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Date: Wed, 27 Oct 2021 04:55:16 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
Jason Wang <jasowang@...hat.com>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH 1/3] virtio: cache indirect desc for split
On Wed, Oct 27, 2021 at 02:19:11PM +0800, Xuan Zhuo wrote:
> In the case of using indirect, indirect desc must be allocated and
> released each time, which increases a lot of cpu overhead.
>
> Here, a cache is added for indirect. If the number of indirect desc to be
> applied for is less than VIRT_QUEUE_CACHE_DESC_NUM, the desc array with
> the size of VIRT_QUEUE_CACHE_DESC_NUM is fixed and cached for reuse.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> ---
> drivers/virtio/virtio.c | 6 ++++
> drivers/virtio/virtio_ring.c | 63 ++++++++++++++++++++++++++++++------
> include/linux/virtio.h | 10 ++++++
> 3 files changed, 70 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 0a5b54034d4b..04bcb74e5b9a 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -431,6 +431,12 @@ bool is_virtio_device(struct device *dev)
> }
> EXPORT_SYMBOL_GPL(is_virtio_device);
>
> +void virtio_use_desc_cache(struct virtio_device *dev, bool val)
> +{
> + dev->desc_cache = val;
> +}
> +EXPORT_SYMBOL_GPL(virtio_use_desc_cache);
> +
> void unregister_virtio_device(struct virtio_device *dev)
> {
> int index = dev->index; /* save for after device release */
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index dd95dfd85e98..0b9a8544b0e8 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -117,6 +117,10 @@ struct vring_virtqueue {
> /* Hint for event idx: already triggered no need to disable. */
> bool event_triggered;
>
> + /* Is indirect cache used? */
> + bool use_desc_cache;
> + void *desc_cache_chain;
> +
> union {
> /* Available for split ring */
> struct {
Let's use llist_head and friends please (I am guessing we want
single-linked to avoid writing into indirect buffer after use,
invalidating the cache, but please document why in a comment). Do not
open-code it.
Also hide all casts in inline wrappers.
> @@ -423,12 +427,47 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
> return extra[i].next;
> }
>
> -static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
> +#define VIRT_QUEUE_CACHE_DESC_NUM 4
Add an inline wrapper for checking sg versus VIRT_QUEUE_CACHE_DESC_NUM.
> +
> +static void desc_cache_chain_free_split(void *chain)
> +{
> + struct vring_desc *desc;
> +
> + while (chain) {
> + desc = chain;
> + chain = (void *)desc->addr;
> + kfree(desc);
> + }
> +}
> +
> +static void desc_cache_put_split(struct vring_virtqueue *vq,
> + struct vring_desc *desc, int n)
> +{
> + if (vq->use_desc_cache && n <= VIRT_QUEUE_CACHE_DESC_NUM) {
> + desc->addr = (u64)vq->desc_cache_chain;
> + vq->desc_cache_chain = desc;
> + } else {
> + kfree(desc);
> + }
> +}
> +
> +static struct vring_desc *alloc_indirect_split(struct vring_virtqueue *vq,
> unsigned int total_sg,
> gfp_t gfp)
> {
> struct vring_desc *desc;
> - unsigned int i;
> + unsigned int i, n;
> +
> + if (vq->use_desc_cache && total_sg <= VIRT_QUEUE_CACHE_DESC_NUM) {
> + if (vq->desc_cache_chain) {
> + desc = vq->desc_cache_chain;
> + vq->desc_cache_chain = (void *)desc->addr;
> + goto got;
> + }
> + n = VIRT_QUEUE_CACHE_DESC_NUM;
Hmm. This will allocate more entries than actually used. Why do it?
> + } else {
> + n = total_sg;
> + }
>
> /*
> * We require lowmem mappings for the descriptors because
> @@ -437,12 +476,13 @@ static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
> */
> gfp &= ~__GFP_HIGHMEM;
>
> - desc = kmalloc_array(total_sg, sizeof(struct vring_desc), gfp);
> + desc = kmalloc_array(n, sizeof(struct vring_desc), gfp);
> if (!desc)
> return NULL;
>
> +got:
> for (i = 0; i < total_sg; i++)
> - desc[i].next = cpu_to_virtio16(_vq->vdev, i + 1);
> + desc[i].next = cpu_to_virtio16(vq->vq.vdev, i + 1);
> return desc;
> }
>
> @@ -508,7 +548,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> head = vq->free_head;
>
> if (virtqueue_use_indirect(_vq, total_sg))
> - desc = alloc_indirect_split(_vq, total_sg, gfp);
> + desc = alloc_indirect_split(vq, total_sg, gfp);
> else {
> desc = NULL;
> WARN_ON_ONCE(total_sg > vq->split.vring.num && !vq->indirect);
> @@ -652,7 +692,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> }
>
> if (indirect)
> - kfree(desc);
> + desc_cache_put_split(vq, desc, total_sg);
>
> END_USE(vq);
> return -ENOMEM;
> @@ -717,7 +757,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
> if (vq->indirect) {
> struct vring_desc *indir_desc =
> vq->split.desc_state[head].indir_desc;
> - u32 len;
> + u32 len, n;
>
> /* Free the indirect table, if any, now that it's unmapped. */
> if (!indir_desc)
> @@ -729,10 +769,12 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
> VRING_DESC_F_INDIRECT));
> BUG_ON(len == 0 || len % sizeof(struct vring_desc));
>
> - for (j = 0; j < len / sizeof(struct vring_desc); j++)
> + n = len / sizeof(struct vring_desc);
> +
> + for (j = 0; j < n; j++)
> vring_unmap_one_split_indirect(vq, &indir_desc[j]);
>
> - kfree(indir_desc);
> + desc_cache_put_split(vq, indir_desc, n);
> vq->split.desc_state[head].indir_desc = NULL;
> } else if (ctx) {
> *ctx = vq->split.desc_state[head].indir_desc;
> @@ -2199,6 +2241,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC) &&
> !context;
> vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
> + vq->desc_cache_chain = NULL;
> + vq->use_desc_cache = vdev->desc_cache;
>
> if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
> vq->weak_barriers = false;
> @@ -2329,6 +2373,7 @@ void vring_del_virtqueue(struct virtqueue *_vq)
> if (!vq->packed_ring) {
> kfree(vq->split.desc_state);
> kfree(vq->split.desc_extra);
> + desc_cache_chain_free_split(vq->desc_cache_chain);
> }
> kfree(vq);
> }
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 41edbc01ffa4..d84b7b8f4070 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -109,6 +109,7 @@ struct virtio_device {
> bool failed;
> bool config_enabled;
> bool config_change_pending;
> + bool desc_cache;
> spinlock_t config_lock;
> spinlock_t vqs_list_lock; /* Protects VQs list access */
> struct device dev;
> @@ -130,6 +131,15 @@ int register_virtio_device(struct virtio_device *dev);
> void unregister_virtio_device(struct virtio_device *dev);
> bool is_virtio_device(struct device *dev);
>
> +/**
> + * virtio_use_desc_cache - virtio ring use desc cache
> + *
> + * virtio will cache the allocated indirect desc.
> + *
> + * This function must be called before find_vqs.
> + */
> +void virtio_use_desc_cache(struct virtio_device *dev, bool val);
> +
> void virtio_break_device(struct virtio_device *dev);
>
> void virtio_config_changed(struct virtio_device *dev);
> --
> 2.31.0
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