp_lock);
		}
		if (__predict_false(TAILQ_EMPTY(&pool->tp_jobs)))
			break;

		/* If there are no threads, we'll have to try to start one.  */
		if (TAILQ_EMPTY(&pool->tp_idle_threads)) {
			TP_LOG(("%s: Got a job, need to create a thread.\n",
			    __func__));
			threadpool_hold(pool);
			mutex_spin_exit(&pool->tp_lock);

			struct threadpool_thread *const thread =
			    pool_cache_get(threadpool_thread_pc, PR_WAITOK);
			thread->tpt_lwp = NULL;
			thread->tpt_pool = pool;
			thread->tpt_job = NULL;
			cv_init(&thread->tpt_cv, "poolthrd");

			ktflags = 0;
			ktflags |= KTHREAD_MPSAFE;
			if (pool->tp_pri < PRI_KERNEL)
				ktflags |= KTHREAD_TS;
			error = kthread_create(pool->tp_pri, ktflags,
			    pool->tp_cpu, &threadpool_thread, thread, &lwp,
			    "poolthread/%d@%d",
			    (pool->tp_cpu ? cpu_index(pool->tp_cpu) : -1),
			    (int)pool->tp_pri);

			mutex_spin_enter(&pool->tp_lock);
			if (error) {
				pool_cache_put(threadpool_thread_pc, thread);
				threadpool_rele(pool);
				/* XXX What to do to wait for memory?  */
				(void)kpause("thrdplcr", false, hz,
				    &pool->tp_lock);
				continue;
			}
			/*
			 * New kthread now owns the reference to the pool
			 * taken above.
			 */
			KASSERT(lwp != NULL);
			TAILQ_INSERT_TAIL(&pool->tp_idle_threads, thread,
			    tpt_entry);
			thread->tpt_lwp = lwp;
			lwp = NULL;
			cv_broadcast(&thread->tpt_cv);
			continue;
		}

		/* There are idle threads, so try giving one a job.  */
		struct threadpool_job *const job = TAILQ_FIRST(&pool->tp_jobs);
		TAILQ_REMOVE(&pool->tp_jobs, job, job_entry);
		/*
		 * Take an extra reference on the job temporarily so that
		 * it won't disappear on us while we have both locks dropped.
		 */
		threadpool_job_hold(job);
		mutex_spin_exit(&pool->tp_lock);

		mutex_enter(job->job_lock);
		/* If the job was cancelled, we'll no longer be its thread.  */
		if (__predict_true(job->job_thread == overseer)) {
			mutex_spin_enter(&pool->tp_lock);
			if (__predict_false(
				    TAILQ_EMPTY(&pool->tp_idle_threads))) {
				/*
				 * Someone else snagged the thread
				 * first.  We'll have to try again.
				 */
				TP_LOG(("%s: '%s' lost race to use idle thread.\n",
				    __func__, job->job_name));
				TAILQ_INSERT_HEAD(&pool->tp_jobs, job,
				    job_entry);
			} else {
				/*
				 * Assign the job to the thread and
				 * wake the thread so it starts work.
				 */
				struct threadpool_thread *const thread =
				    TAILQ_FIRST(&pool->tp_idle_threads);

				TP_LOG(("%s: '%s' gets thread %p\n",
				    __func__, job->job_name, thread));
				KASSERT(thread->tpt_job == NULL);
				TAILQ_REMOVE(&pool->tp_idle_threads, thread,
				    tpt_entry);
				thread->tpt_job = job;
				job->job_thread = thread;
				cv_broadcast(&thread->tpt_cv);
			}
			mutex_spin_exit(&pool->tp_lock);
		}
		threadpool_job_rele(job);
		mutex_exit(job->job_lock);

		mutex_spin_enter(&pool->tp_lock);
	}
	threadpool_rele(pool);
	mutex_spin_exit(&pool->tp_lock);

	TP_LOG(("%s: exiting.\n", __func__));

	kthread_exit(0);
}

/* Thread pool thread */

static void __dead
threadpool_thread(void *arg)
{
	struct threadpool_thread *const thread = arg;
	struct threadpool *const pool = thread->tpt_pool;

	KASSERT((pool->tp_cpu == NULL) || (pool->tp_cpu == curcpu()));

	/* Wait until we're initialized and on the queue.  */
	mutex_spin_enter(&pool->tp_lock);
	while (thread->tpt_lwp == NULL)
		cv_wait(&thread->tpt_cv, &pool->tp_lock);

	TP_LOG(("%s: starting.\n", __func__));

	KASSERT(thread->tpt_lwp == curlwp);
	for (;;) {
		/* Wait until we are assigned a job.  */
		while (thread->tpt_job == NULL) {
			if (ISSET(pool->tp_flags, THREADPOOL_DYING)) {
				TP_LOG(("%s: THREADPOOL_DYING\n",
				    __func__));
				break;
			}
			if (cv_timedwait(&thread->tpt_cv, &pool->tp_lock,
				THREADPOOL_IDLE_TICKS))
				break;
		}
		if (__predict_false(thread->tpt_job == NULL)) {
			TAILQ_REMOVE(&pool->tp_idle_threads, thread,
			    tpt_entry);
			break;
		}

		struct threadpool_job *const job = thread->tpt_job;
		KASSERT(job != NULL);

		/* Set our lwp name to reflect what job we're doing.  */
		lwp_lock(curlwp);
		char *const lwp_name __diagused = curlwp->l_name;
		thread->tpt_lwp_savedname = curlwp->l_name;
		curlwp->l_name = job->job_name;
		lwp_unlock(curlwp);

		mutex_spin_exit(&pool->tp_lock);

		TP_LOG(("%s: running job '%s' on thread %p.\n",
		    __func__, job->job_name, thread));

		/* Run the job.  */
		(*job->job_fn)(job);

		/* lwp name restored in threadpool_job_done(). */
		KASSERTMSG((curlwp->l_name == lwp_name),
		    "someone forgot to call threadpool_job_done()!");

		/*
		 * We can compare pointers, but we can no longer deference
		 * job after this because threadpool_job_done() drops the
		 * last reference on the job while the job is locked.
		 */

		mutex_spin_enter(&pool->tp_lock);
		KASSERT(thread->tpt_job == job);
		thread->tpt_job = NULL;
		TAILQ_INSERT_TAIL(&pool->tp_idle_threads, thread, tpt_entry);
	}
	threadpool_rele(pool);
	mutex_spin_exit(&pool->tp_lock);

	TP_LOG(("%s: thread %p exiting.\n", __func__, thread));

	KASSERT(!cv_has_waiters(&thread->tpt_cv));
	cv_destroy(&thread->tpt_cv);
	pool_cache_put(threadpool_thread_pc, thread);
	kthread_exit(0);
}