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

From: Bandan Das <[email-masked]>
Date: Mon, 10 Aug 2015 16:09:29 -0400

"Michael S. Tsirkin" <[email-masked]> writes:

> On Mon, Jul 13, 2015 at 12:07:32AM -0400, Bandan Das wrote:
>> 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 <[email-masked]>
>> Signed-off-by: Bandan Das <[email-masked]>
>
> BTW, how does this interact with virtio net MQ?
> It would seem that MQ gains from more parallelism and
> CPU locality.

Hm.. Good point. As of this version, this design will always have
one worker thread servicing a guest. Now suppose we have 10 virtio
queues for a guest, surely, we could benefit from spawning off another
worker just like we are doing in case of a new guest/device with
the devs_per_worker parameter.

>> ---
>>  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