[PATCHSET] Replace io_uring workqueues with io-wq

[PATCHSET] Replace io_uring workqueues with io-wq

[Jens Axboe]

Most of the justification for this work is done in the commit messages
themselves, but the tldr is that there are various bits of functionality
that io_uring needs that workqueues don't (and can't) provide. Hence
this adds a small replacement thread pool implementation that caters to
the needs of io_uring, both current and future ones.

Hopefully the sched core changes are palatable. io-wq uses the same
sched in/out hooks as workqueue, and if the task isn't either a
workqueue or io-wq worker, there should be no extra overhead there.

Patches are on top of my for-5.5/io_uring branch, and can also be found
here:

http://git.kernel.dk/cgit/linux-block/log/?h=for-5.5/io_uring-wq

This passes io_uring IO testing and repeated runs of the liburing
regressions suite, and passes tests that would previously deadlock.

 fs/Kconfig            |   3 +
 fs/Makefile           |   1 +
 fs/io-wq.c            | 790 ++++++++++++++++++++++++++++++++++++++++++
 fs/io-wq.h            |  55 +++
 fs/io_uring.c         | 402 +++++----------------
 include/linux/sched.h |   1 +
 init/Kconfig          |   1 +
 kernel/sched/core.c   |  16 +-
 8 files changed, 948 insertions(+), 321 deletions(-)

-- 
Jens Axboe

[PATCH 1/2] io-wq: small threadpool implementation for io_uring

[Jens Axboe]

This adds support for io-wq, a smaller and specialized thread pool
implementation. This is meant to replace workqueues for io_uring. Among
the reasons for this addition are:

- We can assign memory context smarter and more persistently if we
  manage the life time of threads.

- We can drop various work-arounds we have in io_uring, like the
  async_list.

- We can implement hashed work insertion, to manage concurrency of
  buffered writes without needing a) an extra workqueue, or b)
  needlessly making the concurrency of said workqueue very low
  which hurts performance of multiple buffered file writers.

- We can implement cancel through signals, for cancelling
  interruptible work like read/write (or send/recv) to/from sockets.

- We need the above cancel for being able to assign and use file tables
  from a process.

- We can implement a more thorough cancel operation in general.

- We need it to move towards a syslet/threadlet model for even faster
  async execution. For that we need to take ownership of the used
  threads.

This list is just off the top of my head. Performance should be the
same, or better, at least that's what I've seen in my testing. io-wq
supports basic NUMA functionality, setting up a pool per node.

io-wq hooks up to the scheduler schedule in/out just like workqueue
and uses that to drive the need for more/less workers.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/Kconfig            |   3 +
 fs/Makefile           |   1 +
 fs/io-wq.c            | 790 ++++++++++++++++++++++++++++++++++++++++++
 fs/io-wq.h            |  55 +++
 include/linux/sched.h |   1 +
 kernel/sched/core.c   |  16 +-
 6 files changed, 862 insertions(+), 4 deletions(-)
 create mode 100644 fs/io-wq.c
 create mode 100644 fs/io-wq.h

diff --git a/fs/Kconfig b/fs/Kconfig
index 2501e6f1f965..7b623e9fc1b0 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -322,4 +322,7 @@ source "fs/nls/Kconfig"
 source "fs/dlm/Kconfig"
 source "fs/unicode/Kconfig"
 
+config IO_WQ
+	bool
+
 endmenu
diff --git a/fs/Makefile b/fs/Makefile
index 14231b4cf383..1148c555c4d3 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -32,6 +32,7 @@ obj-$(CONFIG_EVENTFD)		+= eventfd.o
 obj-$(CONFIG_USERFAULTFD)	+= userfaultfd.o
 obj-$(CONFIG_AIO)               += aio.o
 obj-$(CONFIG_IO_URING)		+= io_uring.o
+obj-$(CONFIG_IO_WQ)		+= io-wq.o
 obj-$(CONFIG_FS_DAX)		+= dax.o
 obj-$(CONFIG_FS_ENCRYPTION)	+= crypto/
 obj-$(CONFIG_FS_VERITY)		+= verity/
diff --git a/fs/io-wq.c b/fs/io-wq.c
new file mode 100644
index 000000000000..32ae5cdcb726
--- /dev/null
+++ b/fs/io-wq.c
@@ -0,0 +1,790 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Basic worker thread pool for io_uring
+ *
+ * Copyright (C) 2019 Jens Axboe
+ *
+ */
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/errno.h>
+#include <linux/sched/signal.h>
+#include <linux/mm.h>
+#include <linux/mmu_context.h>
+#include <linux/sched/mm.h>
+#include <linux/percpu.h>
+#include <linux/slab.h>
+#include <linux/kthread.h>
+#include <linux/rculist_nulls.h>
+
+#include "io-wq.h"
+
+#define WORKER_IDLE_TIMEOUT	(5 * HZ)
+
+enum {
+	IO_WORKER_F_UP		= 1,	/* up and active */
+	IO_WORKER_F_RUNNING	= 2,	/* account as running */
+	IO_WORKER_F_FREE	= 4,	/* worker on free list */
+	IO_WORKER_F_EXITING	= 8,	/* worker exiting */
+	IO_WORKER_F_FIXED	= 16,	/* static idle worker */
+};
+
+enum {
+	IO_WQ_BIT_EXIT		= 0,	/* wq exiting */
+	IO_WQ_BIT_CANCEL	= 1,	/* cancel work on list */
+};
+
+enum {
+	IO_WQE_BIT_STALLED	= 0,	/* stalled on hash */
+};
+
+/*
+ * One for each thread in a wqe pool
+ */
+struct io_worker {
+	refcount_t ref;
+	unsigned flags;
+	struct hlist_nulls_node nulls_node;
+	struct task_struct *task;
+	wait_queue_head_t wait;
+	struct io_wqe *wqe;
+	struct io_wq_work *cur_work;
+
+	struct rcu_head rcu;
+	struct mm_struct *mm;
+};
+
+struct io_wq_nulls_list {
+	struct hlist_nulls_head head;
+	unsigned long nulls;
+};
+
+#if BITS_PER_LONG == 64
+#define IO_WQ_HASH_ORDER	6
+#else
+#define IO_WQ_HASH_ORDER	5
+#endif
+
+/*
+ * Per-node worker thread pool
+ */
+struct io_wqe {
+	struct {
+		spinlock_t lock;
+		struct list_head work_list;
+		unsigned long state;
+		unsigned long hash_map;
+	} ____cacheline_aligned_in_smp;
+
+	int node;
+	unsigned nr_workers;
+	unsigned max_workers;
+	atomic_t nr_running;
+
+	struct io_wq_nulls_list free_list;
+	struct io_wq_nulls_list busy_list;
+
+	struct io_wq *wq;
+};
+
+/*
+ * Per io_wq state
+  */
+struct io_wq {
+	struct io_wqe **wqes;
+	unsigned long state;
+	unsigned nr_wqes;
+
+	struct task_struct *manager;
+	struct mm_struct *mm;
+	refcount_t refs;
+	struct completion done;
+};
+
+static void io_wq_free_worker(struct rcu_head *head)
+{
+	struct io_worker *worker = container_of(head, struct io_worker, rcu);
+
+	kfree(worker);
+}
+
+static bool io_worker_get(struct io_worker *worker)
+{
+	return refcount_inc_not_zero(&worker->ref);
+}
+
+static void io_worker_release(struct io_worker *worker)
+{
+	if (refcount_dec_and_test(&worker->ref))
+		wake_up_process(worker->task);
+}
+
+static void io_worker_exit(struct io_worker *worker)
+{
+	struct io_wqe *wqe = worker->wqe;
+	bool all_done = false;
+
+	/*
+	 * If we're not at zero, someone else is holding a brief reference
+	 * to the worker. Wait for that to go away.
+	 */
+	set_current_state(TASK_INTERRUPTIBLE);
+	if (!refcount_dec_and_test(&worker->ref))
+		schedule();
+	__set_current_state(TASK_RUNNING);
+
+	preempt_disable();
+	current->flags &= ~PF_IO_WORKER;
+	if (worker->flags & IO_WORKER_F_RUNNING)
+		atomic_dec(&wqe->nr_running);
+	worker->flags = 0;
+	preempt_enable();
+
+	spin_lock(&wqe->lock);
+	hlist_nulls_del_rcu(&worker->nulls_node);
+	wqe->nr_workers--;
+	all_done = !wqe->nr_workers;
+	spin_unlock(&wqe->lock);
+
+	/* all workers gone, wq exit can proceed */
+	if (all_done && refcount_dec_and_test(&wqe->wq->refs))
+		complete(&wqe->wq->done);
+
+	call_rcu(&worker->rcu, io_wq_free_worker);
+}
+
+static void io_worker_start(struct io_wqe *wqe, struct io_worker *worker)
+{
+	allow_kernel_signal(SIGINT);
+
+	current->flags |= PF_IO_WORKER;
+
+	worker->flags |= (IO_WORKER_F_UP | IO_WORKER_F_RUNNING);
+	atomic_inc(&wqe->nr_running);
+}
+
+/*
+ * Worker will start processing some work. Move it to the busy list, if
+ * it's currently on the freelist
+ */
+static void __io_worker_busy(struct io_wqe *wqe, struct io_worker *worker,
+			     struct io_wq_work *work)
+	__must_hold(wqe->lock)
+{
+	if (worker->flags & IO_WORKER_F_FREE) {
+		worker->flags &= ~IO_WORKER_F_FREE;
+		hlist_nulls_del_init_rcu(&worker->nulls_node);
+		hlist_nulls_add_head_rcu(&worker->nulls_node,
+						&wqe->busy_list.head);
+	}
+	worker->cur_work = work;
+}
+
+/*
+ * No work, worker going to sleep. Move to freelist, and unuse mm if we
+ * have one attached. Dropping the mm may potentially sleep, so we drop
+ * the lock in that case and return success. Since the caller has to
+ * retry the loop in that case (we changed task state), we don't regrab
+ * the lock if we return success.
+ */
+static bool __io_worker_idle(struct io_wqe *wqe, struct io_worker *worker)
+	__must_hold(wqe->lock)
+	__releases(wqe->lock)
+{
+	if (!(worker->flags & IO_WORKER_F_FREE)) {
+		worker->flags |= IO_WORKER_F_FREE;
+		hlist_nulls_del_init_rcu(&worker->nulls_node);
+		hlist_nulls_add_head_rcu(&worker->nulls_node,
+						&wqe->free_list.head);
+	}
+	/*
+	 * If we have an active mm, we need to drop the wq lock before unusing
+	 * it. If we do, return true and let the caller retry the idle loop.
+	 */
+	if (worker->mm) {
+		spin_unlock(&wqe->lock);
+		__set_current_state(TASK_RUNNING);
+		set_fs(KERNEL_DS);
+		unuse_mm(worker->mm);
+		mmput(worker->mm);
+		worker->mm = NULL;
+		return true;
+	}
+
+	return false;
+}
+
+static struct io_wq_work *io_get_next_work(struct io_wqe *wqe, unsigned *hash)
+	__must_hold(wqe->lock)
+{
+	struct io_wq_work *work;
+
+	list_for_each_entry(work, &wqe->work_list, list) {
+		/* not hashed, can run anytime */
+		if (!(work->flags & IO_WQ_WORK_HASHED)) {
+			list_del(&work->list);
+			return work;
+		}
+
+		/* hashed, can run if not already running */
+		*hash = work->flags >> IO_WQ_HASH_SHIFT;
+		if (!(wqe->hash_map & (1U << *hash))) {
+			wqe->hash_map |= (1U << *hash);
+			list_del(&work->list);
+			return work;
+		}
+	}
+
+	return NULL;
+}
+
+static void io_worker_handle_work(struct io_worker *worker)
+	__releases(wqe->lock)
+{
+	struct io_wq_work *work, *old_work;
+	struct io_wqe *wqe = worker->wqe;
+	struct io_wq *wq = wqe->wq;
+
+	do {
+		unsigned hash = -1U;
+
+		/*
+		 * If we got some work, mark us as busy. If we didn't, but
+		 * the list isn't empty, it means we stalled on hashed work.
+		 * Mark us stalled so we don't keep looking for work when we
+		 * can't make progress, any work completion or insertion will
+		 * clear the stalled flag.
+		 */
+		work = io_get_next_work(wqe, &hash);
+		if (work)
+			__io_worker_busy(wqe, worker, work);
+		else if (!list_empty(&wqe->work_list))
+			set_bit(IO_WQE_BIT_STALLED, &wqe->state);
+
+		spin_unlock(&wqe->lock);
+		if (!work)
+			break;
+next:
+		if ((work->flags & IO_WQ_WORK_NEEDS_USER) && !worker->mm &&
+		    wq->mm && mmget_not_zero(wq->mm)) {
+			use_mm(wq->mm);
+			set_fs(USER_DS);
+			worker->mm = wq->mm;
+		}
+		if (test_bit(IO_WQ_BIT_CANCEL, &wq->state))
+			work->flags |= IO_WQ_WORK_CANCEL;
+		if (worker->mm)
+			work->flags |= IO_WQ_WORK_HAS_MM;
+
+		old_work = work;
+		work->func(&work);
+
+		spin_lock(&wqe->lock);
+		worker->cur_work = NULL;
+		if (hash != -1U) {
+			wqe->hash_map &= ~(1U << hash);
+			if (test_bit(IO_WQE_BIT_STALLED, &wqe->state))
+				clear_bit(IO_WQE_BIT_STALLED, &wqe->state);
+		}
+		if (work && work != old_work) {
+			spin_unlock(&wqe->lock);
+			goto next;
+		}
+	} while (1);
+}
+
+static inline bool io_wqe_run_queue(struct io_wqe *wqe)
+{
+	if (!list_empty_careful(&wqe->work_list) &&
+	    !test_bit(IO_WQE_BIT_STALLED, &wqe->state))
+		return true;
+	return false;
+}
+
+static int io_wqe_worker(void *data)
+{
+	struct io_worker *worker = data;
+	struct io_wqe *wqe = worker->wqe;
+	struct io_wq *wq = wqe->wq;
+	DEFINE_WAIT(wait);
+
+	io_worker_start(wqe, worker);
+
+	while (!test_bit(IO_WQ_BIT_EXIT, &wq->state)) {
+		prepare_to_wait(&worker->wait, &wait, TASK_INTERRUPTIBLE);
+
+		spin_lock(&wqe->lock);
+		if (io_wqe_run_queue(wqe)) {
+			__set_current_state(TASK_RUNNING);
+			io_worker_handle_work(worker);
+			continue;
+		}
+		/* drops the lock on success, retry */
+		if (__io_worker_idle(wqe, worker))
+			continue;
+		spin_unlock(&wqe->lock);
+		if (signal_pending(current))
+			flush_signals(current);
+		if (schedule_timeout(WORKER_IDLE_TIMEOUT))
+			continue;
+		/* timed out, exit unless we're the fixed worker */
+		if (test_bit(IO_WQ_BIT_EXIT, &wq->state) ||
+		    !(worker->flags & IO_WORKER_F_FIXED))
+			break;
+	}
+
+	finish_wait(&worker->wait, &wait);
+
+	if (test_bit(IO_WQ_BIT_EXIT, &wq->state)) {
+		spin_lock(&wqe->lock);
+		if (!list_empty(&wqe->work_list))
+			io_worker_handle_work(worker);
+		else
+			spin_unlock(&wqe->lock);
+	}
+
+	io_worker_exit(worker);
+	return 0;
+}
+
+/*
+ * Check head of free list for an available worker. If one isn't available,
+ * caller must wake up the wq manager to create one.
+ */
+static bool io_wqe_activate_free_worker(struct io_wqe *wqe)
+	__must_hold(RCU)
+{
+	struct hlist_nulls_node *n;
+	struct io_worker *worker;
+
+	n = rcu_dereference(hlist_nulls_first_rcu(&wqe->free_list.head));
+	if (is_a_nulls(n))
+		return false;
+
+	worker = hlist_nulls_entry(n, struct io_worker, nulls_node);
+	if (io_worker_get(worker)) {
+		wake_up(&worker->wait);
+		io_worker_release(worker);
+		return true;
+	}
+
+	return false;
+}
+
+/*
+ * We need a worker. If we find a free one, we're good. If not, and we're
+ * below the max number of workers, wake up the manager to create one.
+ */
+static void io_wqe_wake_worker(struct io_wqe *wqe)
+{
+	bool ret;
+
+	rcu_read_lock();
+	ret = io_wqe_activate_free_worker(wqe);
+	rcu_read_unlock();
+
+	if (!ret && wqe->nr_workers < wqe->max_workers)
+		wake_up_process(wqe->wq->manager);
+}
+
+/*
+ * Called when a worker is scheduled in. Mark us as currently running.
+ */
+void io_wq_worker_running(struct task_struct *tsk)
+{
+	struct io_worker *worker = kthread_data(tsk);
+	struct io_wqe *wqe = worker->wqe;
+
+	if (!(worker->flags & IO_WORKER_F_UP))
+		return;
+	if (worker->flags & IO_WORKER_F_RUNNING)
+		return;
+	worker->flags |= IO_WORKER_F_RUNNING;
+	atomic_inc(&wqe->nr_running);
+}
+
+/*
+ * Called when worker is going to sleep. If there are no workers currently
+ * running and we have work pending, wake up a free one or have the manager
+ * set one up.
+ */
+void io_wq_worker_sleeping(struct task_struct *tsk)
+{
+	struct io_worker *worker = kthread_data(tsk);
+	struct io_wqe *wqe = worker->wqe;
+
+	if (!(worker->flags & IO_WORKER_F_UP))
+		return;
+	if (!(worker->flags & IO_WORKER_F_RUNNING))
+		return;
+
+	worker->flags &= ~IO_WORKER_F_RUNNING;
+
+	if (atomic_dec_and_test(&wqe->nr_running) && io_wqe_run_queue(wqe))
+		io_wqe_wake_worker(wqe);
+}
+
+static void create_io_worker(struct io_wq *wq, struct io_wqe *wqe)
+{
+	struct io_worker *worker;
+
+	worker = kcalloc_node(1, sizeof(*worker), GFP_KERNEL, wqe->node);
+	if (!worker)
+		return;
+
+	refcount_set(&worker->ref, 1);
+	worker->nulls_node.pprev = NULL;
+	init_waitqueue_head(&worker->wait);
+	worker->wqe = wqe;
+
+	worker->task = kthread_create_on_node(io_wqe_worker, worker, wqe->node,
+						"io_wqe_worker-%d", wqe->node);
+	if (IS_ERR(worker->task)) {
+		kfree(worker);
+		return;
+	}
+
+	spin_lock(&wqe->lock);
+	hlist_nulls_add_head_rcu(&worker->nulls_node, &wqe->free_list.head);
+	worker->flags |= IO_WORKER_F_FREE;
+	if (!wqe->nr_workers)
+		worker->flags |= IO_WORKER_F_FIXED;
+	wqe->nr_workers++;
+	spin_unlock(&wqe->lock);
+
+	wake_up_process(worker->task);
+}
+
+static inline bool io_wqe_need_new_worker(struct io_wqe *wqe)
+	__must_hold(wqe->lock)
+{
+	if (!wqe->nr_workers)
+		return true;
+	if (hlist_nulls_empty(&wqe->free_list.head) &&
+	    wqe->nr_workers < wqe->max_workers && io_wqe_run_queue(wqe))
+		return true;
+
+	return false;
+}
+
+/*
+ * Manager thread. Tasked with creating new workers, if we need them.
+ */
+static int io_wq_manager(void *data)
+{
+	struct io_wq *wq = data;
+
+	while (!kthread_should_stop()) {
+		int i;
+
+		for (i = 0; i < wq->nr_wqes; i++) {
+			struct io_wqe *wqe = wq->wqes[i];
+			bool fork_worker = false;
+
+			spin_lock(&wqe->lock);
+			fork_worker = io_wqe_need_new_worker(wqe);
+			spin_unlock(&wqe->lock);
+			if (fork_worker)
+				create_io_worker(wq, wqe);
+		}
+		set_current_state(TASK_INTERRUPTIBLE);
+		if (schedule_timeout(HZ))
+			continue;
+#ifdef IO_WQ_MANAGER_DEBUG
+		printk("manager: now workers=%d/running=%d/work_empty=%d\n", wq->nr_workers, atomic_read(&wq->nr_running), list_empty(&wq->work_list));
+#endif
+	}
+
+	return 0;
+}
+
+static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
+{
+	spin_lock(&wqe->lock);
+	list_add_tail(&work->list, &wqe->work_list);
+	spin_unlock(&wqe->lock);
+
+	if (test_bit(IO_WQE_BIT_STALLED, &wqe->state))
+		clear_bit(IO_WQE_BIT_STALLED, &wqe->state);
+
+	if (!atomic_read(&wqe->nr_running))
+		io_wqe_wake_worker(wqe);
+}
+
+void io_wq_enqueue(struct io_wq *wq, struct io_wq_work *work)
+{
+	struct io_wqe *wqe = wq->wqes[numa_node_id()];
+
+	io_wqe_enqueue(wqe, work);
+}
+
+/*
+ * Enqueue work, hashed by some key. Work items that hash to the same value
+ * will not be done in parallel. Used to limit concurrent writes, generally
+ * hashed by inode.
+ */
+void io_wq_enqueue_hashed(struct io_wq *wq, struct io_wq_work *work, void *val)
+{
+	struct io_wqe *wqe = wq->wqes[numa_node_id()];
+	unsigned bit;
+
+
+	bit = hash_ptr(val, IO_WQ_HASH_ORDER);
+	work->flags |= (IO_WQ_WORK_HASHED | (bit << IO_WQ_HASH_SHIFT));
+	io_wqe_enqueue(wqe, work);
+}
+
+static bool io_wqe_worker_send_sig(struct io_worker *worker, void *data)
+{
+	send_sig(SIGINT, worker->task, 1);
+	return false;
+}
+
+/*
+ * Iterate the passed in list and call the specific function for each
+ * worker that isn't exiting
+ */
+static bool io_wq_for_each_worker(struct io_wqe *wqe,
+				  struct io_wq_nulls_list *list,
+				  bool (*func)(struct io_worker *, void *),
+				  void *data)
+{
+	struct hlist_nulls_node *n;
+	struct io_worker *worker;
+	bool ret = false;
+
+restart:
+	hlist_nulls_for_each_entry_rcu(worker, n, &list->head, nulls_node) {
+		if (io_worker_get(worker)) {
+			ret = func(worker, data);
+			io_worker_release(worker);
+			if (ret)
+				break;
+		}
+	}
+	if (!ret && get_nulls_value(n) != list->nulls)
+		goto restart;
+	return ret;
+}
+
+void io_wq_cancel_all(struct io_wq *wq)
+{
+	int i;
+
+	set_bit(IO_WQ_BIT_CANCEL, &wq->state);
+
+	/*
+	 * Browse both lists, as there's a gap between handing work off
+	 * to a worker and the worker putting itself on the busy_list
+	 */
+	rcu_read_lock();
+	for (i = 0; i < wq->nr_wqes; i++) {
+		struct io_wqe *wqe = wq->wqes[i];
+
+		io_wq_for_each_worker(wqe, &wqe->busy_list,
+					io_wqe_worker_send_sig, NULL);
+		io_wq_for_each_worker(wqe, &wqe->free_list,
+					io_wqe_worker_send_sig, NULL);
+	}
+	rcu_read_unlock();
+}
+
+static bool io_wq_worker_cancel(struct io_worker *worker, void *data)
+{
+	struct io_wq_work *work = data;
+
+	if (worker->cur_work == work) {
+		send_sig(SIGINT, worker->task, 1);
+		return true;
+	}
+
+	return false;
+}
+
+static enum io_wq_cancel io_wqe_cancel_work(struct io_wqe *wqe,
+					    struct io_wq_work *cwork)
+{
+	struct io_wq_work *work;
+	bool found;
+
+	/*
+	 * First check pending list, if we're lucky we can just remove it
+	 * from there. CANCEL_OK means that the work is returned as-new,
+	 * no completion will be posted for it.
+	 */
+	spin_lock(&wqe->lock);
+	list_for_each_entry(work, &wqe->work_list, list) {
+		if (work == cwork) {
+			list_del(&work->list);
+			spin_unlock(&wqe->lock);
+			return IO_WQ_CANCEL_OK;
+		}
+	}
+	spin_unlock(&wqe->lock);
+
+	/*
+	 * Now check if a free (going busy) or busy worker has the work
+	 * currently running. If we find it there, we'll return CANCEL_RUNNING
+	 * as an indication that we attempte to signal cancellation. The
+	 * completion will run normally in this case.
+	 */
+	rcu_read_lock();
+	found = io_wq_for_each_worker(wqe, &wqe->free_list, io_wq_worker_cancel,
+					cwork);
+	if (found)
+		goto done;
+
+	found = io_wq_for_each_worker(wqe, &wqe->busy_list, io_wq_worker_cancel,
+					cwork);
+done:
+	rcu_read_unlock();
+	return found ? IO_WQ_CANCEL_RUNNING : IO_WQ_CANCEL_NOTFOUND;
+
+}
+
+enum io_wq_cancel io_wq_cancel_work(struct io_wq *wq, struct io_wq_work *cwork)
+{
+	enum io_wq_cancel ret = IO_WQ_CANCEL_NOTFOUND;
+	int i;
+
+	for (i = 0; i < wq->nr_wqes; i++) {
+		struct io_wqe *wqe = wq->wqes[i];
+
+		ret = io_wqe_cancel_work(wqe, cwork);
+		if (ret != IO_WQ_CANCEL_NOTFOUND)
+			break;
+	}
+
+	return ret;
+}
+
+struct io_wq_flush_data {
+	struct io_wq_work work;
+	struct completion done;
+};
+
+static void io_wq_flush_func(struct io_wq_work **workptr)
+{
+	struct io_wq_work *work = *workptr;
+	struct io_wq_flush_data *data;
+
+	data = container_of(work, struct io_wq_flush_data, work);
+	complete(&data->done);
+}
+
+/*
+ * Doesn't wait for previously queued work to finish. When this completes,
+ * it just means that previously queued work was started.
+ */
+void io_wq_flush(struct io_wq *wq)
+{
+	struct io_wq_flush_data data;
+	int i;
+
+	for (i = 0; i < wq->nr_wqes; i++) {
+		struct io_wqe *wqe = wq->wqes[i];
+
+		init_completion(&data.done);
+		INIT_IO_WORK(&data.work, io_wq_flush_func);
+		io_wqe_enqueue(wqe, &data.work);
+		wait_for_completion(&data.done);
+	}
+}
+
+struct io_wq *io_wq_create(unsigned concurrency, struct mm_struct *mm)
+{
+	int ret = -ENOMEM, i, node;
+	struct io_wq *wq;
+
+	wq = kcalloc(1, sizeof(*wq), GFP_KERNEL);
+	if (!wq)
+		return ERR_PTR(-ENOMEM);
+
+	wq->nr_wqes = num_online_nodes();
+	wq->wqes = kcalloc(wq->nr_wqes, sizeof(struct io_wqe *), GFP_KERNEL);
+	if (!wq->wqes) {
+		kfree(wq);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	i = 0;
+	refcount_set(&wq->refs, wq->nr_wqes);
+	for_each_online_node(node) {
+		struct io_wqe *wqe;
+
+		wqe = kcalloc_node(1, sizeof(struct io_wqe), GFP_KERNEL, node);
+		if (!wqe)
+			break;
+		wq->wqes[i] = wqe;
+		wqe->node = node;
+		wqe->max_workers = concurrency;
+		wqe->node = node;
+		wqe->wq = wq;
+		spin_lock_init(&wqe->lock);
+		INIT_LIST_HEAD(&wqe->work_list);
+		INIT_HLIST_NULLS_HEAD(&wqe->free_list.head, 0);
+		wqe->free_list.nulls = 0;
+		INIT_HLIST_NULLS_HEAD(&wqe->busy_list.head, 1);
+		wqe->busy_list.nulls = 1;
+		atomic_set(&wqe->nr_running, 0);
+
+		i++;
+	}
+
+	init_completion(&wq->done);
+
+	if (i != wq->nr_wqes)
+		goto err;
+
+	/* caller must have already done mmgrab() on this mm */
+	wq->mm = mm;
+
+	wq->manager = kthread_create(io_wq_manager, wq, "io_wq_manager");
+	if (!IS_ERR(wq->manager)) {
+		wake_up_process(wq->manager);
+		return wq;
+	}
+
+	ret = PTR_ERR(wq->manager);
+	wq->manager = NULL;
+err:
+	complete(&wq->done);
+	io_wq_destroy(wq);
+	return ERR_PTR(ret);
+}
+
+static bool io_wq_worker_wake(struct io_worker *worker, void *data)
+{
+	wake_up_process(worker->task);
+	return false;
+}
+
+void io_wq_destroy(struct io_wq *wq)
+{
+	int i;
+
+	if (wq->manager) {
+		set_bit(IO_WQ_BIT_EXIT, &wq->state);
+		kthread_stop(wq->manager);
+	}
+
+	rcu_read_lock();
+	for (i = 0; i < wq->nr_wqes; i++) {
+		struct io_wqe *wqe = wq->wqes[i];
+
+		if (!wqe)
+			continue;
+		io_wq_for_each_worker(wqe, &wqe->free_list, io_wq_worker_wake,
+						NULL);
+		io_wq_for_each_worker(wqe, &wqe->busy_list, io_wq_worker_wake,
+						NULL);
+	}
+	rcu_read_unlock();
+
+	wait_for_completion(&wq->done);
+
+	for (i = 0; i < wq->nr_wqes; i++)
+		kfree(wq->wqes[i]);
+	kfree(wq->wqes);
+	kfree(wq);
+}
diff --git a/fs/io-wq.h b/fs/io-wq.h
new file mode 100644
index 000000000000..be8f22c8937b
--- /dev/null
+++ b/fs/io-wq.h
@@ -0,0 +1,55 @@
+#ifndef INTERNAL_IO_WQ_H
+#define INTERNAL_IO_WQ_H
+
+struct io_wq;
+
+enum {
+	IO_WQ_WORK_CANCEL	= 1,
+	IO_WQ_WORK_HAS_MM	= 2,
+	IO_WQ_WORK_HASHED	= 4,
+	IO_WQ_WORK_NEEDS_USER	= 8,
+
+	IO_WQ_HASH_SHIFT	= 24,	/* upper 8 bits are used for hash key */
+};
+
+enum io_wq_cancel {
+	IO_WQ_CANCEL_OK,	/* cancelled before started */
+	IO_WQ_CANCEL_RUNNING,	/* found, running, and attempted cancelled */
+	IO_WQ_CANCEL_NOTFOUND,	/* work not found */
+};
+
+struct io_wq_work {
+	struct list_head list;
+	void (*func)(struct io_wq_work **);
+	unsigned flags;
+};
+
+#define INIT_IO_WORK(work, _func)			\
+	do {						\
+		(work)->func = _func;			\
+		(work)->flags = 0;			\
+	} while (0)					\
+
+struct io_wq *io_wq_create(unsigned concurrency, struct mm_struct *mm);
+void io_wq_destroy(struct io_wq *wq);
+
+void io_wq_enqueue(struct io_wq *wq, struct io_wq_work *work);
+void io_wq_enqueue_hashed(struct io_wq *wq, struct io_wq_work *work, void *val);
+void io_wq_flush(struct io_wq *wq);
+
+void io_wq_cancel_all(struct io_wq *wq);
+enum io_wq_cancel io_wq_cancel_work(struct io_wq *wq, struct io_wq_work *cwork);
+
+#if defined(CONFIG_IO_WQ)
+extern void io_wq_worker_sleeping(struct task_struct *);
+extern void io_wq_worker_running(struct task_struct *);
+#else
+static inline void io_wq_worker_sleeping(struct task_struct *tsk)
+{
+}
+static inline void io_wq_worker_running(struct task_struct *tsk)
+{
+}
+#endif
+
+#endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 67a1d86981a9..6666e25606b7 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1468,6 +1468,7 @@ extern struct pid *cad_pid;
 #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_mask */
 #define PF_MCE_EARLY		0x08000000      /* Early kill for mce process policy */
 #define PF_MEMALLOC_NOCMA	0x10000000	/* All allocation request will have _GFP_MOVABLE cleared */
+#define PF_IO_WORKER		0x20000000	/* Task is an IO worker */
 #define PF_FREEZER_SKIP		0x40000000	/* Freezer should not count it as freezable */
 #define PF_SUSPEND_TASK		0x80000000      /* This thread called freeze_processes() and should not be frozen */
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index dd05a378631a..a95a2f05f3b5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -16,6 +16,7 @@
 #include <asm/tlb.h>
 
 #include "../workqueue_internal.h"
+#include "../../fs/io-wq.h"
 #include "../smpboot.h"
 
 #include "pelt.h"
@@ -4103,9 +4104,12 @@ static inline void sched_submit_work(struct task_struct *tsk)
 	 * we disable preemption to avoid it calling schedule() again
 	 * in the possible wakeup of a kworker.
 	 */
-	if (tsk->flags & PF_WQ_WORKER) {
+	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
 		preempt_disable();
-		wq_worker_sleeping(tsk);
+		if (tsk->flags & PF_WQ_WORKER)
+			wq_worker_sleeping(tsk);
+		else
+			io_wq_worker_sleeping(tsk);
 		preempt_enable_no_resched();
 	}
 
@@ -4122,8 +4126,12 @@ static inline void sched_submit_work(struct task_struct *tsk)
 
 static void sched_update_worker(struct task_struct *tsk)
 {
-	if (tsk->flags & PF_WQ_WORKER)
-		wq_worker_running(tsk);
+	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
+		if (tsk->flags & PF_WQ_WORKER)
+			wq_worker_running(tsk);
+		else
+			io_wq_worker_running(tsk);
+	}
 }
 
 asmlinkage __visible void __sched schedule(void)
-- 
2.17.1

[PATCH 2/2] io_uring: replace workqueue usage with io-wq

[Jens Axboe]

Drop various work-arounds we have for workqueues:

- We no longer need the async_list for tracking sequential IO.

- We don't have to maintain our own mm tracking/setting.

- We don't need a separate workqueue for buffered writes. This didn't
  even work that well to begin with, as it was suboptimal for multiple
  buffered writers on multiple files.

- We can properly cancel pending interruptible work. This fixes
  deadlocks with particularly socket IO, where we cannot cancel them
  when the io_uring is closed. Hence the ring will wait forever for
  these requests to complete, which may never happen. This is different
  from disk IO where we know requests will complete in a finite amount
  of time.

- Due to being able to cancel work interruptible work that is already
  running, we can implement file table support for work. We need that
  for supporting system calls that add to a process file table.

- It gets us one step closer to adding async support for any system
  call.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/io_uring.c | 402 +++++++++++---------------------------------------
 init/Kconfig  |   1 +
 2 files changed, 86 insertions(+), 317 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index fa081c60a49b..de99f97d6629 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -56,7 +56,6 @@
 #include <linux/mmu_context.h>
 #include <linux/percpu.h>
 #include <linux/slab.h>
-#include <linux/workqueue.h>
 #include <linux/kthread.h>
 #include <linux/blkdev.h>
 #include <linux/bvec.h>
@@ -77,6 +76,7 @@
 #include <uapi/linux/io_uring.h>
 
 #include "internal.h"
+#include "io-wq.h"
 
 #define IORING_MAX_ENTRIES	32768
 #define IORING_MAX_CQ_ENTRIES	(2 * IORING_MAX_ENTRIES)
@@ -165,16 +165,6 @@ struct io_mapped_ubuf {
 	unsigned int	nr_bvecs;
 };
 
-struct async_list {
-	spinlock_t		lock;
-	atomic_t		cnt;
-	struct list_head	list;
-
-	struct file		*file;
-	off_t			io_start;
-	size_t			io_len;
-};
-
 struct io_ring_ctx {
 	struct {
 		struct percpu_ref	refs;
@@ -208,7 +198,7 @@ struct io_ring_ctx {
 	} ____cacheline_aligned_in_smp;
 
 	/* IO offload */
-	struct workqueue_struct	*sqo_wq[2];
+	struct io_wq		*io_wq;
 	struct task_struct	*sqo_thread;	/* if using sq thread polling */
 	struct mm_struct	*sqo_mm;
 	wait_queue_head_t	sqo_wait;
@@ -260,8 +250,6 @@ struct io_ring_ctx {
 		struct list_head	cancel_list;
 	} ____cacheline_aligned_in_smp;
 
-	struct async_list	pending_async[2];
-
 #if defined(CONFIG_UNIX)
 	struct socket		*ring_sock;
 #endif
@@ -332,7 +320,7 @@ struct io_kiocb {
 	u32			result;
 	u32			sequence;
 
-	struct work_struct	work;
+	struct io_wq_work	work;
 };
 
 #define IO_PLUG_THRESHOLD		2
@@ -358,7 +346,7 @@ struct io_submit_state {
 	unsigned int		ios_left;
 };
 
-static void io_sq_wq_submit_work(struct work_struct *work);
+static void io_wq_submit_work(struct io_wq_work **workptr);
 static void io_cqring_fill_event(struct io_ring_ctx *ctx, u64 ki_user_data,
 				 long res);
 static void __io_free_req(struct io_kiocb *req);
@@ -390,7 +378,6 @@ static void io_ring_ctx_ref_free(struct percpu_ref *ref)
 static struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 {
 	struct io_ring_ctx *ctx;
-	int i;
 
 	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
 	if (!ctx)
@@ -408,11 +395,6 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 	init_completion(&ctx->sqo_thread_started);
 	mutex_init(&ctx->uring_lock);
 	init_waitqueue_head(&ctx->wait);
-	for (i = 0; i < ARRAY_SIZE(ctx->pending_async); i++) {
-		spin_lock_init(&ctx->pending_async[i].lock);
-		INIT_LIST_HEAD(&ctx->pending_async[i].list);
-		atomic_set(&ctx->pending_async[i].cnt, 0);
-	}
 	spin_lock_init(&ctx->completion_lock);
 	INIT_LIST_HEAD(&ctx->poll_list);
 	INIT_LIST_HEAD(&ctx->cancel_list);
@@ -477,22 +459,37 @@ static void __io_commit_cqring(struct io_ring_ctx *ctx)
 	}
 }
 
+static inline bool io_sqe_needs_user(const struct io_uring_sqe *sqe)
+{
+	u8 opcode = READ_ONCE(sqe->opcode);
+
+	return !(opcode == IORING_OP_READ_FIXED ||
+		 opcode == IORING_OP_WRITE_FIXED);
+}
+
 static inline void io_queue_async_work(struct io_ring_ctx *ctx,
 				       struct io_kiocb *req)
 {
-	int rw = 0;
+	bool do_hashed = false;
 
 	if (req->submit.sqe) {
 		switch (req->submit.sqe->opcode) {
 		case IORING_OP_WRITEV:
 		case IORING_OP_WRITE_FIXED:
-			rw = !(req->rw.ki_flags & IOCB_DIRECT);
+			do_hashed = true;
 			break;
 		}
+		if (io_sqe_needs_user(req->submit.sqe))
+			req->work.flags |= IO_WQ_WORK_NEEDS_USER;
 	}
 
-	trace_io_uring_queue_async_work(ctx, rw, req, &req->work, req->flags);
-	queue_work(ctx->sqo_wq[rw], &req->work);
+	trace_io_uring_queue_async_work(ctx, 0, req, NULL, req->flags);
+	if (!do_hashed) {
+		io_wq_enqueue(ctx->io_wq, &req->work);
+	} else {
+		io_wq_enqueue_hashed(ctx->io_wq, &req->work,
+					file_inode(req->file));
+	}
 }
 
 static void io_kill_timeout(struct io_kiocb *req)
@@ -646,6 +643,7 @@ static struct io_kiocb *io_get_req(struct io_ring_ctx *ctx,
 	/* one is dropped after submission, the other at completion */
 	refcount_set(&req->refs, 2);
 	req->result = 0;
+	INIT_IO_WORK(&req->work, io_wq_submit_work);
 	return req;
 out:
 	percpu_ref_put(&ctx->refs);
@@ -692,12 +690,10 @@ static void io_req_link_next(struct io_kiocb *req, struct io_kiocb **nxtptr)
 		 * If we're in async work, we can continue processing the chain
 		 * in this context instead of having to queue up new async work.
 		 */
-		if (nxtptr && current_work()) {
+		if (nxtptr && current_work())
 			*nxtptr = nxt;
-		} else {
-			INIT_WORK(&nxt->work, io_sq_wq_submit_work);
+		else
 			io_queue_async_work(req->ctx, nxt);
-		}
 	}
 }
 
@@ -756,12 +752,10 @@ static void io_put_req(struct io_kiocb *req, struct io_kiocb **nxtptr)
 
 	nxt = io_put_req_find_next(req);
 	if (nxt) {
-		if (nxtptr) {
+		if (nxtptr)
 			*nxtptr = nxt;
-		} else {
-			INIT_WORK(&nxt->work, io_sq_wq_submit_work);
+		else
 			io_queue_async_work(nxt->ctx, nxt);
-		}
 	}
 }
 
@@ -1308,65 +1302,6 @@ static ssize_t io_import_iovec(struct io_ring_ctx *ctx, int rw,
 	return import_iovec(rw, buf, sqe_len, UIO_FASTIOV, iovec, iter);
 }
 
-static inline bool io_should_merge(struct async_list *al, struct kiocb *kiocb)
-{
-	if (al->file == kiocb->ki_filp) {
-		off_t start, end;
-
-		/*
-		 * Allow merging if we're anywhere in the range of the same
-		 * page. Generally this happens for sub-page reads or writes,
-		 * and it's beneficial to allow the first worker to bring the
-		 * page in and the piggy backed work can then work on the
-		 * cached page.
-		 */
-		start = al->io_start & PAGE_MASK;
-		end = (al->io_start + al->io_len + PAGE_SIZE - 1) & PAGE_MASK;
-		if (kiocb->ki_pos >= start && kiocb->ki_pos <= end)
-			return true;
-	}
-
-	al->file = NULL;
-	return false;
-}
-
-/*
- * Make a note of the last file/offset/direction we punted to async
- * context. We'll use this information to see if we can piggy back a
- * sequential request onto the previous one, if it's still hasn't been
- * completed by the async worker.
- */
-static void io_async_list_note(int rw, struct io_kiocb *req, size_t len)
-{
-	struct async_list *async_list = &req->ctx->pending_async[rw];
-	struct kiocb *kiocb = &req->rw;
-	struct file *filp = kiocb->ki_filp;
-
-	if (io_should_merge(async_list, kiocb)) {
-		unsigned long max_bytes;
-
-		/* Use 8x RA size as a decent limiter for both reads/writes */
-		max_bytes = filp->f_ra.ra_pages << (PAGE_SHIFT + 3);
-		if (!max_bytes)
-			max_bytes = VM_READAHEAD_PAGES << (PAGE_SHIFT + 3);
-
-		/* If max len are exceeded, reset the state */
-		if (async_list->io_len + len <= max_bytes) {
-			req->flags |= REQ_F_SEQ_PREV;
-			async_list->io_len += len;
-		} else {
-			async_list->file = NULL;
-		}
-	}
-
-	/* New file? Reset state. */
-	if (async_list->file != filp) {
-		async_list->io_start = kiocb->ki_pos;
-		async_list->io_len = len;
-		async_list->file = filp;
-	}
-}
-
 /*
  * For files that don't have ->read_iter() and ->write_iter(), handle them
  * by looping over ->read() or ->write() manually.
@@ -1461,13 +1396,10 @@ static int io_read(struct io_kiocb *req, const struct sqe_submit *s,
 		    ret2 > 0 && ret2 < read_size)
 			ret2 = -EAGAIN;
 		/* Catch -EAGAIN return for forced non-blocking submission */
-		if (!force_nonblock || ret2 != -EAGAIN) {
+		if (!force_nonblock || ret2 != -EAGAIN)
 			kiocb_done(kiocb, ret2, nxt, s->in_async);
-		} else {
-			if (!s->in_async)
-				io_async_list_note(READ, req, iov_count);
+		else
 			ret = -EAGAIN;
-		}
 	}
 	kfree(iovec);
 	return ret;
@@ -1501,11 +1433,8 @@ static int io_write(struct io_kiocb *req, const struct sqe_submit *s,
 	iov_count = iov_iter_count(&iter);
 
 	ret = -EAGAIN;
-	if (force_nonblock && !(kiocb->ki_flags & IOCB_DIRECT)) {
-		if (!s->in_async)
-			io_async_list_note(WRITE, req, iov_count);
+	if (force_nonblock && !(kiocb->ki_flags & IOCB_DIRECT))
 		goto out_free;
-	}
 
 	ret = rw_verify_area(WRITE, file, &kiocb->ki_pos, iov_count);
 	if (!ret) {
@@ -1530,13 +1459,10 @@ static int io_write(struct io_kiocb *req, const struct sqe_submit *s,
 			ret2 = call_write_iter(file, kiocb, &iter);
 		else
 			ret2 = loop_rw_iter(WRITE, file, kiocb, &iter);
-		if (!force_nonblock || ret2 != -EAGAIN) {
+		if (!force_nonblock || ret2 != -EAGAIN)
 			kiocb_done(kiocb, ret2, nxt, s->in_async);
-		} else {
-			if (!s->in_async)
-				io_async_list_note(WRITE, req, iov_count);
+		else
 			ret = -EAGAIN;
-		}
 	}
 out_free:
 	kfree(iovec);
@@ -1778,8 +1704,9 @@ static void io_poll_complete(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	io_commit_cqring(ctx);
 }
 
-static void io_poll_complete_work(struct work_struct *work)
+static void io_poll_complete_work(struct io_wq_work **workptr)
 {
+	struct io_wq_work *work = *workptr;
 	struct io_kiocb *req = container_of(work, struct io_kiocb, work);
 	struct io_poll_iocb *poll = &req->poll;
 	struct poll_table_struct pt = { ._key = poll->events };
@@ -1878,7 +1805,7 @@ static int io_poll_add(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		return -EBADF;
 
 	req->submit.sqe = NULL;
-	INIT_WORK(&req->work, io_poll_complete_work);
+	INIT_IO_WORK(&req->work, io_poll_complete_work);
 	events = READ_ONCE(sqe->poll_events);
 	poll->events = demangle_poll(events) | EPOLLERR | EPOLLHUP;
 
@@ -2136,7 +2063,6 @@ static int io_req_defer(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	memcpy(sqe_copy, sqe, sizeof(*sqe_copy));
 	req->submit.sqe = sqe_copy;
 
-	INIT_WORK(&req->work, io_sq_wq_submit_work);
 	trace_io_uring_defer(ctx, req, false);
 	list_add_tail(&req->list, &ctx->defer_list);
 	spin_unlock_irq(&ctx->completion_lock);
@@ -2222,186 +2148,52 @@ static int __io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	return 0;
 }
 
-static struct async_list *io_async_list_from_sqe(struct io_ring_ctx *ctx,
-						 const struct io_uring_sqe *sqe)
-{
-	switch (sqe->opcode) {
-	case IORING_OP_READV:
-	case IORING_OP_READ_FIXED:
-		return &ctx->pending_async[READ];
-	case IORING_OP_WRITEV:
-	case IORING_OP_WRITE_FIXED:
-		return &ctx->pending_async[WRITE];
-	default:
-		return NULL;
-	}
-}
-
-static inline bool io_sqe_needs_user(const struct io_uring_sqe *sqe)
-{
-	u8 opcode = READ_ONCE(sqe->opcode);
-
-	return !(opcode == IORING_OP_READ_FIXED ||
-		 opcode == IORING_OP_WRITE_FIXED);
-}
-
-static void io_sq_wq_submit_work(struct work_struct *work)
+static void io_wq_submit_work(struct io_wq_work **workptr)
 {
+	struct io_wq_work *work = *workptr;
 	struct io_kiocb *req = container_of(work, struct io_kiocb, work);
 	struct io_ring_ctx *ctx = req->ctx;
-	struct mm_struct *cur_mm = NULL;
-	struct async_list *async_list;
-	LIST_HEAD(req_list);
-	mm_segment_t old_fs;
-	int ret;
+	struct sqe_submit *s = &req->submit;
+	const struct io_uring_sqe *sqe = s->sqe;
+	struct io_kiocb *nxt = NULL;
+	int ret = 0;
 
-	async_list = io_async_list_from_sqe(ctx, req->submit.sqe);
-restart:
-	do {
-		struct sqe_submit *s = &req->submit;
-		const struct io_uring_sqe *sqe = s->sqe;
-		unsigned int flags = req->flags;
-		struct io_kiocb *nxt = NULL;
+	/* Ensure we clear previously set non-block flag */
+	req->rw.ki_flags &= ~IOCB_NOWAIT;
 
-		/* Ensure we clear previously set non-block flag */
-		req->rw.ki_flags &= ~IOCB_NOWAIT;
+	if (work->flags & IO_WQ_WORK_CANCEL)
+		ret = -ECANCELED;
 
-		ret = 0;
-		if (io_sqe_needs_user(sqe) && !cur_mm) {
-			if (!mmget_not_zero(ctx->sqo_mm)) {
-				ret = -EFAULT;
-			} else {
-				cur_mm = ctx->sqo_mm;
-				use_mm(cur_mm);
-				old_fs = get_fs();
-				set_fs(USER_DS);
-			}
-		}
+	if (!ret) {
+		s->has_user = (work->flags & IO_WQ_WORK_HAS_MM) != 0;
+		s->in_async = true;
+		do {
+			ret = __io_submit_sqe(ctx, req, s, &nxt, false);
+			/*
+			 * We can get EAGAIN for polled IO even though we're
+			 * forcing a sync submission from here, since we can't
+			 * wait for request slots on the block side.
+			 */
+			if (ret != -EAGAIN)
+				break;
+			cond_resched();
+		} while (1);
+	}
 
-		if (!ret) {
-			s->has_user = cur_mm != NULL;
-			s->in_async = true;
-			do {
-				ret = __io_submit_sqe(ctx, req, s, &nxt, false);
-				/*
-				 * We can get EAGAIN for polled IO even though
-				 * we're forcing a sync submission from here,
-				 * since we can't wait for request slots on the
-				 * block side.
-				 */
-				if (ret != -EAGAIN)
-					break;
-				cond_resched();
-			} while (1);
-		}
+	/* drop submission reference */
+	io_put_req(req, NULL);
 
-		/* drop submission reference */
+	if (ret) {
+		io_cqring_add_event(ctx, sqe->user_data, ret);
 		io_put_req(req, NULL);
-
-		if (ret) {
-			io_cqring_add_event(ctx, sqe->user_data, ret);
-			io_put_req(req, NULL);
-		}
-
-		/* async context always use a copy of the sqe */
-		kfree(sqe);
-
-		/* if a dependent link is ready, do that as the next one */
-		if (!ret && nxt) {
-			req = nxt;
-			continue;
-		}
-
-		/* req from defer and link list needn't decrease async cnt */
-		if (flags & (REQ_F_IO_DRAINED | REQ_F_LINK_DONE))
-			goto out;
-
-		if (!async_list)
-			break;
-		if (!list_empty(&req_list)) {
-			req = list_first_entry(&req_list, struct io_kiocb,
-						list);
-			list_del(&req->list);
-			continue;
-		}
-		if (list_empty(&async_list->list))
-			break;
-
-		req = NULL;
-		spin_lock(&async_list->lock);
-		if (list_empty(&async_list->list)) {
-			spin_unlock(&async_list->lock);
-			break;
-		}
-		list_splice_init(&async_list->list, &req_list);
-		spin_unlock(&async_list->lock);
-
-		req = list_first_entry(&req_list, struct io_kiocb, list);
-		list_del(&req->list);
-	} while (req);
-
-	/*
-	 * Rare case of racing with a submitter. If we find the count has
-	 * dropped to zero AND we have pending work items, then restart
-	 * the processing. This is a tiny race window.
-	 */
-	if (async_list) {
-		ret = atomic_dec_return(&async_list->cnt);
-		while (!ret && !list_empty(&async_list->list)) {
-			spin_lock(&async_list->lock);
-			atomic_inc(&async_list->cnt);
-			list_splice_init(&async_list->list, &req_list);
-			spin_unlock(&async_list->lock);
-
-			if (!list_empty(&req_list)) {
-				req = list_first_entry(&req_list,
-							struct io_kiocb, list);
-				list_del(&req->list);
-				goto restart;
-			}
-			ret = atomic_dec_return(&async_list->cnt);
-		}
-	}
-
-out:
-	if (cur_mm) {
-		set_fs(old_fs);
-		unuse_mm(cur_mm);
-		mmput(cur_mm);
 	}
-}
-
-/*
- * See if we can piggy back onto previously submitted work, that is still
- * running. We currently only allow this if the new request is sequential
- * to the previous one we punted.
- */
-static bool io_add_to_prev_work(struct async_list *list, struct io_kiocb *req)
-{
-	bool ret;
 
-	if (!list)
-		return false;
-	if (!(req->flags & REQ_F_SEQ_PREV))
-		return false;
-	if (!atomic_read(&list->cnt))
-		return false;
+	/* async context always use a copy of the sqe */
+	kfree(sqe);
 
-	ret = true;
-	spin_lock(&list->lock);
-	list_add_tail(&req->list, &list->list);
-	/*
-	 * Ensure we see a simultaneous modification from io_sq_wq_submit_work()
-	 */
-	smp_mb();
-	if (!atomic_read(&list->cnt)) {
-		list_del_init(&req->list);
-		ret = false;
-	}
-	spin_unlock(&list->lock);
-
-	trace_io_uring_add_to_prev(req, ret);
-	return ret;
+	/* if a dependent link is ready, pass it back */
+	if (!ret && nxt)
+		*workptr = &nxt->work;
 }
 
 static bool io_op_needs_file(const struct io_uring_sqe *sqe)
@@ -2475,17 +2267,9 @@ static int __io_queue_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
 
 		sqe_copy = kmemdup(s->sqe, sizeof(*sqe_copy), GFP_KERNEL);
 		if (sqe_copy) {
-			struct async_list *list;
-
 			s->sqe = sqe_copy;
 			memcpy(&req->submit, s, sizeof(*s));
-			list = io_async_list_from_sqe(ctx, s->sqe);
-			if (!io_add_to_prev_work(list, req)) {
-				if (list)
-					atomic_inc(&list->cnt);
-				INIT_WORK(&req->work, io_sq_wq_submit_work);
-				io_queue_async_work(ctx, req);
-			}
+			io_queue_async_work(ctx, req);
 
 			/*
 			 * Queued up for async execution, worker will release
@@ -3090,15 +2874,11 @@ static void io_sq_thread_stop(struct io_ring_ctx *ctx)
 
 static void io_finish_async(struct io_ring_ctx *ctx)
 {
-	int i;
-
 	io_sq_thread_stop(ctx);
 
-	for (i = 0; i < ARRAY_SIZE(ctx->sqo_wq); i++) {
-		if (ctx->sqo_wq[i]) {
-			destroy_workqueue(ctx->sqo_wq[i]);
-			ctx->sqo_wq[i] = NULL;
-		}
+	if (ctx->io_wq) {
+		io_wq_destroy(ctx->io_wq);
+		ctx->io_wq = NULL;
 	}
 }
 
@@ -3106,11 +2886,9 @@ static void io_finish_async(struct io_ring_ctx *ctx)
 static void io_destruct_skb(struct sk_buff *skb)
 {
 	struct io_ring_ctx *ctx = skb->sk->sk_user_data;
-	int i;
 
-	for (i = 0; i < ARRAY_SIZE(ctx->sqo_wq); i++)
-		if (ctx->sqo_wq[i])
-			flush_workqueue(ctx->sqo_wq[i]);
+	if (ctx->io_wq)
+		io_wq_flush(ctx->io_wq);
 
 	unix_destruct_scm(skb);
 }
@@ -3454,6 +3232,7 @@ static int io_sqe_files_update(struct io_ring_ctx *ctx, void __user *arg,
 static int io_sq_offload_start(struct io_ring_ctx *ctx,
 			       struct io_uring_params *p)
 {
+	unsigned concurrency;
 	int ret;
 
 	init_waitqueue_head(&ctx->sqo_wait);
@@ -3497,25 +3276,10 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
 		goto err;
 	}
 
-	/* Do QD, or 2 * CPUS, whatever is smallest */
-	ctx->sqo_wq[0] = alloc_workqueue("io_ring-wq",
-			WQ_UNBOUND | WQ_FREEZABLE,
-			min(ctx->sq_entries - 1, 2 * num_online_cpus()));
-	if (!ctx->sqo_wq[0]) {
-		ret = -ENOMEM;
-		goto err;
-	}
-
-	/*
-	 * This is for buffered writes, where we want to limit the parallelism
-	 * due to file locking in file systems. As "normal" buffered writes
-	 * should parellelize on writeout quite nicely, limit us to having 2
-	 * pending. This avoids massive contention on the inode when doing
-	 * buffered async writes.
-	 */
-	ctx->sqo_wq[1] = alloc_workqueue("io_ring-write-wq",
-						WQ_UNBOUND | WQ_FREEZABLE, 2);
-	if (!ctx->sqo_wq[1]) {
+	/* Do QD, or 4 * CPUS, whatever is smallest */
+	concurrency = min(ctx->sq_entries, 4 * num_online_cpus());
+	ctx->io_wq = io_wq_create(concurrency, ctx->sqo_mm);
+	if (!ctx->io_wq) {
 		ret = -ENOMEM;
 		goto err;
 	}
@@ -3900,6 +3664,10 @@ static void io_ring_ctx_wait_and_kill(struct io_ring_ctx *ctx)
 
 	io_kill_timeouts(ctx);
 	io_poll_remove_all(ctx);
+
+	if (ctx->io_wq)
+		io_wq_cancel_all(ctx->io_wq);
+
 	io_iopoll_reap_events(ctx);
 	wait_for_completion(&ctx->ctx_done);
 	io_ring_ctx_free(ctx);
diff --git a/init/Kconfig b/init/Kconfig
index b4daad2bac23..4d8d145c41d2 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1548,6 +1548,7 @@ config AIO
 config IO_URING
 	bool "Enable IO uring support" if EXPERT
 	select ANON_INODES
+	select IO_WQ
 	default y
 	help
 	  This option enables support for the io_uring interface, enabling
-- 
2.17.1

[PATCHSET v2 0/2] Replace io_uring workqueues with io-wq

[Jens Axboe]

Hopefully the sched core changes are palatable. io-wq uses the same
sched in/out hooks as workqueue, and if the task isn't either a
workqueue or io-wq worker, there should be no extra overhead there.

Patches are on top of my for-5.5/io_uring branch, and can also be found
here:

http://git.kernel.dk/cgit/linux-block/log/?h=for-5.5/io_uring-wq

This passes io_uring IO testing and repeated runs of the liburing
regressions suite, and passes tests that would previously deadlock.

Also see LWN article here:

https://lwn.net/Articles/803070/

Changes since v1:

- Fix dependent work hash
- Turn wqe->state into wqe->flags
- Fix issue with unusing mm/files if worker got busy at exit timo
- Fix issue with hashed work bit mangling (Colin)
- Fix issue with dependent work
- Rebase

 fs/Kconfig                      |   3 +
 fs/Makefile                     |   1 +
 fs/io-wq.c                      | 805 ++++++++++++++++++++++++++++++++
 fs/io-wq.h                      |  55 +++
 fs/io_uring.c                   | 414 ++++------------
 include/linux/sched.h           |   1 +
 include/trace/events/io_uring.h |  12 +-
 init/Kconfig                    |   1 +
 kernel/sched/core.c             |  16 +-
 9 files changed, 981 insertions(+), 327 deletions(-)

-- 
Jens Axboe

[PATCH 1/2] io-wq: small threadpool implementation for io_uring

[Jens Axboe]

This adds support for io-wq, a smaller and specialized thread pool
implementation. This is meant to replace workqueues for io_uring. Among
the reasons for this addition are:

- We can assign memory context smarter and more persistently if we
  manage the life time of threads.

- We can drop various work-arounds we have in io_uring, like the
  async_list.

- We can implement hashed work insertion, to manage concurrency of
  buffered writes without needing a) an extra workqueue, or b)
  needlessly making the concurrency of said workqueue very low
  which hurts performance of multiple buffered file writers.

- We can implement cancel through signals, for cancelling
  interruptible work like read/write (or send/recv) to/from sockets.

- We need the above cancel for being able to assign and use file tables
  from a process.

- We can implement a more thorough cancel operation in general.

- We need it to move towards a syslet/threadlet model for even faster
  async execution. For that we need to take ownership of the used
  threads.

This list is just off the top of my head. Performance should be the
same, or better, at least that's what I've seen in my testing. io-wq
supports basic NUMA functionality, setting up a pool per node.

io-wq hooks up to the scheduler schedule in/out just like workqueue
and uses that to drive the need for more/less workers.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/Kconfig            |   3 +
 fs/Makefile           |   1 +
 fs/io-wq.c            | 805 ++++++++++++++++++++++++++++++++++++++++++
 fs/io-wq.h            |  55 +++
 include/linux/sched.h |   1 +
 kernel/sched/core.c   |  16 +-
 6 files changed, 877 insertions(+), 4 deletions(-)
 create mode 100644 fs/io-wq.c
 create mode 100644 fs/io-wq.h

diff --git a/fs/Kconfig b/fs/Kconfig
index 2501e6f1f965..7b623e9fc1b0 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -322,4 +322,7 @@ source "fs/nls/Kconfig"
 source "fs/dlm/Kconfig"
 source "fs/unicode/Kconfig"
 
+config IO_WQ
+	bool
+
 endmenu
diff --git a/fs/Makefile b/fs/Makefile
index 14231b4cf383..1148c555c4d3 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -32,6 +32,7 @@ obj-$(CONFIG_EVENTFD)		+= eventfd.o
 obj-$(CONFIG_USERFAULTFD)	+= userfaultfd.o
 obj-$(CONFIG_AIO)               += aio.o
 obj-$(CONFIG_IO_URING)		+= io_uring.o
+obj-$(CONFIG_IO_WQ)		+= io-wq.o
 obj-$(CONFIG_FS_DAX)		+= dax.o
 obj-$(CONFIG_FS_ENCRYPTION)	+= crypto/
 obj-$(CONFIG_FS_VERITY)		+= verity/
diff --git a/fs/io-wq.c b/fs/io-wq.c
new file mode 100644
index 000000000000..99ac5e338d99
--- /dev/null
+++ b/fs/io-wq.c
@@ -0,0 +1,805 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Basic worker thread pool for io_uring
+ *
+ * Copyright (C) 2019 Jens Axboe
+ *
+ */
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/errno.h>
+#include <linux/sched/signal.h>
+#include <linux/mm.h>
+#include <linux/mmu_context.h>
+#include <linux/sched/mm.h>
+#include <linux/percpu.h>
+#include <linux/slab.h>
+#include <linux/kthread.h>
+#include <linux/rculist_nulls.h>
+
+#include "io-wq.h"
+
+#define WORKER_IDLE_TIMEOUT	(5 * HZ)
+
+enum {
+	IO_WORKER_F_UP		= 1,	/* up and active */
+	IO_WORKER_F_RUNNING	= 2,	/* account as running */
+	IO_WORKER_F_FREE	= 4,	/* worker on free list */
+	IO_WORKER_F_EXITING	= 8,	/* worker exiting */
+	IO_WORKER_F_FIXED	= 16,	/* static idle worker */
+};
+
+enum {
+	IO_WQ_BIT_EXIT		= 0,	/* wq exiting */
+	IO_WQ_BIT_CANCEL	= 1,	/* cancel work on list */
+};
+
+enum {
+	IO_WQE_FLAG_STALLED	= 1,	/* stalled on hash */
+};
+
+/*
+ * One for each thread in a wqe pool
+ */
+struct io_worker {
+	refcount_t ref;
+	unsigned flags;
+	struct hlist_nulls_node nulls_node;
+	struct task_struct *task;
+	wait_queue_head_t wait;
+	struct io_wqe *wqe;
+	struct io_wq_work *cur_work;
+
+	struct rcu_head rcu;
+	struct mm_struct *mm;
+};
+
+struct io_wq_nulls_list {
+	struct hlist_nulls_head head;
+	unsigned long nulls;
+};
+
+#if BITS_PER_LONG == 64
+#define IO_WQ_HASH_ORDER	6
+#else
+#define IO_WQ_HASH_ORDER	5
+#endif
+
+/*
+ * Per-node worker thread pool
+ */
+struct io_wqe {
+	struct {
+		spinlock_t lock;
+		struct list_head work_list;
+		unsigned long hash_map;
+		unsigned flags;
+	} ____cacheline_aligned_in_smp;
+
+	int node;
+	unsigned nr_workers;
+	unsigned max_workers;
+	atomic_t nr_running;
+
+	struct io_wq_nulls_list free_list;
+	struct io_wq_nulls_list busy_list;
+
+	struct io_wq *wq;
+};
+
+/*
+ * Per io_wq state
+  */
+struct io_wq {
+	struct io_wqe **wqes;
+	unsigned long state;
+	unsigned nr_wqes;
+
+	struct task_struct *manager;
+	struct mm_struct *mm;
+	refcount_t refs;
+	struct completion done;
+};
+
+static void io_wq_free_worker(struct rcu_head *head)
+{
+	struct io_worker *worker = container_of(head, struct io_worker, rcu);
+
+	kfree(worker);
+}
+
+static bool io_worker_get(struct io_worker *worker)
+{
+	return refcount_inc_not_zero(&worker->ref);
+}
+
+static void io_worker_release(struct io_worker *worker)
+{
+	if (refcount_dec_and_test(&worker->ref))
+		wake_up_process(worker->task);
+}
+
+/*
+ * Note: drops the wqe->lock if returning true! The caller must re-acquire
+ * the lock in that case. Some callers need to restart handling if this
+ * happens, so we can't just re-acquire the lock on behalf of the caller.
+ */
+static bool __io_worker_unuse(struct io_wqe *wqe, struct io_worker *worker)
+	__must_hold(wqe->lock)
+	__releases(wqe->lock)
+{
+	/*
+	 * If we have an active mm, we need to drop the wq lock before unusing
+	 * it. If we do, return true and let the caller retry the idle loop.
+	 */
+	if (worker->mm) {
+		spin_unlock(&wqe->lock);
+		__set_current_state(TASK_RUNNING);
+		set_fs(KERNEL_DS);
+		unuse_mm(worker->mm);
+		mmput(worker->mm);
+		worker->mm = NULL;
+		return true;
+	}
+
+	return false;
+}
+
+static void io_worker_exit(struct io_worker *worker)
+{
+	struct io_wqe *wqe = worker->wqe;
+	bool all_done = false;
+
+	/*
+	 * If we're not at zero, someone else is holding a brief reference
+	 * to the worker. Wait for that to go away.
+	 */
+	set_current_state(TASK_INTERRUPTIBLE);
+	if (!refcount_dec_and_test(&worker->ref))
+		schedule();
+	__set_current_state(TASK_RUNNING);
+
+	preempt_disable();
+	current->flags &= ~PF_IO_WORKER;
+	if (worker->flags & IO_WORKER_F_RUNNING)
+		atomic_dec(&wqe->nr_running);
+	worker->flags = 0;
+	preempt_enable();
+
+	spin_lock(&wqe->lock);
+	hlist_nulls_del_rcu(&worker->nulls_node);
+	if (__io_worker_unuse(wqe, worker))
+		spin_lock(&wqe->lock);
+	wqe->nr_workers--;
+	all_done = !wqe->nr_workers;
+	spin_unlock(&wqe->lock);
+
+	/* all workers gone, wq exit can proceed */
+	if (all_done && refcount_dec_and_test(&wqe->wq->refs))
+		complete(&wqe->wq->done);
+
+	call_rcu(&worker->rcu, io_wq_free_worker);
+}
+
+static void io_worker_start(struct io_wqe *wqe, struct io_worker *worker)
+{
+	allow_kernel_signal(SIGINT);
+
+	current->flags |= PF_IO_WORKER;
+
+	worker->flags |= (IO_WORKER_F_UP | IO_WORKER_F_RUNNING);
+	atomic_inc(&wqe->nr_running);
+}
+
+/*
+ * Worker will start processing some work. Move it to the busy list, if
+ * it's currently on the freelist
+ */
+static void __io_worker_busy(struct io_wqe *wqe, struct io_worker *worker,
+			     struct io_wq_work *work)
+	__must_hold(wqe->lock)
+{
+	if (worker->flags & IO_WORKER_F_FREE) {
+		worker->flags &= ~IO_WORKER_F_FREE;
+		hlist_nulls_del_init_rcu(&worker->nulls_node);
+		hlist_nulls_add_head_rcu(&worker->nulls_node,
+						&wqe->busy_list.head);
+	}
+	worker->cur_work = work;
+}
+
+/*
+ * No work, worker going to sleep. Move to freelist, and unuse mm if we
+ * have one attached. Dropping the mm may potentially sleep, so we drop
+ * the lock in that case and return success. Since the caller has to
+ * retry the loop in that case (we changed task state), we don't regrab
+ * the lock if we return success.
+ */
+static bool __io_worker_idle(struct io_wqe *wqe, struct io_worker *worker)
+{
+	if (!(worker->flags & IO_WORKER_F_FREE)) {
+		worker->flags |= IO_WORKER_F_FREE;
+		hlist_nulls_del_init_rcu(&worker->nulls_node);
+		hlist_nulls_add_head_rcu(&worker->nulls_node,
+						&wqe->free_list.head);
+	}
+
+	return __io_worker_unuse(wqe, worker);
+}
+
+static struct io_wq_work *io_get_next_work(struct io_wqe *wqe, unsigned *hash)
+	__must_hold(wqe->lock)
+{
+	struct io_wq_work *work;
+
+	list_for_each_entry(work, &wqe->work_list, list) {
+		/* not hashed, can run anytime */
+		if (!(work->flags & IO_WQ_WORK_HASHED)) {
+			list_del(&work->list);
+			return work;
+		}
+
+		/* hashed, can run if not already running */
+		*hash = work->flags >> IO_WQ_HASH_SHIFT;
+		if (!(wqe->hash_map & BIT_ULL(*hash))) {
+			wqe->hash_map |= BIT_ULL(*hash);
+			list_del(&work->list);
+			return work;
+		}
+	}
+
+	return NULL;
+}
+
+static void io_worker_handle_work(struct io_worker *worker)
+	__releases(wqe->lock)
+{
+	struct io_wq_work *work, *old_work;
+	struct io_wqe *wqe = worker->wqe;
+	struct io_wq *wq = wqe->wq;
+
+	do {
+		unsigned hash = -1U;
+
+		/*
+		 * If we got some work, mark us as busy. If we didn't, but
+		 * the list isn't empty, it means we stalled on hashed work.
+		 * Mark us stalled so we don't keep looking for work when we
+		 * can't make progress, any work completion or insertion will
+		 * clear the stalled flag.
+		 */
+		work = io_get_next_work(wqe, &hash);
+		if (work)
+			__io_worker_busy(wqe, worker, work);
+		else if (!list_empty(&wqe->work_list))
+			wqe->flags |= IO_WQE_FLAG_STALLED;
+
+		spin_unlock(&wqe->lock);
+		if (!work)
+			break;
+next:
+		if ((work->flags & IO_WQ_WORK_NEEDS_USER) && !worker->mm &&
+		    wq->mm && mmget_not_zero(wq->mm)) {
+			use_mm(wq->mm);
+			set_fs(USER_DS);
+			worker->mm = wq->mm;
+		}
+		if (test_bit(IO_WQ_BIT_CANCEL, &wq->state))
+			work->flags |= IO_WQ_WORK_CANCEL;
+		if (worker->mm)
+			work->flags |= IO_WQ_WORK_HAS_MM;
+
+		old_work = work;
+		work->func(&work);
+
+		spin_lock(&wqe->lock);
+		worker->cur_work = NULL;
+		if (hash != -1U) {
+			wqe->hash_map &= ~BIT_ULL(hash);
+			wqe->flags &= ~IO_WQE_FLAG_STALLED;
+		}
+		if (work && work != old_work) {
+			spin_unlock(&wqe->lock);
+			/* dependent work not hashed */
+			hash = -1U;
+			goto next;
+		}
+	} while (1);
+}
+
+static inline bool io_wqe_run_queue(struct io_wqe *wqe)
+	__must_hold(wqe->lock)
+{
+	if (!list_empty_careful(&wqe->work_list) &&
+	    !(wqe->flags & IO_WQE_FLAG_STALLED))
+		return true;
+	return false;
+}
+
+static int io_wqe_worker(void *data)
+{
+	struct io_worker *worker = data;
+	struct io_wqe *wqe = worker->wqe;
+	struct io_wq *wq = wqe->wq;
+	DEFINE_WAIT(wait);
+
+	io_worker_start(wqe, worker);
+
+	while (!test_bit(IO_WQ_BIT_EXIT, &wq->state)) {
+		prepare_to_wait(&worker->wait, &wait, TASK_INTERRUPTIBLE);
+
+		spin_lock(&wqe->lock);
+		if (io_wqe_run_queue(wqe)) {
+			__set_current_state(TASK_RUNNING);
+			io_worker_handle_work(worker);
+			continue;
+		}
+		/* drops the lock on success, retry */
+		if (__io_worker_idle(wqe, worker))
+			continue;
+		spin_unlock(&wqe->lock);
+		if (signal_pending(current))
+			flush_signals(current);
+		if (schedule_timeout(WORKER_IDLE_TIMEOUT))
+			continue;
+		/* timed out, exit unless we're the fixed worker */
+		if (test_bit(IO_WQ_BIT_EXIT, &wq->state) ||
+		    !(worker->flags & IO_WORKER_F_FIXED))
+			break;
+	}
+
+	finish_wait(&worker->wait, &wait);
+
+	if (test_bit(IO_WQ_BIT_EXIT, &wq->state)) {
+		spin_lock(&wqe->lock);
+		if (!list_empty(&wqe->work_list))
+			io_worker_handle_work(worker);
+		else
+			spin_unlock(&wqe->lock);
+	}
+
+	io_worker_exit(worker);
+	return 0;
+}
+
+/*
+ * Check head of free list for an available worker. If one isn't available,
+ * caller must wake up the wq manager to create one.
+ */
+static bool io_wqe_activate_free_worker(struct io_wqe *wqe)
+	__must_hold(RCU)
+{
+	struct hlist_nulls_node *n;
+	struct io_worker *worker;
+
+	n = rcu_dereference(hlist_nulls_first_rcu(&wqe->free_list.head));
+	if (is_a_nulls(n))
+		return false;
+
+	worker = hlist_nulls_entry(n, struct io_worker, nulls_node);
+	if (io_worker_get(worker)) {
+		wake_up(&worker->wait);
+		io_worker_release(worker);
+		return true;
+	}
+
+	return false;
+}
+
+/*
+ * We need a worker. If we find a free one, we're good. If not, and we're
+ * below the max number of workers, wake up the manager to create one.
+ */
+static void io_wqe_wake_worker(struct io_wqe *wqe)
+{
+	bool ret;
+
+	rcu_read_lock();
+	ret = io_wqe_activate_free_worker(wqe);
+	rcu_read_unlock();
+
+	if (!ret && wqe->nr_workers < wqe->max_workers)
+		wake_up_process(wqe->wq->manager);
+}
+
+/*
+ * Called when a worker is scheduled in. Mark us as currently running.
+ */
+void io_wq_worker_running(struct task_struct *tsk)
+{
+	struct io_worker *worker = kthread_data(tsk);
+	struct io_wqe *wqe = worker->wqe;
+
+	if (!(worker->flags & IO_WORKER_F_UP))
+		return;
+	if (worker->flags & IO_WORKER_F_RUNNING)
+		return;
+	worker->flags |= IO_WORKER_F_RUNNING;
+	atomic_inc(&wqe->nr_running);
+}
+
+/*
+ * Called when worker is going to sleep. If there are no workers currently
+ * running and we have work pending, wake up a free one or have the manager
+ * set one up.
+ */
+void io_wq_worker_sleeping(struct task_struct *tsk)
+{
+	struct io_worker *worker = kthread_data(tsk);
+	struct io_wqe *wqe = worker->wqe;
+
+	if (!(worker->flags & IO_WORKER_F_UP))
+		return;
+	if (!(worker->flags & IO_WORKER_F_RUNNING))
+		return;
+
+	worker->flags &= ~IO_WORKER_F_RUNNING;
+
+	if (atomic_dec_and_test(&wqe->nr_running) && io_wqe_run_queue(wqe))
+		io_wqe_wake_worker(wqe);
+}
+
+static void create_io_worker(struct io_wq *wq, struct io_wqe *wqe)
+{
+	struct io_worker *worker;
+
+	worker = kcalloc_node(1, sizeof(*worker), GFP_KERNEL, wqe->node);
+	if (!worker)
+		return;
+
+	refcount_set(&worker->ref, 1);
+	worker->nulls_node.pprev = NULL;
+	init_waitqueue_head(&worker->wait);
+	worker->wqe = wqe;
+
+	worker->task = kthread_create_on_node(io_wqe_worker, worker, wqe->node,
+						"io_wqe_worker-%d", wqe->node);
+	if (IS_ERR(worker->task)) {
+		kfree(worker);
+		return;
+	}
+
+	spin_lock(&wqe->lock);
+	hlist_nulls_add_head_rcu(&worker->nulls_node, &wqe->free_list.head);
+	worker->flags |= IO_WORKER_F_FREE;
+	if (!wqe->nr_workers)
+		worker->flags |= IO_WORKER_F_FIXED;
+	wqe->nr_workers++;
+	spin_unlock(&wqe->lock);
+
+	wake_up_process(worker->task);
+}
+
+static inline bool io_wqe_need_new_worker(struct io_wqe *wqe)
+	__must_hold(wqe->lock)
+{
+	if (!wqe->nr_workers)
+		return true;
+	if (hlist_nulls_empty(&wqe->free_list.head) &&
+	    wqe->nr_workers < wqe->max_workers && io_wqe_run_queue(wqe))
+		return true;
+
+	return false;
+}
+
+/*
+ * Manager thread. Tasked with creating new workers, if we need them.
+ */
+static int io_wq_manager(void *data)
+{
+	struct io_wq *wq = data;
+
+	while (!kthread_should_stop()) {
+		int i;
+
+		for (i = 0; i < wq->nr_wqes; i++) {
+			struct io_wqe *wqe = wq->wqes[i];
+			bool fork_worker = false;
+
+			spin_lock(&wqe->lock);
+			fork_worker = io_wqe_need_new_worker(wqe);
+			spin_unlock(&wqe->lock);
+			if (fork_worker)
+				create_io_worker(wq, wqe);
+		}
+		set_current_state(TASK_INTERRUPTIBLE);
+		if (schedule_timeout(HZ))
+			continue;
+#ifdef IO_WQ_MANAGER_DEBUG
+		printk("manager: now workers=%d/running=%d/work_empty=%d\n", wq->nr_workers, atomic_read(&wq->nr_running), list_empty(&wq->work_list));
+#endif
+	}
+
+	return 0;
+}
+
+static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
+{
+	spin_lock(&wqe->lock);
+	list_add_tail(&work->list, &wqe->work_list);
+	wqe->flags &= ~IO_WQE_FLAG_STALLED;
+	spin_unlock(&wqe->lock);
+
+	if (!atomic_read(&wqe->nr_running))
+		io_wqe_wake_worker(wqe);
+}
+
+void io_wq_enqueue(struct io_wq *wq, struct io_wq_work *work)
+{
+	struct io_wqe *wqe = wq->wqes[numa_node_id()];
+
+	io_wqe_enqueue(wqe, work);
+}
+
+/*
+ * Enqueue work, hashed by some key. Work items that hash to the same value
+ * will not be done in parallel. Used to limit concurrent writes, generally
+ * hashed by inode.
+ */
+void io_wq_enqueue_hashed(struct io_wq *wq, struct io_wq_work *work, void *val)
+{
+	struct io_wqe *wqe = wq->wqes[numa_node_id()];
+	unsigned bit;
+
+
+	bit = hash_ptr(val, IO_WQ_HASH_ORDER);
+	work->flags |= (IO_WQ_WORK_HASHED | (bit << IO_WQ_HASH_SHIFT));
+	io_wqe_enqueue(wqe, work);
+}
+
+static bool io_wqe_worker_send_sig(struct io_worker *worker, void *data)
+{
+	send_sig(SIGINT, worker->task, 1);
+	return false;
+}
+
+/*
+ * Iterate the passed in list and call the specific function for each
+ * worker that isn't exiting
+ */
+static bool io_wq_for_each_worker(struct io_wqe *wqe,
+				  struct io_wq_nulls_list *list,
+				  bool (*func)(struct io_worker *, void *),
+				  void *data)
+{
+	struct hlist_nulls_node *n;
+	struct io_worker *worker;
+	bool ret = false;
+
+restart:
+	hlist_nulls_for_each_entry_rcu(worker, n, &list->head, nulls_node) {
+		if (io_worker_get(worker)) {
+			ret = func(worker, data);
+			io_worker_release(worker);
+			if (ret)
+				break;
+		}
+	}
+	if (!ret && get_nulls_value(n) != list->nulls)
+		goto restart;
+	return ret;
+}
+
+void io_wq_cancel_all(struct io_wq *wq)
+{
+	int i;
+
+	set_bit(IO_WQ_BIT_CANCEL, &wq->state);
+
+	/*
+	 * Browse both lists, as there's a gap between handing work off
+	 * to a worker and the worker putting itself on the busy_list
+	 */
+	rcu_read_lock();
+	for (i = 0; i < wq->nr_wqes; i++) {
+		struct io_wqe *wqe = wq->wqes[i];
+
+		io_wq_for_each_worker(wqe, &wqe->busy_list,
+					io_wqe_worker_send_sig, NULL);
+		io_wq_for_each_worker(wqe, &wqe->free_list,
+					io_wqe_worker_send_sig, NULL);
+	}
+	rcu_read_unlock();
+}
+
+static bool io_wq_worker_cancel(struct io_worker *worker, void *data)
+{
+	struct io_wq_work *work = data;
+
+	if (worker->cur_work == work) {
+		send_sig(SIGINT, worker->task, 1);
+		return true;
+	}
+
+	return false;
+}
+
+static enum io_wq_cancel io_wqe_cancel_work(struct io_wqe *wqe,
+					    struct io_wq_work *cwork)
+{
+	struct io_wq_work *work;
+	bool found;
+
+	cwork->flags |= IO_WQ_WORK_CANCEL;
+
+	/*
+	 * First check pending list, if we're lucky we can just remove it
+	 * from there. CANCEL_OK means that the work is returned as-new,
+	 * no completion will be posted for it.
+	 */
+	spin_lock(&wqe->lock);
+	list_for_each_entry(work, &wqe->work_list, list) {
+		if (work == cwork) {
+			list_del(&work->list);
+			spin_unlock(&wqe->lock);
+			return IO_WQ_CANCEL_OK;
+		}
+	}
+	spin_unlock(&wqe->lock);
+
+	/*
+	 * Now check if a free (going busy) or busy worker has the work
+	 * currently running. If we find it there, we'll return CANCEL_RUNNING
+	 * as an indication that we attempte to signal cancellation. The
+	 * completion will run normally in this case.
+	 */
+	rcu_read_lock();
+	found = io_wq_for_each_worker(wqe, &wqe->free_list, io_wq_worker_cancel,
+					cwork);
+	if (found)
+		goto done;
+
+	found = io_wq_for_each_worker(wqe, &wqe->busy_list, io_wq_worker_cancel,
+					cwork);
+done:
+	rcu_read_unlock();
+	return found ? IO_WQ_CANCEL_RUNNING : IO_WQ_CANCEL_NOTFOUND;
+
+}
+
+enum io_wq_cancel io_wq_cancel_work(struct io_wq *wq, struct io_wq_work *cwork)
+{
+	enum io_wq_cancel ret = IO_WQ_CANCEL_NOTFOUND;
+	int i;
+
+	for (i = 0; i < wq->nr_wqes; i++) {
+		struct io_wqe *wqe = wq->wqes[i];
+
+		ret = io_wqe_cancel_work(wqe, cwork);
+		if (ret != IO_WQ_CANCEL_NOTFOUND)
+			break;
+	}
+
+	return ret;
+}
+
+struct io_wq_flush_data {
+	struct io_wq_work work;
+	struct completion done;
+};
+
+static void io_wq_flush_func(struct io_wq_work **workptr)
+{
+	struct io_wq_work *work = *workptr;
+	struct io_wq_flush_data *data;
+
+	data = container_of(work, struct io_wq_flush_data, work);
+	complete(&data->done);
+}
+
+/*
+ * Doesn't wait for previously queued work to finish. When this completes,
+ * it just means that previously queued work was started.
+ */
+void io_wq_flush(struct io_wq *wq)
+{
+	struct io_wq_flush_data data;
+	int i;
+
+	for (i = 0; i < wq->nr_wqes; i++) {
+		struct io_wqe *wqe = wq->wqes[i];
+
+		init_completion(&data.done);
+		INIT_IO_WORK(&data.work, io_wq_flush_func);
+		io_wqe_enqueue(wqe, &data.work);
+		wait_for_completion(&data.done);
+	}
+}
+
+struct io_wq *io_wq_create(unsigned concurrency, struct mm_struct *mm)
+{
+	int ret = -ENOMEM, i, node;
+	struct io_wq *wq;
+
+	wq = kcalloc(1, sizeof(*wq), GFP_KERNEL);
+	if (!wq)
+		return ERR_PTR(-ENOMEM);
+
+	wq->nr_wqes = num_online_nodes();
+	wq->wqes = kcalloc(wq->nr_wqes, sizeof(struct io_wqe *), GFP_KERNEL);
+	if (!wq->wqes) {
+		kfree(wq);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	i = 0;
+	refcount_set(&wq->refs, wq->nr_wqes);
+	for_each_online_node(node) {
+		struct io_wqe *wqe;
+
+		wqe = kcalloc_node(1, sizeof(struct io_wqe), GFP_KERNEL, node);
+		if (!wqe)
+			break;
+		wq->wqes[i] = wqe;
+		wqe->node = node;
+		wqe->max_workers = concurrency;
+		wqe->node = node;
+		wqe->wq = wq;
+		spin_lock_init(&wqe->lock);
+		INIT_LIST_HEAD(&wqe->work_list);
+		INIT_HLIST_NULLS_HEAD(&wqe->free_list.head, 0);
+		wqe->free_list.nulls = 0;
+		INIT_HLIST_NULLS_HEAD(&wqe->busy_list.head, 1);
+		wqe->busy_list.nulls = 1;
+		atomic_set(&wqe->nr_running, 0);
+
+		i++;
+	}
+
+	init_completion(&wq->done);
+
+	if (i != wq->nr_wqes)
+		goto err;
+
+	/* caller must have already done mmgrab() on this mm */
+	wq->mm = mm;
+
+	wq->manager = kthread_create(io_wq_manager, wq, "io_wq_manager");
+	if (!IS_ERR(wq->manager)) {
+		wake_up_process(wq->manager);
+		return wq;
+	}
+
+	ret = PTR_ERR(wq->manager);
+	wq->manager = NULL;
+err:
+	complete(&wq->done);
+	io_wq_destroy(wq);
+	return ERR_PTR(ret);
+}
+
+static bool io_wq_worker_wake(struct io_worker *worker, void *data)
+{
+	wake_up_process(worker->task);
+	return false;
+}
+
+void io_wq_destroy(struct io_wq *wq)
+{
+	int i;
+
+	if (wq->manager) {
+		set_bit(IO_WQ_BIT_EXIT, &wq->state);
+		kthread_stop(wq->manager);
+	}
+
+	rcu_read_lock();
+	for (i = 0; i < wq->nr_wqes; i++) {
+		struct io_wqe *wqe = wq->wqes[i];
+
+		if (!wqe)
+			continue;
+		io_wq_for_each_worker(wqe, &wqe->free_list, io_wq_worker_wake,
+						NULL);
+		io_wq_for_each_worker(wqe, &wqe->busy_list, io_wq_worker_wake,
+						NULL);
+	}
+	rcu_read_unlock();
+
+	wait_for_completion(&wq->done);
+
+	for (i = 0; i < wq->nr_wqes; i++)
+		kfree(wq->wqes[i]);
+	kfree(wq->wqes);
+	kfree(wq);
+}
diff --git a/fs/io-wq.h b/fs/io-wq.h
new file mode 100644
index 000000000000..be8f22c8937b
--- /dev/null
+++ b/fs/io-wq.h
@@ -0,0 +1,55 @@
+#ifndef INTERNAL_IO_WQ_H
+#define INTERNAL_IO_WQ_H
+
+struct io_wq;
+
+enum {
+	IO_WQ_WORK_CANCEL	= 1,
+	IO_WQ_WORK_HAS_MM	= 2,
+	IO_WQ_WORK_HASHED	= 4,
+	IO_WQ_WORK_NEEDS_USER	= 8,
+
+	IO_WQ_HASH_SHIFT	= 24,	/* upper 8 bits are used for hash key */
+};
+
+enum io_wq_cancel {
+	IO_WQ_CANCEL_OK,	/* cancelled before started */
+	IO_WQ_CANCEL_RUNNING,	/* found, running, and attempted cancelled */
+	IO_WQ_CANCEL_NOTFOUND,	/* work not found */
+};
+
+struct io_wq_work {
+	struct list_head list;
+	void (*func)(struct io_wq_work **);
+	unsigned flags;
+};
+
+#define INIT_IO_WORK(work, _func)			\
+	do {						\
+		(work)->func = _func;			\
+		(work)->flags = 0;			\
+	} while (0)					\
+
+struct io_wq *io_wq_create(unsigned concurrency, struct mm_struct *mm);
+void io_wq_destroy(struct io_wq *wq);
+
+void io_wq_enqueue(struct io_wq *wq, struct io_wq_work *work);
+void io_wq_enqueue_hashed(struct io_wq *wq, struct io_wq_work *work, void *val);
+void io_wq_flush(struct io_wq *wq);
+
+void io_wq_cancel_all(struct io_wq *wq);
+enum io_wq_cancel io_wq_cancel_work(struct io_wq *wq, struct io_wq_work *cwork);
+
+#if defined(CONFIG_IO_WQ)
+extern void io_wq_worker_sleeping(struct task_struct *);
+extern void io_wq_worker_running(struct task_struct *);
+#else
+static inline void io_wq_worker_sleeping(struct task_struct *tsk)
+{
+}
+static inline void io_wq_worker_running(struct task_struct *tsk)
+{
+}
+#endif
+
+#endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 67a1d86981a9..6666e25606b7 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1468,6 +1468,7 @@ extern struct pid *cad_pid;
 #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_mask */
 #define PF_MCE_EARLY		0x08000000      /* Early kill for mce process policy */
 #define PF_MEMALLOC_NOCMA	0x10000000	/* All allocation request will have _GFP_MOVABLE cleared */
+#define PF_IO_WORKER		0x20000000	/* Task is an IO worker */
 #define PF_FREEZER_SKIP		0x40000000	/* Freezer should not count it as freezable */
 #define PF_SUSPEND_TASK		0x80000000      /* This thread called freeze_processes() and should not be frozen */
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index dd05a378631a..a95a2f05f3b5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -16,6 +16,7 @@
 #include <asm/tlb.h>
 
 #include "../workqueue_internal.h"
+#include "../../fs/io-wq.h"
 #include "../smpboot.h"
 
 #include "pelt.h"
@@ -4103,9 +4104,12 @@ static inline void sched_submit_work(struct task_struct *tsk)
 	 * we disable preemption to avoid it calling schedule() again
 	 * in the possible wakeup of a kworker.
 	 */
-	if (tsk->flags & PF_WQ_WORKER) {
+	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
 		preempt_disable();
-		wq_worker_sleeping(tsk);
+		if (tsk->flags & PF_WQ_WORKER)
+			wq_worker_sleeping(tsk);
+		else
+			io_wq_worker_sleeping(tsk);
 		preempt_enable_no_resched();
 	}
 
@@ -4122,8 +4126,12 @@ static inline void sched_submit_work(struct task_struct *tsk)
 
 static void sched_update_worker(struct task_struct *tsk)
 {
-	if (tsk->flags & PF_WQ_WORKER)
-		wq_worker_running(tsk);
+	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
+		if (tsk->flags & PF_WQ_WORKER)
+			wq_worker_running(tsk);
+		else
+			io_wq_worker_running(tsk);
+	}
 }
 
 asmlinkage __visible void __sched schedule(void)
-- 
2.17.1

Re: [PATCH 1/2] io-wq: small threadpool implementation for io_uring

[Peter Zijlstra]

On Fri, Oct 25, 2019 at 11:22:50AM -0600, Jens Axboe wrote:

>  include/linux/sched.h |   1 +
>  kernel/sched/core.c   |  16 +-

This all seems pretty harmless, it makes the wq paths wee bit slower,
but everything else should be unaffected.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 67a1d86981a9..6666e25606b7 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1468,6 +1468,7 @@ extern struct pid *cad_pid;
>  #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_mask */
>  #define PF_MCE_EARLY		0x08000000      /* Early kill for mce process policy */
>  #define PF_MEMALLOC_NOCMA	0x10000000	/* All allocation request will have _GFP_MOVABLE cleared */
> +#define PF_IO_WORKER		0x20000000	/* Task is an IO worker */
>  #define PF_FREEZER_SKIP		0x40000000	/* Freezer should not count it as freezable */
>  #define PF_SUSPEND_TASK		0x80000000      /* This thread called freeze_processes() and should not be frozen */
>  
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index dd05a378631a..a95a2f05f3b5 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -16,6 +16,7 @@
>  #include <asm/tlb.h>
>  
>  #include "../workqueue_internal.h"
> +#include "../../fs/io-wq.h"
>  #include "../smpboot.h"
>  
>  #include "pelt.h"
> @@ -4103,9 +4104,12 @@ static inline void sched_submit_work(struct task_struct *tsk)
>  	 * we disable preemption to avoid it calling schedule() again
>  	 * in the possible wakeup of a kworker.
>  	 */
> -	if (tsk->flags & PF_WQ_WORKER) {
> +	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
>  		preempt_disable();
> -		wq_worker_sleeping(tsk);
> +		if (tsk->flags & PF_WQ_WORKER)
> +			wq_worker_sleeping(tsk);
> +		else
> +			io_wq_worker_sleeping(tsk);
>  		preempt_enable_no_resched();
>  	}
>  
> @@ -4122,8 +4126,12 @@ static inline void sched_submit_work(struct task_struct *tsk)
>  
>  static void sched_update_worker(struct task_struct *tsk)
>  {
> -	if (tsk->flags & PF_WQ_WORKER)
> -		wq_worker_running(tsk);
> +	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
> +		if (tsk->flags & PF_WQ_WORKER)
> +			wq_worker_running(tsk);
> +		else
> +			io_wq_worker_running(tsk);
> +	}
>  }
>  
>  asmlinkage __visible void __sched schedule(void)
> -- 
> 2.17.1
> 

Re: [PATCH 1/2] io-wq: small threadpool implementation for io_uring

[Jens Axboe]

On 10/28/19 11:10 AM, Peter Zijlstra wrote:
> On Fri, Oct 25, 2019 at 11:22:50AM -0600, Jens Axboe wrote:
> 
>>   include/linux/sched.h |   1 +
>>   kernel/sched/core.c   |  16 +-
> 
> This all seems pretty harmless, it makes the wq paths wee bit slower,
> but everything else should be unaffected.
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Great, thanks Peter!

-- 
Jens Axboe