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SubjectRe: [PATCH RFC v1 1/2] vhost: option to fetch descriptors through an independent struct
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On 2019/10/11 下午9:45, Michael S. Tsirkin wrote:
> The idea is to support multiple ring formats by converting
> to a format-independent array of descriptors.
>
> This costs extra cycles, but we gain in ability
> to fetch a batch of descriptors in one go, which
> is good for code cache locality.
>
> To simplify benchmarking, I kept the old code
> around so one can switch back and forth by
> writing into a module parameter.
> This will go away in the final submission.
>
> This patch causes a minor performance degradation,
> it's been kept as simple as possible for ease of review.
> Next patch gets us back the performance by adding batching.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
> drivers/vhost/test.c | 17 ++-
> drivers/vhost/vhost.c | 299 +++++++++++++++++++++++++++++++++++++++++-
> drivers/vhost/vhost.h | 16 +++
> 3 files changed, 327 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> index 056308008288..39a018a7af2d 100644
> --- a/drivers/vhost/test.c
> +++ b/drivers/vhost/test.c
> @@ -18,6 +18,9 @@
> #include "test.h"
> #include "vhost.h"
>
> +static int newcode = 0;
> +module_param(newcode, int, 0644);
> +
> /* Max number of bytes transferred before requeueing the job.
> * Using this limit prevents one virtqueue from starving others. */
> #define VHOST_TEST_WEIGHT 0x80000
> @@ -58,10 +61,16 @@ static void handle_vq(struct vhost_test *n)
> vhost_disable_notify(&n->dev, vq);
>
> for (;;) {
> - head = vhost_get_vq_desc(vq, vq->iov,
> - ARRAY_SIZE(vq->iov),
> - &out, &in,
> - NULL, NULL);
> + if (newcode)
> + head = vhost_get_vq_desc_batch(vq, vq->iov,
> + ARRAY_SIZE(vq->iov),
> + &out, &in,
> + NULL, NULL);
> + else
> + head = vhost_get_vq_desc(vq, vq->iov,
> + ARRAY_SIZE(vq->iov),
> + &out, &in,
> + NULL, NULL);
> /* On error, stop handling until the next kick. */
> if (unlikely(head < 0))
> break;
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 36ca2cf419bf..36661d6cb51f 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -301,6 +301,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> struct vhost_virtqueue *vq)
> {
> vq->num = 1;
> + vq->ndescs = 0;
> vq->desc = NULL;
> vq->avail = NULL;
> vq->used = NULL;
> @@ -369,6 +370,9 @@ static int vhost_worker(void *data)
>
> static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
> {
> + kfree(vq->descs);
> + vq->descs = NULL;
> + vq->max_descs = 0;
> kfree(vq->indirect);
> vq->indirect = NULL;
> kfree(vq->log);
> @@ -385,6 +389,10 @@ 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->descs = kmalloc_array(vq->max_descs,
> + sizeof(*vq->descs),
> + GFP_KERNEL);


Is iov_limit too much here? It can obviously increase the footprint. I
guess the batching can only be done for descriptor without indirect or
next set. Then we may batch 16 or 64.

Thanks

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