Subject: Re: [PATCH 1/3] virtio: cache indirect desc for split

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 27 Oct 2021 13:07:19 -0400

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 <[email-masked]>
> ---
>  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 {
> @@ -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
> +
> +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);
> +	}
> +}
> +

So I have a question here. What happens if we just do:

if (n <= VIRT_QUEUE_CACHE_DESC_NUM) {
	return kmem_cache_alloc(VIRT_QUEUE_CACHE_DESC_NUM * sizeof desc, gfp)
} else {
	return kmalloc_arrat(n, sizeof desc, gfp)
}

A small change and won't we reap most performance benefits?

-- 
MST