Date:	Wed, 29 Aug 2012 19:14:01 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, avi@...hat.com, kvm@...r.kernel.org
Subject: Re: [PATCH v2 2/2] virtio-ring: Allocate indirect buffers from cache
 when possible

On 08/29/2012 05:38 PM, Michael S. Tsirkin wrote:
> On Tue, Aug 28, 2012 at 03:04:03PM +0200, Sasha Levin wrote:
>> Currently if VIRTIO_RING_F_INDIRECT_DESC is enabled we will
>> use indirect descriptors and allocate them using a simple
>> kmalloc().
>>
>> This patch adds a cache which will allow indirect buffers under
>> a configurable size to be allocated from that cache instead.
>>
>> Signed-off-by: Sasha Levin <levinsasha928@...il.com>
> 
> The API is ugly - how does driver know what to set?
> Is this a typical request size? Then let's call it that.

We've discussed it during the RFC phase, the idea is that we don't know what
would be a good number to use as threshold - which is why I'd like to keep it
disabled as default until it gets more serious tests.

The driver doesn't know what to set, the plan was to make it a dynamic
algorithms which would change it based on current load. Since I can't really do
testing which will provide the correct values for that, the decision was to do
it this way as the first stage and modify it later.

> Also this is really per VQ, right?

Right, we keep it per-device at this stage to keep it simple.

> What is a good default for net? I guess max sg?

I think that it depends on the workload. I'd say we should keep the default to 0
(disabled) unless we can have a good way to adjust it to the load.

>> ---
>>
>> Changes in v2:
>>
>>  - Free correctly indirect buffers.
>>
>>  drivers/block/virtio_blk.c          |  4 ++++
>>  drivers/char/hw_random/virtio-rng.c |  4 ++++
>>  drivers/char/virtio_console.c       |  4 ++++
>>  drivers/net/virtio_net.c            |  4 ++++
>>  drivers/virtio/virtio_balloon.c     |  4 ++++
>>  drivers/virtio/virtio_ring.c        | 28 ++++++++++++++++++++++++----
>>  include/linux/virtio.h              |  1 +
>>  net/9p/trans_virtio.c               |  5 +++++
>>  8 files changed, 50 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>> index 13b8ae9..7f670af 100644
>> --- a/drivers/block/virtio_blk.c
>> +++ b/drivers/block/virtio_blk.c
>> @@ -25,6 +25,9 @@ struct workqueue_struct *virtblk_wq;
>>  static unsigned int indirect_thresh;
>>  module_param(indirect_thresh, uint, S_IRUGO);
>>  
>> +static unsigned int indirect_alloc_thresh;
>> +module_param(indirect_alloc_thresh, uint, S_IRUGO);
>> +
>>  struct virtio_blk
>>  {
>>  	struct virtio_device *vdev;
>> @@ -739,6 +742,7 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
>>  	INIT_WORK(&vblk->config_work, virtblk_config_changed_work);
>>  	vblk->config_enable = true;
>>  	vdev->indirect_thresh = indirect_thresh;
>> +	vdev->indirect_alloc_thresh = indirect_alloc_thresh;
>>  
>>  	err = init_vq(vblk);
>>  	if (err)
>> diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
>> index 02d8421..8475ece 100644
>> --- a/drivers/char/hw_random/virtio-rng.c
>> +++ b/drivers/char/hw_random/virtio-rng.c
>> @@ -28,6 +28,9 @@
>>  static unsigned int indirect_thresh;
>>  module_param(indirect_thresh, uint, S_IRUGO);
>>  
>> +static unsigned int indirect_alloc_thresh;
>> +module_param(indirect_alloc_thresh, uint, S_IRUGO);
>> +
>>  static struct virtqueue *vq;
>>  static unsigned int data_avail;
>>  static DECLARE_COMPLETION(have_data);
>> @@ -97,6 +100,7 @@ static int probe_common(struct virtio_device *vdev)
>>  
>>  	/* We expect a single virtqueue. */
>>  	vdev->indirect_thresh = indirect_thresh;
>> +	vdev->indirect_alloc_thresh = indirect_alloc_thresh;
>>  	vq = virtio_find_single_vq(vdev, random_recv_done, "input");
>>  	if (IS_ERR(vq))
>>  		return PTR_ERR(vq);
>> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
>> index fc14e7f..c6f24a7 100644
>> --- a/drivers/char/virtio_console.c
>> +++ b/drivers/char/virtio_console.c
>> @@ -42,6 +42,9 @@
>>  static unsigned int indirect_thresh;
>>  module_param(indirect_thresh, uint, S_IRUGO);
>>  
>> +static unsigned int indirect_alloc_thresh;
>> +module_param(indirect_alloc_thresh, uint, S_IRUGO);
>> +
>>  /*
>>   * This is a global struct for storing common data for all the devices
>>   * this driver handles.
>> @@ -1891,6 +1894,7 @@ static int __devinit virtcons_probe(struct virtio_device *vdev)
>>  			      &portdev->config.max_nr_ports) == 0)
>>  		multiport = true;
>>  	vdev->indirect_thresh = indirect_thresh;
>> +	vdev->indirect_alloc_thresh = indirect_alloc_thresh;
>>  
>>  	err = init_vqs(portdev);
>>  	if (err < 0) {
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 64a8321..c091efd 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -37,6 +37,9 @@ module_param(gso, bool, 0444);
>>  static unsigned int indirect_thresh;
>>  module_param(indirect_thresh, uint, S_IRUGO);
>>  
>> +static unsigned int indirect_alloc_thresh;
>> +module_param(indirect_alloc_thresh, uint, S_IRUGO);
>> +
>>  /* FIXME: MTU in config. */
>>  #define MAX_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
>>  #define GOOD_COPY_LEN	128
>> @@ -1132,6 +1135,7 @@ static int virtnet_probe(struct virtio_device *vdev)
>>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF))
>>  		vi->mergeable_rx_bufs = true;
>>  	vdev->indirect_thresh = indirect_thresh;
>> +	vdev->indirect_alloc_thresh = indirect_alloc_thresh;
>>  
>>  	err = init_vqs(vi);
>>  	if (err)
>> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
>> index fce7347..ccf87db 100644
>> --- a/drivers/virtio/virtio_balloon.c
>> +++ b/drivers/virtio/virtio_balloon.c
>> @@ -38,6 +38,9 @@
>>  static unsigned int indirect_thresh;
>>  module_param(indirect_thresh, uint, S_IRUGO);
>>  
>> +static unsigned int indirect_alloc_thresh;
>> +module_param(indirect_alloc_thresh, uint, S_IRUGO);
>> +
>>  struct virtio_balloon
>>  {
>>  	struct virtio_device *vdev;
>> @@ -360,6 +363,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
>>  	vb->vdev = vdev;
>>  	vb->need_stats_update = 0;
>>  	vdev->indirect_thresh = indirect_thresh;
>> +	vdev->indirect_alloc_thresh = indirect_alloc_thresh;
>>  
>>  	err = init_vqs(vb);
>>  	if (err)
>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>> index 99a64a7..e8b9c54 100644
>> --- a/drivers/virtio/virtio_ring.c
>> +++ b/drivers/virtio/virtio_ring.c
>> @@ -93,6 +93,10 @@ struct vring_virtqueue
>>  	 */
>>  	unsigned int indirect_thresh;
>>  
>> +	/* Buffers below this size will be allocated from cache */
>> +	unsigned int indirect_alloc_thresh;
>> +	struct kmem_cache *indirect_cache;
>> +
>>  	/* Host publishes avail event idx */
>>  	bool event;
>>  
>> @@ -135,7 +139,10 @@ static int vring_add_indirect(struct vring_virtqueue *vq,
>>  	unsigned head;
>>  	int i;
>>  
>> -	desc = kmalloc((out + in) * sizeof(struct vring_desc), gfp);
>> +	if ((out + in) <= vq->indirect_alloc_thresh)
>> +		desc = kmem_cache_alloc(vq->indirect_cache, gfp);
>> +	else
>> +		desc = kmalloc((out + in) * sizeof(struct vring_desc), gfp);
>>  	if (!desc)
>>  		return -ENOMEM;
> 
> So this means that drivers need to know how large each
> descriptor is to avoid trying to allocate 32Mbyte chunks :)

Right, this interface isn't perfect and would hopefully be removed in favour of
a dynamic algorithm in the driver.

>>  
>> @@ -384,8 +391,13 @@ static void detach_buf(struct vring_virtqueue *vq, unsigned int head)
>>  	i = head;
>>  
>>  	/* Free the indirect table */
>> -	if (vq->vring.desc[i].flags & VRING_DESC_F_INDIRECT)
>> -		kfree(phys_to_virt(vq->vring.desc[i].addr));
>> +	if (vq->vring.desc[i].flags & VRING_DESC_F_INDIRECT) {
>> +		if (vq->vring.desc[i].len > vq->indirect_alloc_thresh)
> 
> This looks wrong. indirect_alloc_thresh is in entries but len is in
> bytes, isn't it?

Uh, yes. It's supposed to be '.len/sizeof(struct vring_desc)'.

>> +			kfree(phys_to_virt(vq->vring.desc[i].addr));
>> +		else
>> +			kmem_cache_free(vq->indirect_cache,
>> +					phys_to_virt(vq->vring.desc[i].addr));
>> +	}
>>  
>>  	while (vq->vring.desc[i].flags & VRING_DESC_F_NEXT) {
>>  		i = vq->vring.desc[i].next;
>> @@ -654,14 +666,20 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
>>  	vq->last_used_idx = 0;
>>  	vq->num_added = 0;
>>  	vq->indirect_thresh = 0;
>> +	vq->indirect_alloc_thresh = 0;
>> +	vq->indirect_cache = NULL;
>>  	list_add_tail(&vq->vq.list, &vdev->vqs);
>>  #ifdef DEBUG
>>  	vq->in_use = false;
>>  	vq->last_add_time_valid = false;
>>  #endif
>>  
>> -	if (virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC))
>> +	if (virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC)) {
>>  		vq->indirect_thresh = vdev->indirect_thresh;
>> +		vq->indirect_alloc_thresh = vdev->indirect_alloc_thresh;
>> +		if (vq->indirect_alloc_thresh)
>> +			vq->indirect_cache = KMEM_CACHE(vring_desc[vq->indirect_alloc_thresh],
>> 0);
> 
> I am not a purist but this line looks way too long.
> Also - no need to check cache creation succeeded?
> On failure - disable caching?
> 
> Also - let's check that values are sane before passing them on?
> They come from user after all.

will fix these two.

> Also - should not threshold be per VQ? E.g. for -net we do not
> need the cache for RX unless in legacy big packet mode.

We've discussed it two months ago over IRC (at least thats what I have in my
notes) - the plan was to keep it simple per-device until something more advanced
to deal with the threshold shows up.


Thanks,
Sasha

> 
>> +	}
>>  
>>  	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
>>  
>> @@ -685,6 +703,8 @@ EXPORT_SYMBOL_GPL(vring_new_virtqueue);
>>  void vring_del_virtqueue(struct virtqueue *vq)
>>  {
>>  	list_del(&vq->list);
>> +	if (to_vvq(vq)->indirect_cache)
>> +		kmem_cache_destroy(to_vvq(vq)->indirect_cache);
>>  	kfree(to_vvq(vq));
>>  }
>>  EXPORT_SYMBOL_GPL(vring_del_virtqueue);
>> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
>> index 48bc457..3261c02 100644
>> --- a/include/linux/virtio.h
>> +++ b/include/linux/virtio.h
>> @@ -70,6 +70,7 @@ struct virtio_device {
>>  	unsigned long features[1];
>>  	void *priv;
>>  	unsigned int indirect_thresh;
>> +	unsigned int indirect_alloc_thresh;
>>  };
>>  
>>  #define dev_to_virtio(dev) container_of(dev, struct virtio_device, dev)
>> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
>> index fc93962..3044c86 100644
>> --- a/net/9p/trans_virtio.c
>> +++ b/net/9p/trans_virtio.c
>> @@ -55,6 +55,9 @@
>>  static unsigned int indirect_thresh;
>>  module_param(indirect_thresh, uint, S_IRUGO);
>>  
>> +static unsigned int indirect_alloc_thresh;
>> +module_param(indirect_alloc_thresh, uint, S_IRUGO);
>> +
>>  /* a single mutex to manage channel initialization and attachment */
>>  static DEFINE_MUTEX(virtio_9p_lock);
>>  static DECLARE_WAIT_QUEUE_HEAD(vp_wq);
>> @@ -505,6 +508,8 @@ static int p9_virtio_probe(struct virtio_device *vdev)
>>  
>>  	/* We expect one virtqueue, for requests. */
>>  	vdev->indirect_thresh = indirect_thresh;
>> +	vdev->indirect_alloc_thresh = indirect_alloc_thresh;
>> +
>>  	chan->vq = virtio_find_single_vq(vdev, req_done, "requests");
>>  	if (IS_ERR(chan->vq)) {
>>  		err = PTR_ERR(chan->vq);
>> -- 
>> 1.7.12

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