Re: [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@xxxxxxxxxx>
---
  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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