Re: [PATCH net-next RFC 2/5] vhost: introduce helper to prefetch desc index

Jason Wang
2017-09-28T07:44:50Z

On 2017年09月28日 08:47, Willem de Bruijn wrote:
> On Fri, Sep 22, 2017 at 4:02 AM, Jason Wang <jasowang@redhat.com> wrote:
>> This patch introduces vhost_prefetch_desc_indices() which could batch
>> descriptor indices fetching and used ring updating. This intends to
>> reduce the cache misses of indices fetching and updating and reduce
>> cache line bounce when virtqueue is almost full. copy_to_user() was
>> used in order to benefit from modern cpus that support fast string
>> copy. Batched virtqueue processing will be the first user.
>>
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>> ---
>> drivers/vhost/vhost.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++
>> drivers/vhost/vhost.h | 3 +++
>> 2 files changed, 58 insertions(+)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index f87ec75..8424166d 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -2437,6 +2437,61 @@ struct vhost_msg_node *vhost_dequeue_msg(struct vhost_dev *dev,
>> }
>> EXPORT_SYMBOL_GPL(vhost_dequeue_msg);
>>
>> +int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq,
>> + struct vring_used_elem *heads,
>> + u16 num, bool used_update)
>> +{
>> + int ret, ret2;
>> + u16 last_avail_idx, last_used_idx, total, copied;
>> + __virtio16 avail_idx;
>> + struct vring_used_elem __user *used;
>> + int i;
>> +
>> + if (unlikely(vhost_get_avail(vq, avail_idx, &vq->avail->idx))) {
>> + vq_err(vq, "Failed to access avail idx at %p\n",
>> + &vq->avail->idx);
>> + return -EFAULT;
>> + }
>> + last_avail_idx = vq->last_avail_idx & (vq->num - 1);
>> + vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>> + total = vq->avail_idx - vq->last_avail_idx;
>> + ret = total = min(total, num);
>> +
>> + for (i = 0; i < ret; i++) {
>> + ret2 = vhost_get_avail(vq, heads[i].id,
>> + &vq->avail->ring[last_avail_idx]);
>> + if (unlikely(ret2)) {
>> + vq_err(vq, "Failed to get descriptors\n");
>> + return -EFAULT;
>> + }
>> + last_avail_idx = (last_avail_idx + 1) & (vq->num - 1);
>> + }
> This is understandably very similar to the existing logic in vhost_get_vq_desc.
> Can that be extracted to a helper to avoid code duplication?
>
> Perhaps one helper to update vq->avail_idx and return num, and
> another to call vhost_get_avail one or more times.

Yes it can.

>
>> +
>> + if (!used_update)
>> + return ret;
>> +
>> + last_used_idx = vq->last_used_idx & (vq->num - 1);
>> + while (total) {
>> + copied = min((u16)(vq->num - last_used_idx), total);
>> + ret2 = vhost_copy_to_user(vq,
>> + &vq->used->ring[last_used_idx],
>> + &heads[ret - total],
>> + copied * sizeof(*used));
>> +
>> + if (unlikely(ret2)) {
>> + vq_err(vq, "Failed to update used ring!\n");
>> + return -EFAULT;
>> + }
>> +
>> + last_used_idx = 0;
>> + total -= copied;
>> + }
> This second part seems unrelated and could be a separate function?

Yes.

>
> Also, no need for ret2 and double assignment "ret = total =" if not
> modifying total
> in the the second loop:
>
> for (i = 0; i < total; ) {
> ...
> i += copied;
> }

Right, will do this in V2.

Thanks