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

[Jason Wang]

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 <m...@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)