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 17:05:13 -0400

Bandan Das <[email-masked]> writes:

> "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 <razya-7z/[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.

So, I did a quick smoke test with virtio-net and the Elvis patches.
virtio net MQ already spawns a new worker thread for every queue,
it seems ? So, the above setup already works! :) I will run some tests and
post back the results.

>>> ---
>>>  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
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