Re: [PATCH RFC v7 03/14] vhost: use batched get_vq_desc version

Eugenio Perez Martin
2020-06-10T12:38:37Z

On Wed, Jun 10, 2020 at 1:36 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> As testing shows no performance change, switch to that now.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> Link: https://lore.kernel.org/r/20200401183118.8334-3-eperezma@redhat.com
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
> drivers/vhost/test.c | 2 +-
> drivers/vhost/vhost.c | 318 ++++++++----------------------------------
> drivers/vhost/vhost.h | 7 +-
> 3 files changed, 65 insertions(+), 262 deletions(-)
>
> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> index 0466921f4772..7d69778aaa26 100644
> --- a/drivers/vhost/test.c
> +++ b/drivers/vhost/test.c
> @@ -119,7 +119,7 @@ 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, true, NULL);
>
> f->private_data = n;
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 11433d709651..28f324fd77df 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -304,6 +304,7 @@ 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;
> @@ -372,6 +373,11 @@ static int vhost_worker(void *data)
> return 0;
> }
>
> +static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
> +{
> + return vq->max_descs - UIO_MAXIOV;
> +}
> +
> static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
> {
> kfree(vq->descs);
> @@ -394,6 +400,9 @@ 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;
> + if (vhost_vq_num_batch_descs(vq) < 0) {
> + return -EINVAL;
> + }
> vq->descs = kmalloc_array(vq->max_descs,
> sizeof(*vq->descs),
> GFP_KERNEL);
> @@ -1610,6 +1619,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
> vq->last_avail_idx = s.num;
> /* Forget the cached index value. */
> vq->avail_idx = vq->last_avail_idx;
> + vq->ndescs = vq->first_desc = 0;
> break;
> case VHOST_GET_VRING_BASE:
> s.index = idx;
> @@ -2078,253 +2088,6 @@ static unsigned next_desc(struct vhost_virtqueue *vq, struct vring_desc *desc)
> return next;
> }
>
> -static int get_indirect(struct vhost_virtqueue *vq,
> - struct iovec iov[], unsigned int iov_size,
> - unsigned int *out_num, unsigned int *in_num,
> - struct vhost_log *log, unsigned int *log_num,
> - struct vring_desc *indirect)
> -{
> - struct vring_desc desc;
> - unsigned int i = 0, count, found = 0;
> - u32 len = vhost32_to_cpu(vq, indirect->len);
> - struct iov_iter from;
> - int ret, access;
> -
> - /* Sanity check */
> - if (unlikely(len % sizeof desc)) {
> - vq_err(vq, "Invalid length in indirect descriptor: "
> - "len 0x%llx not multiple of 0x%zx\n",
> - (unsigned long long)len,
> - sizeof desc);
> - return -EINVAL;
> - }
> -
> - ret = translate_desc(vq, vhost64_to_cpu(vq, indirect->addr), len, vq->indirect,
> - UIO_MAXIOV, VHOST_ACCESS_RO);
> - if (unlikely(ret < 0)) {
> - if (ret != -EAGAIN)
> - vq_err(vq, "Translation failure %d in indirect.\n", ret);
> - return ret;
> - }
> - iov_iter_init(&from, READ, vq->indirect, ret, len);
> -
> - /* We will use the result as an address to read from, so most
> - * architectures only need a compiler barrier here. */
> - read_barrier_depends();
> -
> - count = len / sizeof desc;
> - /* Buffers are chained via a 16 bit next field, so
> - * we can have at most 2^16 of these. */
> - if (unlikely(count > USHRT_MAX + 1)) {
> - vq_err(vq, "Indirect buffer length too big: %d\n",
> - indirect->len);
> - return -E2BIG;
> - }
> -
> - do {
> - unsigned iov_count = *in_num + *out_num;
> - if (unlikely(++found > count)) {
> - vq_err(vq, "Loop detected: last one at %u "
> - "indirect size %u\n",
> - i, count);
> - return -EINVAL;
> - }
> - if (unlikely(!copy_from_iter_full(&desc, sizeof(desc), &from))) {
> - vq_err(vq, "Failed indirect descriptor: idx %d, %zx\n",
> - i, (size_t)vhost64_to_cpu(vq, indirect->addr) + i * sizeof desc);
> - return -EINVAL;
> - }
> - if (unlikely(desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_INDIRECT))) {
> - vq_err(vq, "Nested indirect descriptor: idx %d, %zx\n",
> - i, (size_t)vhost64_to_cpu(vq, indirect->addr) + i * sizeof desc);
> - return -EINVAL;
> - }
> -
> - if (desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_WRITE))
> - access = VHOST_ACCESS_WO;
> - else
> - access = VHOST_ACCESS_RO;
> -
> - ret = translate_desc(vq, vhost64_to_cpu(vq, desc.addr),
> - vhost32_to_cpu(vq, desc.len), iov + iov_count,
> - iov_size - iov_count, access);
> - if (unlikely(ret < 0)) {
> - if (ret != -EAGAIN)
> - vq_err(vq, "Translation failure %d indirect idx %d\n",
> - ret, i);
> - return ret;
> - }
> - /* If this is an input descriptor, increment that count. */
> - if (access == VHOST_ACCESS_WO) {
> - *in_num += ret;
> - if (unlikely(log && ret)) {
> - log[*log_num].addr = vhost64_to_cpu(vq, desc.addr);
> - log[*log_num].len = vhost32_to_cpu(vq, desc.len);
> - ++*log_num;
> - }
> - } else {
> - /* If it's an output descriptor, they're all supposed
> - * to come before any input descriptors. */
> - if (unlikely(*in_num)) {
> - vq_err(vq, "Indirect descriptor "
> - "has out after in: idx %d\n", i);
> - return -EINVAL;
> - }
> - *out_num += ret;
> - }
> - } while ((i = next_desc(vq, &desc)) != -1);
> - return 0;
> -}
> -
> -/* 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
> - * iovecs, but we pack them into one and note how many of each there were.
> - *
> - * This function returns the descriptor number found, or vq->num (which is
> - * never a valid descriptor number) if none was found. A negative code is
> - * returned on error. */
> -int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> - struct iovec iov[], unsigned int iov_size,
> - unsigned int *out_num, unsigned int *in_num,
> - struct vhost_log *log, unsigned int *log_num)
> -{
> - struct vring_desc desc;
> - unsigned int i, head, found = 0;
> - u16 last_avail_idx;
> - __virtio16 avail_idx;
> - __virtio16 ring_head;
> - int ret, access;
> -
> - /* 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(vhost_get_avail_idx(vq, &avail_idx))) {
> - vq_err(vq, "Failed to access avail idx at %p\n",
> - &vq->avail->idx);
> - return -EFAULT;
> - }
> - vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> -
> - if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
> - vq_err(vq, "Guest moved used index from %u to %u",
> - last_avail_idx, vq->avail_idx);
> - return -EFAULT;
> - }
> -
> - /* If there's nothing new since last we looked, return
> - * invalid.
> - */
> - if (vq->avail_idx == last_avail_idx)
> - return vq->num;
> -
> - /* Only get avail ring entries after they have been
> - * exposed by guest.
> - */
> - smp_rmb();
> - }
> -
> - /* Grab the next descriptor number they're advertising, and increment
> - * the index we've seen. */
> - if (unlikely(vhost_get_avail_head(vq, &ring_head, last_avail_idx))) {
> - vq_err(vq, "Failed to read head: idx %d address %p\n",
> - last_avail_idx,
> - &vq->avail->ring[last_avail_idx % vq->num]);
> - return -EFAULT;
> - }
> -
> - head = vhost16_to_cpu(vq, ring_head);
> -
> - /* If their number is silly, that's an error. */
> - if (unlikely(head >= vq->num)) {
> - vq_err(vq, "Guest says index %u > %u is available",
> - head, vq->num);
> - return -EINVAL;
> - }
> -
> - /* When we start there are none of either input nor output. */
> - *out_num = *in_num = 0;
> - if (unlikely(log))
> - *log_num = 0;
> -
> - i = head;
> - do {
> - unsigned iov_count = *in_num + *out_num;
> - if (unlikely(i >= vq->num)) {
> - vq_err(vq, "Desc index is %u > %u, head = %u",
> - i, vq->num, head);
> - return -EINVAL;
> - }
> - if (unlikely(++found > vq->num)) {
> - vq_err(vq, "Loop detected: last one at %u "
> - "vq size %u head %u\n",
> - i, vq->num, head);
> - return -EINVAL;
> - }
> - ret = vhost_get_desc(vq, &desc, i);
> - if (unlikely(ret)) {
> - vq_err(vq, "Failed to get descriptor: idx %d addr %p\n",
> - i, vq->desc + i);
> - return -EFAULT;
> - }
> - if (desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_INDIRECT)) {
> - ret = get_indirect(vq, iov, iov_size,
> - out_num, in_num,
> - log, log_num, &desc);
> - if (unlikely(ret < 0)) {
> - if (ret != -EAGAIN)
> - vq_err(vq, "Failure detected "
> - "in indirect descriptor at idx %d\n", i);
> - return ret;
> - }
> - continue;
> - }
> -
> - if (desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_WRITE))
> - access = VHOST_ACCESS_WO;
> - else
> - access = VHOST_ACCESS_RO;
> - ret = translate_desc(vq, vhost64_to_cpu(vq, desc.addr),
> - vhost32_to_cpu(vq, desc.len), iov + iov_count,
> - iov_size - iov_count, access);
> - if (unlikely(ret < 0)) {
> - if (ret != -EAGAIN)
> - vq_err(vq, "Translation failure %d descriptor idx %d\n",
> - ret, i);
> - return ret;
> - }
> - if (access == VHOST_ACCESS_WO) {
> - /* If this is an input descriptor,
> - * increment that count. */
> - *in_num += ret;
> - if (unlikely(log && ret)) {
> - log[*log_num].addr = vhost64_to_cpu(vq, desc.addr);
> - log[*log_num].len = vhost32_to_cpu(vq, desc.len);
> - ++*log_num;
> - }
> - } else {
> - /* If it's an output descriptor, they're all supposed
> - * to come before any input descriptors. */
> - if (unlikely(*in_num)) {
> - vq_err(vq, "Descriptor has out after in: "
> - "idx %d\n", i);
> - return -EINVAL;
> - }
> - *out_num += ret;
> - }
> - } while ((i = next_desc(vq, &desc)) != -1);
> -
> - /* On success, increment avail index. */
> - vq->last_avail_idx++;
> -
> - /* Assume notifications from guest are disabled at this point,
> - * if they aren't we would need to update avail_event index. */
> - BUG_ON(!(vq->used_flags & VRING_USED_F_NO_NOTIFY));
> - return head;
> -}
> -EXPORT_SYMBOL_GPL(vhost_get_vq_desc);
> -
> static struct vhost_desc *peek_split_desc(struct vhost_virtqueue *vq)
> {
> BUG_ON(!vq->ndescs);
> @@ -2428,7 +2191,7 @@ static int fetch_indirect_descs(struct vhost_virtqueue *vq,
>
> /* This function returns a value > 0 if a descriptor was found, or 0 if none were found.
> * A negative code is returned on error. */
> -static int fetch_descs(struct vhost_virtqueue *vq)
> +static int fetch_buf(struct vhost_virtqueue *vq)
> {
> unsigned int i, head, found = 0;
> struct vhost_desc *last;
> @@ -2441,7 +2204,11 @@ 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 1;
> +
> if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
> vq_err(vq, "Failed to access avail idx at %p\n",
> &vq->avail->idx);
> @@ -2532,6 +2299,41 @@ static int fetch_descs(struct vhost_virtqueue *vq)
> return 1;
> }
>
> +/* This function returns a value > 0 if a descriptor was found, or 0 if none were found.
> + * A negative code is returned on error. */
> +static int fetch_descs(struct vhost_virtqueue *vq)
> +{
> + int ret;
> +
> + if (unlikely(vq->first_desc >= vq->ndescs)) {
> + vq->first_desc = 0;
> + vq->ndescs = 0;
> + }
> +
> + if (vq->ndescs)
> + return 1;
> +
> + for (ret = 1;
> + ret > 0 && vq->ndescs <= vhost_vq_num_batch_descs(vq);
> + ret = fetch_buf(vq))
> + ;

(Expanding comment in V6):

We get an infinite loop this way:
* vq->ndescs == 0, so we call fetch_buf() here
* fetch_buf gets less than vhost_vq_num_batch_descs(vq); descriptors. ret = 1
* This loop calls again fetch_buf, but vq->ndescs > 0 (and avail_vq ==
last_avail_vq), so it just return 1

I think that we should check vq->ndescs == 0 instead of ndescs <=
vhost_vq_num_batch_descs(vq). However, this could cause less
descriptors to be fetches/cached, so less thoughput. Another
possibility is to compare with vhost_vq_num_batch_descs(vq) for the
"early return", or to find a sensible limit (increasing latency?).

In my local version:

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 28f324fd77df..50b258a46cef 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2310,12 +2310,7 @@ static int fetch_descs(struct vhost_virtqueue *vq)
vq->ndescs = 0;
}

- if (vq->ndescs)
- return 1;
-
- for (ret = 1;
- ret > 0 && vq->ndescs <= vhost_vq_num_batch_descs(vq);
- ret = fetch_buf(vq))
+ for (ret = 1; ret > 0 && vq->ndescs == 0; ret = fetch_buf(vq))
;

/* On success we expect some descs */
---

> +
> + /* On success we expect some descs */
> + BUG_ON(ret > 0 && !vq->ndescs);
> + return ret;
> +}
> +
> +/* Reverse the effects of fetch_descs */
> +static void unfetch_descs(struct vhost_virtqueue *vq)
> +{
> + int i;
> +
> + for (i = vq->first_desc; i < vq->ndescs; ++i)
> + if (!(vq->descs[i].flags & VRING_DESC_F_NEXT))
> + vq->last_avail_idx -= 1;
> + vq->ndescs = 0;
> +}
> +
> /* 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
> @@ -2540,7 +2342,7 @@ static int fetch_descs(struct vhost_virtqueue *vq)
> * This function returns the descriptor number found, or vq->num (which is
> * never a valid descriptor number) if none was found. A negative code is
> * returned on error. */
> -int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
> +int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> struct iovec iov[], unsigned int iov_size,
> unsigned int *out_num, unsigned int *in_num,
> struct vhost_log *log, unsigned int *log_num)
> @@ -2549,7 +2351,7 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
> int i;
>
> if (ret <= 0)
> - goto err_fetch;
> + goto err;
>
> /* Now convert to IOV */
> /* When we start there are none of either input nor output. */
> @@ -2557,7 +2359,7 @@ 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;
> @@ -2603,24 +2405,26 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
> }
>
> ret = desc->id;
> +
> + if (!(desc->flags & VRING_DESC_F_NEXT))
> + break;
> }
>
> - vq->ndescs = 0;
> + vq->first_desc = i + 1;
>
> return ret;
>
> err:
> - vhost_discard_vq_desc(vq, 1);
> -err_fetch:
> - vq->ndescs = 0;
> + unfetch_descs(vq);
>
> return ret ? ret : vq->num;
> }
> -EXPORT_SYMBOL_GPL(vhost_get_vq_desc_batch);
> +EXPORT_SYMBOL_GPL(vhost_get_vq_desc);
>
> /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
> void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n)
> {
> + unfetch_descs(vq);
> vq->last_avail_idx -= n;
> }
> EXPORT_SYMBOL_GPL(vhost_discard_vq_desc);
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 87089d51490d..fed36af5c444 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -81,6 +81,7 @@ struct vhost_virtqueue {
>
> struct vhost_desc *descs;
> int ndescs;
> + int first_desc;
> int max_descs;
>
> struct file *kick;
> @@ -189,10 +190,6 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
> bool vhost_vq_access_ok(struct vhost_virtqueue *vq);
> bool vhost_log_access_ok(struct vhost_dev *);
>
> -int vhost_get_vq_desc_batch(struct vhost_virtqueue *,
> - struct iovec iov[], unsigned int iov_count,
> - unsigned int *out_num, unsigned int *in_num,
> - struct vhost_log *log, unsigned int *log_num);
> int vhost_get_vq_desc(struct vhost_virtqueue *,
> struct iovec iov[], unsigned int iov_count,
> unsigned int *out_num, unsigned int *in_num,
> @@ -261,6 +258,8 @@ static inline void vhost_vq_set_backend(struct vhost_virtqueue *vq,
> void *private_data)
> {
> vq->private_data = private_data;
> + vq->ndescs = 0;
> + vq->first_desc = 0;
> }
>
> /**
> --
> MST
>