Subject: Re: [PATCH 1/2] vhost: allow multiple workers threads

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 19 Feb 2012 16:41:45 +0200

On Fri, Feb 17, 2012 at 05:02:05PM -0600, Anthony Liguori wrote:
> This patch allows vhost to have multiple worker threads for devices such as
> virtio-net which may have multiple virtqueues.
> 
> Since virtqueues are a lockless ring queue, in an ideal world data is being
> produced by the producer as fast as data is being consumed by the consumer.
> These loops will continue to consume data until none is left.
> 
> vhost currently multiplexes the consumer side of the queue on a single thread
> by attempting to read from the queue until everything is read or it cannot
> process anymore.  This means that activity on one queue may stall another queue.

There's actually an attempt to address this: look up
VHOST_NET_WEIGHT in the code. I take it, this isn't effective?

> This is exacerbated when using any form of polling to read from the queues (as
> we'll introduce in the next patch).  By spawning a thread per-virtqueue, this
> is addressed.
> 
> The only problem with this patch right now is how the wake up of the threads is
> done.  It's essentially a broadcast and we have seen lock contention as a
> result.

On which lock?

> We've tried some approaches to signal a single thread but I'm not
> confident that that code is correct yet so I'm only sending the broadcast
> version.

Yes, that looks like an obvious question.

> Here are some performance results from this change.  There's a modest
> improvement with stream although a fair bit of variability too.
> 
> With RR, there's pretty significant improvements as the instance rate drives up.

Interesting. This was actually tested at one time and we saw
a significant performance improvement from using
a single thread especially with a single
stream in the guest. Profiling indicated that
with a single thread we get too many context
switches between TX and RX, since guest networking
tends to run TX and RX processing on the same
guest VCPU.

Maybe we were wrong or maybe this went away
for some reason. I'll see if this can be reproduced.

> 
> Test, Size, Instance, Baseline, Patch, Relative
> 
> TCP_RR
> 	Tx:256 Rx:256
> 		1	9,639.66	10,164.06	105.44%
> 		10	62,819.55	54,059.78	86.06%
> 		30	84,715.60	131,241.86	154.92%
> 		60	124,614.71	148,720.66	119.34%
> 
> UDP_RR
> 	Tx:256 Rx:256
> 		1	9,652.50	10,343.72	107.16%
> 		10	53,830.26	58,235.90	108.18%
> 		30	89,471.01	97,634.53	109.12%
> 		60	103,640.59	164,035.01	158.27%
> 
> TCP_STREAM
> 	Tx: 256
> 		1	2,622.63	2,610.71	99.55%
> 		4	4,928.02	4,812.05	97.65%
> 
> 	Tx: 1024
> 		1	5,639.89	5,751.28	101.97%
> 		4	5,874.72	6,575.55	111.93%
> 
> 	Tx: 4096
> 		1	6,257.42	7,655.22	122.34%
> 		4	5,370.78	6,044.83	112.55%
> 
> 	Tx: 16384
> 		1	6,346.63	7,267.44	114.51%
> 		4	5,198.02	5,657.12	108.83%
> 
> TCP_MAERTS
> 	Rx: 256
> 		1	2,091.38	1,765.62	84.42%
> 		4	5,319.52	5,619.49	105.64%
> 
> 	Rx: 1024
> 		1	7,030.66	7,593.61	108.01%
> 		4	9,040.53	7,275.84	80.48%
> 
> 	Rx: 4096
> 		1	9,160.93	9,318.15	101.72%
> 		4	9,372.49	8,875.63	94.70%
> 
> 	Rx: 16384
> 		1	9,183.28	9,134.02	99.46%
> 		4	9,377.17	8,877.52	94.67%
> 
> 							106.46%

One point missing here is the ratio of
throughput divided by host CPU utilization.

The concern being to avoid hurting
heavily loaded systems.

These numbers also tend to be less variable as
they depend less on host scheduler decisions.

> Cc: Tom Lendacky <[email-masked]>
> Cc: Cristian Viana <[email-masked]>
> Signed-off-by: Anthony Liguori <[email-masked]>
> ---
>  drivers/vhost/net.c   |    6 ++++-
>  drivers/vhost/vhost.c |   51 ++++++++++++++++++++++++++++++------------------
>  drivers/vhost/vhost.h |    8 +++++-
>  3 files changed, 43 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 9dab1f5..47175cd 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -33,6 +33,10 @@ static int experimental_zcopytx;
>  module_param(experimental_zcopytx, int, 0444);
>  MODULE_PARM_DESC(experimental_zcopytx, "Enable Experimental Zero Copy TX");
>  
> +static int workers = 2;
> +module_param(workers, int, 0444);
> +MODULE_PARM_DESC(workers, "Set the number of worker threads");
> +
>  /* Max number of bytes transferred before requeueing the job.
>   * Using this limit prevents one virtqueue from starving others. */
>  #define VHOST_NET_WEIGHT 0x80000
> @@ -504,7 +508,7 @@ static int vhost_net_open(struct inode *inode, struct file *f)
>  	dev = &n->dev;
>  	n->vqs[VHOST_NET_VQ_TX].handle_kick = handle_tx_kick;
>  	n->vqs[VHOST_NET_VQ_RX].handle_kick = handle_rx_kick;
> -	r = vhost_dev_init(dev, n->vqs, VHOST_NET_VQ_MAX);
> +	r = vhost_dev_init(dev, n->vqs, workers, VHOST_NET_VQ_MAX);
>  	if (r < 0) {
>  		kfree(n);
>  		return r;
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index c14c42b..676cead 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -144,9 +144,12 @@ static inline void vhost_work_queue(struct vhost_dev *dev,
>  
>  	spin_lock_irqsave(&dev->work_lock, flags);
>  	if (list_empty(&work->node)) {
> +		int i;
> +
>  		list_add_tail(&work->node, &dev->work_list);
>  		work->queue_seq++;
> -		wake_up_process(dev->worker);
> +		for (i = 0; i < dev->nworkers; i++)
> +			wake_up_process(dev->workers[i]);
>  	}
>  	spin_unlock_irqrestore(&dev->work_lock, flags);
>  }
> @@ -287,7 +290,7 @@ static void vhost_dev_free_iovecs(struct vhost_dev *dev)
>  }
>  
>  long vhost_dev_init(struct vhost_dev *dev,
> -		    struct vhost_virtqueue *vqs, int nvqs)
> +		    struct vhost_virtqueue *vqs, int nworkers, int nvqs)
>  {
>  	int i;
>  
> @@ -300,7 +303,9 @@ long vhost_dev_init(struct vhost_dev *dev,
>  	dev->mm = NULL;
>  	spin_lock_init(&dev->work_lock);
>  	INIT_LIST_HEAD(&dev->work_list);
> -	dev->worker = NULL;
> +	dev->nworkers = min(nworkers, VHOST_MAX_WORKERS);
> +	for (i = 0; i < dev->nworkers; i++)
> +		dev->workers[i] = NULL;
>  
>  	for (i = 0; i < dev->nvqs; ++i) {
>  		dev->vqs[i].log = NULL;
> @@ -354,7 +359,7 @@ static int vhost_attach_cgroups(struct vhost_dev *dev)
>  static long vhost_dev_set_owner(struct vhost_dev *dev)
>  {
>  	struct task_struct *worker;
> -	int err;
> +	int err, i;
>  
>  	/* Is there an owner already? */
>  	if (dev->mm) {
> @@ -364,28 +369,34 @@ static 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;
> -	}
> +	for (i = 0; i < dev->nworkers; i++) {
> +		worker = kthread_create(vhost_worker, dev, "vhost-%d.%d", current->pid, i);
> +		if (IS_ERR(worker)) {
> +			err = PTR_ERR(worker);
> +			goto err_worker;
> +		}
>  
> -	dev->worker = worker;
> -	wake_up_process(worker);	/* avoid contributing to loadavg */
> +		dev->workers[i] = worker;
> +		wake_up_process(worker);	/* avoid contributing to loadavg */
> +	}
>  
>  	err = vhost_attach_cgroups(dev);
>  	if (err)
> -		goto err_cgroup;
> +		goto err_worker;
>  
>  	err = vhost_dev_alloc_iovecs(dev);
>  	if (err)
> -		goto err_cgroup;
> +		goto err_worker;
>  
>  	return 0;
> -err_cgroup:
> -	kthread_stop(worker);
> -	dev->worker = NULL;
> +
>  err_worker:
> +	for (i = 0; i < dev->nworkers; i++) {
> +		if (dev->workers[i]) {
> +			kthread_stop(dev->workers[i]);
> +			dev->workers[i] = NULL;
> +		}
> +	}
>  	if (dev->mm)
>  		mmput(dev->mm);
>  	dev->mm = NULL;
> @@ -475,9 +486,11 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
>  					lockdep_is_held(&dev->mutex)));
>  	RCU_INIT_POINTER(dev->memory, NULL);
>  	WARN_ON(!list_empty(&dev->work_list));
> -	if (dev->worker) {
> -		kthread_stop(dev->worker);
> -		dev->worker = NULL;
> +	for (i = 0; i < dev->nworkers; i++) {
> +		if (dev->workers[i]) {
> +			kthread_stop(dev->workers[i]);
> +			dev->workers[i] = NULL;
> +		}
>  	}
>  	if (dev->mm)
>  		mmput(dev->mm);
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index a801e28..fa5e75e 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -18,6 +18,9 @@
>  #define VHOST_DMA_DONE_LEN	1
>  #define VHOST_DMA_CLEAR_LEN	0
>  
> +/* Maximum number of worker threads per-vhost device */
> +#define VHOST_MAX_WORKERS	8
> +
>  struct vhost_device;
>  
>  struct vhost_work;
> @@ -157,10 +160,11 @@ struct vhost_dev {
>  	struct eventfd_ctx *log_ctx;
>  	spinlock_t work_lock;
>  	struct list_head work_list;
> -	struct task_struct *worker;
> +	int nworkers;
> +	struct task_struct *workers[VHOST_MAX_WORKERS];
>  };
>  
> -long vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue *vqs, int nvqs);
> +long vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue *vqs, int nworkers, int nvqs);
>  long vhost_dev_check_owner(struct vhost_dev *);
>  long vhost_dev_reset_owner(struct vhost_dev *);
>  void vhost_dev_cleanup(struct vhost_dev *);
> -- 
> 1.7.4.1