Re: [PATCH] rpmsg: char: Use preallocated SKBs.

[Bjorn Andersson]

On Tue, Dec 06, 2022 at 10:38:41AM +0100, Piotr Wojtaszczyk wrote:
> On a message reception copy the message to a SKB taken from preallocated
> pool instead of allocating a new SKB each time.
> During high rpmsg traffic this reduces consumed CPU time noticeably.
> 
> Signed-off-by: Piotr Wojtaszczyk <piotr.wojtaszczyk@xxxxxxxxxxx>
> ---
>  drivers/rpmsg/rpmsg_char.c       | 46 +++++++++++++++++++++++++++++---
>  drivers/rpmsg/rpmsg_internal.h   | 21 +++++++++++++++
>  drivers/rpmsg/virtio_rpmsg_bus.c | 21 ---------------
>  3 files changed, 64 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/rpmsg/rpmsg_char.c b/drivers/rpmsg/rpmsg_char.c
> index 3e0b8f3496ed..51b1b077687e 100644
> --- a/drivers/rpmsg/rpmsg_char.c
> +++ b/drivers/rpmsg/rpmsg_char.c
> @@ -66,10 +66,37 @@ struct rpmsg_eptdev {
>  
>  	spinlock_t queue_lock;
>  	struct sk_buff_head queue;
> +	struct sk_buff_head skb_pool;
>  	wait_queue_head_t readq;
>  
>  };
>  
> +static inline
> +struct sk_buff *rpmsg_eptdev_get_skb(struct rpmsg_eptdev *eptdev)
> +{
> +	struct sk_buff *skb;
> +
> +	skb = skb_dequeue(&eptdev->skb_pool);
> +	if (!skb)
> +		skb = alloc_skb(MAX_RPMSG_BUF_SIZE, GFP_ATOMIC);
> +	return skb;
> +}
> +
> +static inline
> +void rpmsg_eptdev_put_skb(struct rpmsg_eptdev *eptdev, struct sk_buff *skb)
> +{
> +	/* Recycle the skb */
> +	skb->tail = 0;
> +	skb->len = 0;
> +	skb_queue_head(&eptdev->skb_pool, skb);
> +}
> +
> +static void rpmsg_eptdev_free_all_skb(struct rpmsg_eptdev *eptdev)
> +{
> +	skb_queue_purge(&eptdev->queue);
> +	skb_queue_purge(&eptdev->skb_pool);
> +}
> +
>  int rpmsg_chrdev_eptdev_destroy(struct device *dev, void *data)
>  {
>  	struct rpmsg_eptdev *eptdev = dev_to_eptdev(dev);
> @@ -99,7 +126,7 @@ static int rpmsg_ept_cb(struct rpmsg_device *rpdev, void *buf, int len,
>  	struct rpmsg_eptdev *eptdev = priv;
>  	struct sk_buff *skb;
>  
> -	skb = alloc_skb(len, GFP_ATOMIC);
> +	skb = rpmsg_eptdev_get_skb(eptdev);
>  	if (!skb)
>  		return -ENOMEM;
>  
> @@ -121,6 +148,18 @@ static int rpmsg_eptdev_open(struct inode *inode, struct file *filp)
>  	struct rpmsg_endpoint *ept;
>  	struct rpmsg_device *rpdev = eptdev->rpdev;
>  	struct device *dev = &eptdev->dev;
> +	struct sk_buff *skb;
> +	int i;
> +
> +	/* Preallocate 8 SKBs */
> +	for (i = 0; i < 8; i++) {
> +		skb = rpmsg_eptdev_get_skb(eptdev);
> +		if (!skb) {
> +			rpmsg_eptdev_free_all_skb(eptdev);
> +			return -ENOMEM;
> +		}
> +		rpmsg_eptdev_put_skb(eptdev, skb);
> +	}
>  
>  	mutex_lock(&eptdev->ept_lock);
>  	if (eptdev->ept) {
> @@ -168,7 +207,7 @@ static int rpmsg_eptdev_release(struct inode *inode, struct file *filp)
>  	mutex_unlock(&eptdev->ept_lock);
>  
>  	/* Discard all SKBs */
> -	skb_queue_purge(&eptdev->queue);
> +	rpmsg_eptdev_free_all_skb(eptdev);
>  
>  	put_device(dev);
>  
> @@ -217,7 +256,7 @@ static ssize_t rpmsg_eptdev_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  	if (copy_to_iter(skb->data, use, to) != use)
>  		use = -EFAULT;
>  
> -	kfree_skb(skb);
> +	rpmsg_eptdev_put_skb(eptdev, skb);
>  
>  	return use;
>  }
> @@ -370,6 +409,7 @@ static struct rpmsg_eptdev *rpmsg_chrdev_eptdev_alloc(struct rpmsg_device *rpdev
>  	mutex_init(&eptdev->ept_lock);
>  	spin_lock_init(&eptdev->queue_lock);
>  	skb_queue_head_init(&eptdev->queue);
> +	skb_queue_head_init(&eptdev->skb_pool);
>  	init_waitqueue_head(&eptdev->readq);
>  
>  	device_initialize(dev);
> diff --git a/drivers/rpmsg/rpmsg_internal.h b/drivers/rpmsg/rpmsg_internal.h
> index 39b646d0d40d..b30bfe01db69 100644
> --- a/drivers/rpmsg/rpmsg_internal.h
> +++ b/drivers/rpmsg/rpmsg_internal.h
> @@ -15,6 +15,27 @@
>  #include <linux/rpmsg.h>
>  #include <linux/poll.h>
>  
> +/*
> + * We're allocating buffers of 512 bytes each for communications. The
> + * number of buffers will be computed from the number of buffers supported
> + * by the vring, upto a maximum of 512 buffers (256 in each direction).
> + *
> + * Each buffer will have 16 bytes for the msg header and 496 bytes for
> + * the payload.
> + *
> + * This will utilize a maximum total space of 256KB for the buffers.
> + *
> + * We might also want to add support for user-provided buffers in time.
> + * This will allow bigger buffer size flexibility, and can also be used
> + * to achieve zero-copy messaging.
> + *
> + * Note that these numbers are purely a decision of this driver - we
> + * can change this without changing anything in the firmware of the remote
> + * processor.
> + */
> +#define MAX_RPMSG_NUM_BUFS	(512)
> +#define MAX_RPMSG_BUF_SIZE	(512)

This is a limitation of the virtio rpmsg implementation, there has
been efforts to make this dynamic and/or configurable. So I don't think
it's suitable to lift this limitation out of the virtio rpmsg
implementation.

Regards,
Bjorn

> +
>  #define to_rpmsg_device(d) container_of(d, struct rpmsg_device, dev)
>  #define to_rpmsg_driver(d) container_of(d, struct rpmsg_driver, drv)
>  
> diff --git a/drivers/rpmsg/virtio_rpmsg_bus.c b/drivers/rpmsg/virtio_rpmsg_bus.c
> index 905ac7910c98..5369669d3327 100644
> --- a/drivers/rpmsg/virtio_rpmsg_bus.c
> +++ b/drivers/rpmsg/virtio_rpmsg_bus.c
> @@ -109,27 +109,6 @@ struct virtio_rpmsg_channel {
>  #define to_virtio_rpmsg_channel(_rpdev) \
>  	container_of(_rpdev, struct virtio_rpmsg_channel, rpdev)
>  
> -/*
> - * We're allocating buffers of 512 bytes each for communications. The
> - * number of buffers will be computed from the number of buffers supported
> - * by the vring, upto a maximum of 512 buffers (256 in each direction).
> - *
> - * Each buffer will have 16 bytes for the msg header and 496 bytes for
> - * the payload.
> - *
> - * This will utilize a maximum total space of 256KB for the buffers.
> - *
> - * We might also want to add support for user-provided buffers in time.
> - * This will allow bigger buffer size flexibility, and can also be used
> - * to achieve zero-copy messaging.
> - *
> - * Note that these numbers are purely a decision of this driver - we
> - * can change this without changing anything in the firmware of the remote
> - * processor.
> - */
> -#define MAX_RPMSG_NUM_BUFS	(512)
> -#define MAX_RPMSG_BUF_SIZE	(512)
> -
>  /*
>   * Local addresses are dynamically allocated on-demand.
>   * We do not dynamically assign addresses from the low 1024 range,
> -- 
> 2.38.1
>