[RFC PATCH 1/4] vhost: Introduce a universal thread to serve all users

[Bandan Das <bsd@redhat.com>]

vhost threads are per-device, but in most cases a single thread
is enough. This change creates a single thread that is used to
serve all guests.

However, this complicates cgroups associations. The current policy
is to attach the per-device thread to all cgroups of the parent process
that the device is associated it. This is no longer possible if we
have a single thread. So, we end up moving the thread around to
cgroups of whichever device that needs servicing. This is a very
inefficient protocol but seems to be the only way to integrate
cgroups support.

Signed-off-by: Razya Ladelsky <razya@il.ibm.com>
Signed-off-by: Bandan Das <bsd@redhat.com>
---
 drivers/vhost/scsi.c  |  15 +++--
 drivers/vhost/vhost.c | 150 ++++++++++++++++++++++++--------------------------
 drivers/vhost/vhost.h |  19 +++++--
 3 files changed, 97 insertions(+), 87 deletions(-)

diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
index ea32b38..6c42936 100644
--- a/drivers/vhost/scsi.c
+++ b/drivers/vhost/scsi.c
@@ -535,7 +535,7 @@ static void vhost_scsi_complete_cmd(struct vhost_scsi_cmd *cmd)
 
 	llist_add(&cmd->tvc_completion_list, &vs->vs_completion_list);
 
-	vhost_work_queue(&vs->dev, &vs->vs_completion_work);
+	vhost_work_queue(vs->dev.worker, &vs->vs_completion_work);
 }
 
 static int vhost_scsi_queue_data_in(struct se_cmd *se_cmd)
@@ -1282,7 +1282,7 @@ vhost_scsi_send_evt(struct vhost_scsi *vs,
 	}
 
 	llist_add(&evt->list, &vs->vs_event_list);
-	vhost_work_queue(&vs->dev, &vs->vs_event_work);
+	vhost_work_queue(vs->dev.worker, &vs->vs_event_work);
 }
 
 static void vhost_scsi_evt_handle_kick(struct vhost_work *work)
@@ -1335,8 +1335,8 @@ static void vhost_scsi_flush(struct vhost_scsi *vs)
 	/* Flush both the vhost poll and vhost work */
 	for (i = 0; i < VHOST_SCSI_MAX_VQ; i++)
 		vhost_scsi_flush_vq(vs, i);
-	vhost_work_flush(&vs->dev, &vs->vs_completion_work);
-	vhost_work_flush(&vs->dev, &vs->vs_event_work);
+	vhost_work_flush(vs->dev.worker, &vs->vs_completion_work);
+	vhost_work_flush(vs->dev.worker, &vs->vs_event_work);
 
 	/* Wait for all reqs issued before the flush to be finished */
 	for (i = 0; i < VHOST_SCSI_MAX_VQ; i++)
@@ -1584,8 +1584,11 @@ static int vhost_scsi_open(struct inode *inode, struct file *f)
 	if (!vqs)
 		goto err_vqs;
 
-	vhost_work_init(&vs->vs_completion_work, vhost_scsi_complete_cmd_work);
-	vhost_work_init(&vs->vs_event_work, vhost_scsi_evt_work);
+	vhost_work_init(&vs->dev, &vs->vs_completion_work,
+			vhost_scsi_complete_cmd_work);
+
+	vhost_work_init(&vs->dev, &vs->vs_event_work,
+			vhost_scsi_evt_work);
 
 	vs->vs_events_nr = 0;
 	vs->vs_events_missed = false;
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 2ee2826..951c96b 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -11,6 +11,8 @@
  * Generic code for virtio server in host kernel.
  */
 
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
 #include <linux/eventfd.h>
 #include <linux/vhost.h>
 #include <linux/uio.h>
@@ -28,6 +30,9 @@
 
 #include "vhost.h"
 
+/* Just one worker thread to service all devices */
+static struct vhost_worker *worker;
+
 enum {
 	VHOST_MEMORY_MAX_NREGIONS = 64,
 	VHOST_MEMORY_F_LOG = 0x1,
@@ -58,13 +63,15 @@ static int vhost_poll_wakeup(wait_queue_t *wait, unsigned mode, int sync,
 	return 0;
 }
 
-void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn)
+void vhost_work_init(struct vhost_dev *dev,
+		     struct vhost_work *work, vhost_work_fn_t fn)
 {
 	INIT_LIST_HEAD(&work->node);
 	work->fn = fn;
 	init_waitqueue_head(&work->done);
 	work->flushing = 0;
 	work->queue_seq = work->done_seq = 0;
+	work->dev = dev;
 }
 EXPORT_SYMBOL_GPL(vhost_work_init);
 
@@ -78,7 +85,7 @@ void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
 	poll->dev = dev;
 	poll->wqh = NULL;
 
-	vhost_work_init(&poll->work, fn);
+	vhost_work_init(dev, &poll->work, fn);
 }
 EXPORT_SYMBOL_GPL(vhost_poll_init);
 
@@ -116,30 +123,30 @@ void vhost_poll_stop(struct vhost_poll *poll)
 }
 EXPORT_SYMBOL_GPL(vhost_poll_stop);
 
-static bool vhost_work_seq_done(struct vhost_dev *dev, struct vhost_work *work,
-				unsigned seq)
+static bool vhost_work_seq_done(struct vhost_worker *worker,
+				struct vhost_work *work, unsigned seq)
 {
 	int left;
 
-	spin_lock_irq(&dev->work_lock);
+	spin_lock_irq(&worker->work_lock);
 	left = seq - work->done_seq;
-	spin_unlock_irq(&dev->work_lock);
+	spin_unlock_irq(&worker->work_lock);
 	return left <= 0;
 }
 
-void vhost_work_flush(struct vhost_dev *dev, struct vhost_work *work)
+void vhost_work_flush(struct vhost_worker *worker, struct vhost_work *work)
 {
 	unsigned seq;
 	int flushing;
 
-	spin_lock_irq(&dev->work_lock);
+	spin_lock_irq(&worker->work_lock);
 	seq = work->queue_seq;
 	work->flushing++;
-	spin_unlock_irq(&dev->work_lock);
-	wait_event(work->done, vhost_work_seq_done(dev, work, seq));
-	spin_lock_irq(&dev->work_lock);
+	spin_unlock_irq(&worker->work_lock);
+	wait_event(work->done, vhost_work_seq_done(worker, work, seq));
+	spin_lock_irq(&worker->work_lock);
 	flushing = --work->flushing;
-	spin_unlock_irq(&dev->work_lock);
+	spin_unlock_irq(&worker->work_lock);
 	BUG_ON(flushing < 0);
 }
 EXPORT_SYMBOL_GPL(vhost_work_flush);
@@ -148,29 +155,30 @@ EXPORT_SYMBOL_GPL(vhost_work_flush);
  * locks that are also used by the callback. */
 void vhost_poll_flush(struct vhost_poll *poll)
 {
-	vhost_work_flush(poll->dev, &poll->work);
+	vhost_work_flush(poll->dev->worker, &poll->work);
 }
 EXPORT_SYMBOL_GPL(vhost_poll_flush);
 
-void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work)
+void vhost_work_queue(struct vhost_worker *worker,
+		      struct vhost_work *work)
 {
 	unsigned long flags;
 
-	spin_lock_irqsave(&dev->work_lock, flags);
+	spin_lock_irqsave(&worker->work_lock, flags);
 	if (list_empty(&work->node)) {
-		list_add_tail(&work->node, &dev->work_list);
+		list_add_tail(&work->node, &worker->work_list);
 		work->queue_seq++;
-		spin_unlock_irqrestore(&dev->work_lock, flags);
-		wake_up_process(dev->worker);
+		spin_unlock_irqrestore(&worker->work_lock, flags);
+		wake_up_process(worker->thread);
 	} else {
-		spin_unlock_irqrestore(&dev->work_lock, flags);
+		spin_unlock_irqrestore(&worker->work_lock, flags);
 	}
 }
 EXPORT_SYMBOL_GPL(vhost_work_queue);
 
 void vhost_poll_queue(struct vhost_poll *poll)
 {
-	vhost_work_queue(poll->dev, &poll->work);
+	vhost_work_queue(poll->dev->worker, &poll->work);
 }
 EXPORT_SYMBOL_GPL(vhost_poll_queue);
 
@@ -203,19 +211,18 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 
 static int vhost_worker(void *data)
 {
-	struct vhost_dev *dev = data;
+	struct vhost_worker *worker = data;
 	struct vhost_work *work = NULL;
 	unsigned uninitialized_var(seq);
 	mm_segment_t oldfs = get_fs();
 
 	set_fs(USER_DS);
-	use_mm(dev->mm);
 
 	for (;;) {
 		/* mb paired w/ kthread_stop */
 		set_current_state(TASK_INTERRUPTIBLE);
 
-		spin_lock_irq(&dev->work_lock);
+		spin_lock_irq(&worker->work_lock);
 		if (work) {
 			work->done_seq = seq;
 			if (work->flushing)
@@ -223,21 +230,35 @@ static int vhost_worker(void *data)
 		}
 
 		if (kthread_should_stop()) {
-			spin_unlock_irq(&dev->work_lock);
+			spin_unlock_irq(&worker->work_lock);
 			__set_current_state(TASK_RUNNING);
 			break;
 		}
-		if (!list_empty(&dev->work_list)) {
-			work = list_first_entry(&dev->work_list,
+		if (!list_empty(&worker->work_list)) {
+			work = list_first_entry(&worker->work_list,
 						struct vhost_work, node);
 			list_del_init(&work->node);
 			seq = work->queue_seq;
 		} else
 			work = NULL;
-		spin_unlock_irq(&dev->work_lock);
+		spin_unlock_irq(&worker->work_lock);
 
 		if (work) {
+			struct vhost_dev *dev = work->dev;
+
 			__set_current_state(TASK_RUNNING);
+
+			if (current->mm != dev->mm) {
+				unuse_mm(current->mm);
+				use_mm(dev->mm);
+			}
+
+			/* TODO: Consider a more elegant solution */
+			if (worker->owner != dev->owner) {
+				/* Should check for return value */
+				cgroup_attach_task_all(dev->owner, current);
+				worker->owner = dev->owner;
+			}
 			work->fn(work);
 			if (need_resched())
 				schedule();
@@ -245,7 +266,6 @@ static int vhost_worker(void *data)
 			schedule();
 
 	}
-	unuse_mm(dev->mm);
 	set_fs(oldfs);
 	return 0;
 }
@@ -304,9 +324,8 @@ void vhost_dev_init(struct vhost_dev *dev,
 	dev->log_file = NULL;
 	dev->memory = NULL;
 	dev->mm = NULL;
-	spin_lock_init(&dev->work_lock);
-	INIT_LIST_HEAD(&dev->work_list);
-	dev->worker = NULL;
+	dev->worker = worker;
+	dev->owner = current;
 
 	for (i = 0; i < dev->nvqs; ++i) {
 		vq = dev->vqs[i];
@@ -331,31 +350,6 @@ long vhost_dev_check_owner(struct vhost_dev *dev)
 }
 EXPORT_SYMBOL_GPL(vhost_dev_check_owner);
 
-struct vhost_attach_cgroups_struct {
-	struct vhost_work work;
-	struct task_struct *owner;
-	int ret;
-};
-
-static void vhost_attach_cgroups_work(struct vhost_work *work)
-{
-	struct vhost_attach_cgroups_struct *s;
-
-	s = container_of(work, struct vhost_attach_cgroups_struct, work);
-	s->ret = cgroup_attach_task_all(s->owner, current);
-}
-
-static int vhost_attach_cgroups(struct vhost_dev *dev)
-{
-	struct vhost_attach_cgroups_struct attach;
-
-	attach.owner = current;
-	vhost_work_init(&attach.work, vhost_attach_cgroups_work);
-	vhost_work_queue(dev, &attach.work);
-	vhost_work_flush(dev, &attach.work);
-	return attach.ret;
-}
-
 /* Caller should have device mutex */
 bool vhost_dev_has_owner(struct vhost_dev *dev)
 {
@@ -366,7 +360,6 @@ EXPORT_SYMBOL_GPL(vhost_dev_has_owner);
 /* Caller should have device mutex */
 long vhost_dev_set_owner(struct vhost_dev *dev)
 {
-	struct task_struct *worker;
 	int err;
 
 	/* Is there an owner already? */
@@ -377,28 +370,15 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
 
 	/* No owner, become one */
 	dev->mm = get_task_mm(current);
-	worker = kthread_create(vhost_worker, dev, "vhost-%d", current->pid);
-	if (IS_ERR(worker)) {
-		err = PTR_ERR(worker);
-		goto err_worker;
-	}
-
 	dev->worker = worker;
-	wake_up_process(worker);	/* avoid contributing to loadavg */
-
-	err = vhost_attach_cgroups(dev);
-	if (err)
-		goto err_cgroup;
 
 	err = vhost_dev_alloc_iovecs(dev);
 	if (err)
-		goto err_cgroup;
+		goto err_alloc;
 
 	return 0;
-err_cgroup:
-	kthread_stop(worker);
+err_alloc:
 	dev->worker = NULL;
-err_worker:
 	if (dev->mm)
 		mmput(dev->mm);
 	dev->mm = NULL;
@@ -472,11 +452,6 @@ void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
 	/* No one will access memory at this point */
 	kfree(dev->memory);
 	dev->memory = NULL;
-	WARN_ON(!list_empty(&dev->work_list));
-	if (dev->worker) {
-		kthread_stop(dev->worker);
-		dev->worker = NULL;
-	}
 	if (dev->mm)
 		mmput(dev->mm);
 	dev->mm = NULL;
@@ -1567,11 +1542,32 @@ EXPORT_SYMBOL_GPL(vhost_disable_notify);
 
 static int __init vhost_init(void)
 {
+	struct vhost_worker *w =
+		kzalloc(sizeof(*w), GFP_KERNEL);
+	if (!w)
+		return -ENOMEM;
+
+	w->thread = kthread_create(vhost_worker,
+				   w, "vhost-worker");
+	if (IS_ERR(w->thread))
+		return PTR_ERR(w->thread);
+
+	worker = w;
+	spin_lock_init(&worker->work_lock);
+	INIT_LIST_HEAD(&worker->work_list);
+	wake_up_process(worker->thread);
+	pr_info("Created universal thread to service requests\n");
+
 	return 0;
 }
 
 static void __exit vhost_exit(void)
 {
+	if (worker) {
+		kthread_stop(worker->thread);
+		WARN_ON(!list_empty(&worker->work_list));
+		kfree(worker);
+	}
 }
 
 module_init(vhost_init);
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index 8c1c792..2f204ce 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -22,6 +22,7 @@ struct vhost_work {
 	int			  flushing;
 	unsigned		  queue_seq;
 	unsigned		  done_seq;
+	struct vhost_dev          *dev;
 };
 
 /* Poll a file (eventfd or socket) */
@@ -35,8 +36,8 @@ struct vhost_poll {
 	struct vhost_dev	 *dev;
 };
 
-void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn);
-void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work);
+void vhost_work_init(struct vhost_dev *dev,
+		     struct vhost_work *work, vhost_work_fn_t fn);
 
 void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
 		     unsigned long mask, struct vhost_dev *dev);
@@ -44,7 +45,6 @@ int vhost_poll_start(struct vhost_poll *poll, struct file *file);
 void vhost_poll_stop(struct vhost_poll *poll);
 void vhost_poll_flush(struct vhost_poll *poll);
 void vhost_poll_queue(struct vhost_poll *poll);
-void vhost_work_flush(struct vhost_dev *dev, struct vhost_work *work);
 long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp);
 
 struct vhost_log {
@@ -116,11 +116,22 @@ struct vhost_dev {
 	int nvqs;
 	struct file *log_file;
 	struct eventfd_ctx *log_ctx;
+	/* vhost shared worker */
+	struct vhost_worker *worker;
+	/* for cgroup support */
+	struct task_struct *owner;
+};
+
+struct vhost_worker {
 	spinlock_t work_lock;
 	struct list_head work_list;
-	struct task_struct *worker;
+	struct task_struct *thread;
+	struct task_struct *owner;
 };
 
+void vhost_work_queue(struct vhost_worker *worker,
+		      struct vhost_work *work);
+void vhost_work_flush(struct vhost_worker *worker, struct vhost_work *work);
 void vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue **vqs, int nvqs);
 long vhost_dev_set_owner(struct vhost_dev *dev);
 bool vhost_dev_has_owner(struct vhost_dev *dev);
-- 
2.4.3