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Date: Fri, 29 Oct 2021 10:20:04 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: virtualization <virtualization@...ts.linux-foundation.org>,
netdev <netdev@...r.kernel.org>,
"Michael S. Tsirkin" <mst@...hat.com>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH v2 1/3] virtio: cache indirect desc for split
On Thu, Oct 28, 2021 at 6:49 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> 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.
The commit log looks out of date? It's actually desc_cache_thr now.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> ---
> drivers/virtio/virtio.c | 6 +++
> drivers/virtio/virtio_ring.c | 77 ++++++++++++++++++++++++++++++++----
> include/linux/virtio.h | 14 +++++++
> 3 files changed, 89 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 0a5b54034d4b..1047149ac2a4 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_set_desc_cache(struct virtio_device *dev, u32 thr)
> +{
> + dev->desc_cache_thr = thr;
> +}
> +EXPORT_SYMBOL_GPL(virtio_set_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..0ebcd4f12d3b 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -117,6 +117,15 @@ struct vring_virtqueue {
> /* Hint for event idx: already triggered no need to disable. */
> bool event_triggered;
>
> + /* desc cache threshold
> + * 0 - disable desc cache
> + * > 0 - enable desc cache. As the threshold of the desc cache.
> + */
> + u32 desc_cache_thr;
> +
> + /* desc cache chain */
> + struct list_head desc_cache;
> +
> union {
> /* Available for split ring */
> struct {
> @@ -423,7 +432,53 @@ 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,
> +static void desc_cache_free(struct list_head *head)
> +{
> + struct list_head *n, *pos;
> +
> + BUILD_BUG_ON(sizeof(struct list_head) > sizeof(struct vring_desc));
> + BUILD_BUG_ON(sizeof(struct list_head) > sizeof(struct vring_packed_desc));
> +
> + list_for_each_prev_safe(pos, n, head)
> + kfree(pos);
> +}
> +
> +static void __desc_cache_put(struct vring_virtqueue *vq,
> + struct list_head *node, int n)
> +{
> + if (n <= vq->desc_cache_thr)
> + list_add(node, &vq->desc_cache);
> + else
> + kfree(node);
> +}
> +
> +#define desc_cache_put(vq, desc, n) \
> + __desc_cache_put(vq, (struct list_head *)desc, n)
> +
> +static void *desc_cache_get(struct vring_virtqueue *vq,
> + int size, int n, gfp_t gfp)
> +{
> + struct list_head *node;
> +
> + if (n > vq->desc_cache_thr)
> + return kmalloc_array(n, size, gfp);
> +
> + if (!list_empty(&vq->desc_cache)) {
> + node = vq->desc_cache.next;
> + list_del(node);
> + return node;
> + }
> +
> + return kmalloc_array(vq->desc_cache_thr, size, gfp);
Instead of doing the if .. kmalloc_array here. I wonder if we can
simply pre-allocate per buffer indirect descriptors array.
E.g if we use MAX_SKB_FRAGS + 1, it will be ~64KB for 256 queue size
and ~272KB for 1024 queue size.
Then we avoid list manipulation and kmalloc in datapath with much more
simpler codes.
Thanks
> +}
> +
> +#define _desc_cache_get(vq, n, gfp, tp) \
> + ((tp *)desc_cache_get(vq, (sizeof(tp)), n, gfp))
> +
> +#define desc_cache_get_split(vq, n, gfp) \
> + _desc_cache_get(vq, n, gfp, struct vring_desc)
> +
> +static struct vring_desc *alloc_indirect_split(struct vring_virtqueue *vq,
> unsigned int total_sg,
> gfp_t gfp)
> {
> @@ -437,12 +492,12 @@ static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
> */
> gfp &= ~__GFP_HIGHMEM;
>
> - desc = kmalloc_array(total_sg, sizeof(struct vring_desc), gfp);
> + desc = desc_cache_get_split(vq, total_sg, gfp);
> if (!desc)
> return NULL;
>
> 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 +563,7 @@ 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 +707,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> }
>
> if (indirect)
> - kfree(desc);
> + desc_cache_put(vq, desc, total_sg);
>
> END_USE(vq);
> return -ENOMEM;
> @@ -717,7 +772,7 @@ 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 +784,12 @@ 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(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 +2256,9 @@ 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_thr = vdev->desc_cache_thr;
> +
> + INIT_LIST_HEAD(&vq->desc_cache);
>
> if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
> vq->weak_barriers = false;
> @@ -2329,6 +2389,7 @@ void vring_del_virtqueue(struct virtqueue *_vq)
> if (!vq->packed_ring) {
> kfree(vq->split.desc_state);
> kfree(vq->split.desc_extra);
> + desc_cache_free(&vq->desc_cache);
> }
> kfree(vq);
> }
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 41edbc01ffa4..bda6f9853e97 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -118,6 +118,7 @@ struct virtio_device {
> struct list_head vqs;
> u64 features;
> void *priv;
> + u32 desc_cache_thr;
> };
>
> static inline struct virtio_device *dev_to_virtio(struct device *_dev)
> @@ -130,6 +131,19 @@ int register_virtio_device(struct virtio_device *dev);
> void unregister_virtio_device(struct virtio_device *dev);
> bool is_virtio_device(struct device *dev);
>
> +/**
> + * virtio_set_desc_cache - set virtio ring desc cache threshold
> + *
> + * virtio will cache the allocated indirect desc.
> + *
> + * This function must be called before find_vqs.
> + *
> + * @thr:
> + * 0 - disable desc cache
> + * > 0 - enable desc cache. As the threshold of the desc cache.
> + */
> +void virtio_set_desc_cache(struct virtio_device *dev, u32 thr);
> +
> void virtio_break_device(struct virtio_device *dev);
>
> void virtio_config_changed(struct virtio_device *dev);
> --
> 2.31.0
>
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