Re: [PATCH RFC v1 2/2] vhost: batching fetches

[Jason Wang <jasowang@redhat.com>]

On 2019/10/13 上午4:36, Michael S. Tsirkin wrote:
> On Sat, Oct 12, 2019 at 03:30:52PM +0800, Jason Wang wrote:
>> On 2019/10/11 下午9:46, Michael S. Tsirkin wrote:
>>> With this patch applied, new and old code perform identically.
>>>
>>> Lots of extra optimizations are now possible, e.g.
>>> we can fetch multiple heads with copy_from/to_user now.
>>> We can get rid of maintaining the log array.  Etc etc.
>>>
>>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>>> ---
>>>    drivers/vhost/test.c  |  2 +-
>>>    drivers/vhost/vhost.c | 50 ++++++++++++++++++++++++++++++++++++-------
>>>    drivers/vhost/vhost.h |  4 +++-
>>>    3 files changed, 46 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
>>> index 39a018a7af2d..e3a8e9db22cd 100644
>>> --- a/drivers/vhost/test.c
>>> +++ b/drivers/vhost/test.c
>>> @@ -128,7 +128,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
>>>    	dev = &n->dev;
>>>    	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
>>>    	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
>>> -	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
>>> +	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
>>>    		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT);
>>>    	f->private_data = n;
>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>>> index 36661d6cb51f..aa383e847865 100644
>>> --- a/drivers/vhost/vhost.c
>>> +++ b/drivers/vhost/vhost.c
>>> @@ -302,6 +302,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>>>    {
>>>    	vq->num = 1;
>>>    	vq->ndescs = 0;
>>> +	vq->first_desc = 0;
>>>    	vq->desc = NULL;
>>>    	vq->avail = NULL;
>>>    	vq->used = NULL;
>>> @@ -390,6 +391,7 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
>>>    	for (i = 0; i < dev->nvqs; ++i) {
>>>    		vq = dev->vqs[i];
>>>    		vq->max_descs = dev->iov_limit;
>>> +		vq->batch_descs = dev->iov_limit - UIO_MAXIOV;
>>>    		vq->descs = kmalloc_array(vq->max_descs,
>>>    					  sizeof(*vq->descs),
>>>    					  GFP_KERNEL);
>>> @@ -2366,6 +2368,8 @@ static void pop_split_desc(struct vhost_virtqueue *vq)
>>>    	--vq->ndescs;
>>>    }
>>> +#define VHOST_DESC_FLAGS (VRING_DESC_F_INDIRECT | VRING_DESC_F_WRITE | \
>>> +			  VRING_DESC_F_NEXT)
>>>    static int push_split_desc(struct vhost_virtqueue *vq, struct vring_desc *desc, u16 id)
>>>    {
>>>    	struct vhost_desc *h;
>>> @@ -2375,7 +2379,7 @@ static int push_split_desc(struct vhost_virtqueue *vq, struct vring_desc *desc,
>>>    	h = &vq->descs[vq->ndescs++];
>>>    	h->addr = vhost64_to_cpu(vq, desc->addr);
>>>    	h->len = vhost32_to_cpu(vq, desc->len);
>>> -	h->flags = vhost16_to_cpu(vq, desc->flags);
>>> +	h->flags = vhost16_to_cpu(vq, desc->flags) & VHOST_DESC_FLAGS;
>>>    	h->id = id;
>>>    	return 0;
>>> @@ -2450,7 +2454,7 @@ static int fetch_indirect_descs(struct vhost_virtqueue *vq,
>>>    	return 0;
>>>    }
>>> -static int fetch_descs(struct vhost_virtqueue *vq)
>>> +static int fetch_buf(struct vhost_virtqueue *vq)
>>>    {
>>>    	struct vring_desc desc;
>>>    	unsigned int i, head, found = 0;
>>> @@ -2462,7 +2466,11 @@ static int fetch_descs(struct vhost_virtqueue *vq)
>>>    	/* Check it isn't doing very strange things with descriptor numbers. */
>>>    	last_avail_idx = vq->last_avail_idx;
>>> -	if (vq->avail_idx == vq->last_avail_idx) {
>>> +	if (unlikely(vq->avail_idx == vq->last_avail_idx)) {
>>> +		/* If we already have work to do, don't bother re-checking. */
>>> +		if (likely(vq->ndescs))
>>> +			return vq->num;
>>> +
>>>    		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
>>>    			vq_err(vq, "Failed to access avail idx at %p\n",
>>>    				&vq->avail->idx);
>>> @@ -2541,6 +2549,24 @@ static int fetch_descs(struct vhost_virtqueue *vq)
>>>    	return 0;
>>>    }
>>> +static int fetch_descs(struct vhost_virtqueue *vq)
>>> +{
>>> +	int ret = 0;
>>> +
>>> +	if (unlikely(vq->first_desc >= vq->ndescs)) {
>>> +		vq->first_desc = 0;
>>> +		vq->ndescs = 0;
>>> +	}
>>> +
>>> +	if (vq->ndescs)
>>> +		return 0;
>>> +
>>> +	while (!ret && vq->ndescs <= vq->batch_descs)
>>> +		ret = fetch_buf(vq);
>>
>> It looks to me descriptor chaining might be broken here.
> It should work because fetch_buf fetches a whole buf, following
> the chain. Seems to work in a small test ... what issues do you see?


Consider the case when fetch_buf return -EINVAL when push_split_desc() 
fail. This means we only have partial descriptors.

Do we need decreasing last_avail_idx and vq->ndescs here? Or having a 
head,tail instead of first_desc?

Thanks


>
>>> +
>>> +	return vq->ndescs ? 0 : ret;
>>> +}
>>> +
>>>    /* This looks in the virtqueue and for the first available buffer, and converts
>>>     * it to an iovec for convenient access.  Since descriptors consist of some
>>>     * number of output then some number of input descriptors, it's actually two
>>> @@ -2562,6 +2588,8 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
>>>    	if (ret)
>>>    		return ret;
>>> +	/* Note: indirect descriptors are not batched */
>>> +	/* TODO: batch up to a limit */
>>>    	last = peek_split_desc(vq);
>>>    	id = last->id;
>>> @@ -2584,12 +2612,12 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
>>>    	if (unlikely(log))
>>>    		*log_num = 0;
>>> -	for (i = 0; i < vq->ndescs; ++i) {
>>> +	for (i = vq->first_desc; i < vq->ndescs; ++i) {
>>>    		unsigned iov_count = *in_num + *out_num;
>>>    		struct vhost_desc *desc = &vq->descs[i];
>>>    		int access;
>>> -		if (desc->flags & ~(VRING_DESC_F_INDIRECT | VRING_DESC_F_WRITE)) {
>>> +		if (desc->flags & ~VHOST_DESC_FLAGS) {
>>>    			vq_err(vq, "Unexpected flags: 0x%x at descriptor id 0x%x\n",
>>>    			       desc->flags, desc->id);
>>>    			ret = -EINVAL;
>>> @@ -2628,15 +2656,21 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
>>>    			}
>>>    			*out_num += ret;
>>>    		}
>>> +
>>> +		ret = desc->id;
>>> +
>>> +		if (!(desc->flags & VRING_DESC_F_NEXT))
>>> +			break;
>>>    	}
>>
>> What happens if we reach vq->ndescs but VRING_DESC_F_NEXT is still set?
>>
>> Thanks
> This can't happen: descriptors are pushed by push_split_desc each time
> we go through a loop in fetch_buf. The only way to exit the loop
> with return code 0 is if next_desc return -1 that is when VRING_DESC_F_NEXT
> is clear.
>
> But it's a good idea to add a BUG_ON here, I'll do it in the next version.
>
>
>>> -	ret = id;
>>> -	vq->ndescs = 0;
>>> +	vq->first_desc = i + 1;
>>>    	return ret;
>>>    err:
>>> -	vhost_discard_vq_desc(vq, 1);
>>> +	for (i = vq->first_desc; i < vq->ndescs; ++i)
>>> +		if (!(desc->flags & VRING_DESC_F_NEXT))
>>> +			vhost_discard_vq_desc(vq, 1);
>>>    	vq->ndescs = 0;
>>>    	return ret;
>>> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
>>> index 1724f61b6c2d..8b88e0c903da 100644
>>> --- a/drivers/vhost/vhost.h
>>> +++ b/drivers/vhost/vhost.h
>>> @@ -100,7 +100,9 @@ struct vhost_virtqueue {
>>>    	struct vhost_desc *descs;
>>>    	int ndescs;
>>> +	int first_desc;
>>>    	int max_descs;
>>> +	int batch_descs;
>>>    	const struct vhost_umem_node *meta_iotlb[VHOST_NUM_ADDRS];
>>>    	struct file *kick;
>>> @@ -245,7 +247,7 @@ ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
>>>    int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled);
>>>    #define vq_err(vq, fmt, ...) do {                                  \
>>> -		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
>>> +		pr_err(pr_fmt(fmt), ##__VA_ARGS__);       \
>>>    		if ((vq)->error_ctx)                               \
>>>    				eventfd_signal((vq)->error_ctx, 1);\
>>>    	} while (0)