Re: [PATCH net-next v6 3/5] page_pool: Allow drivers to hint on SKB recycling

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2021/5/22 0:15, Matteo Croce wrote:
> From: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> 
> Up to now several high speed NICs have custom mechanisms of recycling
> the allocated memory they use for their payloads.
> Our page_pool API already has recycling capabilities that are always
> used when we are running in 'XDP mode'. So let's tweak the API and the
> kernel network stack slightly and allow the recycling to happen even
> during the standard operation.
> The API doesn't take into account 'split page' policies used by those
> drivers currently, but can be extended once we have users for that.
> 
> The idea is to be able to intercept the packet on skb_release_data().
> If it's a buffer coming from our page_pool API recycle it back to the
> pool for further usage or just release the packet entirely.
> 
> To achieve that we introduce a bit in struct sk_buff (pp_recycle:1) and
> a field in struct page (page->pp) to store the page_pool pointer.
> Storing the information in page->pp allows us to recycle both SKBs and
> their fragments.
> We could have skipped the skb bit entirely, since identical information
> can bederived from struct page. However, in an effort to affect the free path
> as less as possible, reading a single bit in the skb which is already
> in cache, is better that trying to derive identical information for the
> page stored data.
> 
> The driver or page_pool has to take care of the sync operations on it's own
> during the buffer recycling since the buffer is, after opting-in to the
> recycling, never unmapped.
> 
> Since the gain on the drivers depends on the architecture, we are not
> enabling recycling by default if the page_pool API is used on a driver.
> In order to enable recycling the driver must call skb_mark_for_recycle()
> to store the information we need for recycling in page->pp and
> enabling the recycling bit, or page_pool_store_mem_info() for a fragment.

The state of this patch in patchwork is "Not Applicable", so
you may need to respin it again.

Some minor comment below:

> 
> Co-developed-by: Jesper Dangaard Brouer <brouer@redhat.com>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> Co-developed-by: Matteo Croce <mcroce@microsoft.com>
> Signed-off-by: Matteo Croce <mcroce@microsoft.com>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> ---
>  include/linux/skbuff.h  | 28 +++++++++++++++++++++++++---
>  include/net/page_pool.h |  9 +++++++++
>  net/core/page_pool.c    | 23 +++++++++++++++++++++++
>  net/core/skbuff.c       | 24 ++++++++++++++++++++----
>  4 files changed, 77 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 7fcfea7e7b21..057b40ad29bd 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -40,6 +40,9 @@
>  #if IS_ENABLED(CONFIG_NF_CONNTRACK)
>  #include <linux/netfilter/nf_conntrack_common.h>
>  #endif
> +#ifdef CONFIG_PAGE_POOL
> +#include <net/page_pool.h>
> +#endif
>  
>  /* The interface for checksum offload between the stack and networking drivers
>   * is as follows...
> @@ -667,6 +670,8 @@ typedef unsigned char *sk_buff_data_t;
>   *	@head_frag: skb was allocated from page fragments,
>   *		not allocated by kmalloc() or vmalloc().
>   *	@pfmemalloc: skbuff was allocated from PFMEMALLOC reserves
> + *	@pp_recycle: mark the packet for recycling instead of freeing (implies
> + *		page_pool support on driver)
>   *	@active_extensions: active extensions (skb_ext_id types)
>   *	@ndisc_nodetype: router type (from link layer)
>   *	@ooo_okay: allow the mapping of a socket to a queue to be changed
> @@ -791,10 +796,12 @@ struct sk_buff {
>  				fclone:2,
>  				peeked:1,
>  				head_frag:1,
> -				pfmemalloc:1;
> +				pfmemalloc:1,
> +				pp_recycle:1; /* page_pool recycle indicator */

The about comment seems unnecessary, for there is comment
added above in this patch to explain that.

>  #ifdef CONFIG_SKB_EXTENSIONS
>  	__u8			active_extensions;
>  #endif
> +

Unnecessary change?

>  	/* fields enclosed in headers_start/headers_end are copied
>  	 * using a single memcpy() in __copy_skb_header()
>  	 */
> @@ -3088,7 +3095,13 @@ static inline void skb_frag_ref(struct sk_buff *skb, int f)
>   */
>  static inline void __skb_frag_unref(skb_frag_t *frag, bool recycle)
>  {
> -	put_page(skb_frag_page(frag));
> +	struct page *page = skb_frag_page(frag);
> +
> +#ifdef CONFIG_PAGE_POOL
> +	if (recycle && page_pool_return_skb_page(page_address(page)))
> +		return;
> +#endif
> +	put_page(page);
>  }
>  
>  /**
> @@ -3100,7 +3113,7 @@ static inline void __skb_frag_unref(skb_frag_t *frag, bool recycle)
>   */
>  static inline void skb_frag_unref(struct sk_buff *skb, int f)
>  {
> -	__skb_frag_unref(&skb_shinfo(skb)->frags[f], false);
> +	__skb_frag_unref(&skb_shinfo(skb)->frags[f], skb->pp_recycle);
>  }
>  
>  /**
> @@ -4699,5 +4712,14 @@ static inline u64 skb_get_kcov_handle(struct sk_buff *skb)
>  #endif
>  }
>  
> +#ifdef CONFIG_PAGE_POOL
> +static inline void skb_mark_for_recycle(struct sk_buff *skb, struct page *page,
> +					struct page_pool *pp)
> +{
> +	skb->pp_recycle = 1;
> +	page_pool_store_mem_info(page, pp);
> +}
> +#endif
> +
>  #endif	/* __KERNEL__ */
>  #endif	/* _LINUX_SKBUFF_H */
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index b4b6de909c93..7b9b6a1c61f5 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -146,6 +146,8 @@ inline enum dma_data_direction page_pool_get_dma_dir(struct page_pool *pool)
>  	return pool->p.dma_dir;
>  }
>  
> +bool page_pool_return_skb_page(void *data);
> +
>  struct page_pool *page_pool_create(const struct page_pool_params *params);
>  
>  #ifdef CONFIG_PAGE_POOL
> @@ -251,4 +253,11 @@ static inline void page_pool_ring_unlock(struct page_pool *pool)
>  		spin_unlock_bh(&pool->ring.producer_lock);
>  }
>  
> +/* Store mem_info on struct page and use it while recycling skb frags */
> +static inline
> +void page_pool_store_mem_info(struct page *page, struct page_pool *pp)

The conventional practice seems to put "struct page_pool" before other
parameter in page_pool.h.

> +{
> +	page->pp = pp;
> +}
> +
>  #endif /* _NET_PAGE_POOL_H */
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index e1321bc9d316..2a020cca489f 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -628,3 +628,26 @@ void page_pool_update_nid(struct page_pool *pool, int new_nid)
>  	}
>  }
>  EXPORT_SYMBOL(page_pool_update_nid);
> +
> +bool page_pool_return_skb_page(void *data)
> +{
> +	struct page_pool *pp;
> +	struct page *page;
> +
> +	page = virt_to_head_page(data);
> +	if (unlikely(page->pp_magic != PP_SIGNATURE))
> +		return false;
> +
> +	pp = (struct page_pool *)page->pp;
> +
> +	/* Driver set this to memory recycling info. Reset it on recycle.
> +	 * This will *not* work for NIC using a split-page memory model.
> +	 * The page will be returned to the pool here regardless of the
> +	 * 'flipped' fragment being in use or not.
> +	 */

I am not sure I understand how does the last part of comment related
to the code below, as there is no driver using split-page memory model
will reach here because those driver will not call skb_mark_for_recycle(),
right?

> +	page->pp = NULL;
> +	page_pool_put_full_page(pp, virt_to_head_page(data), false);
> +
> +	return true;
> +}
> +EXPORT_SYMBOL(page_pool_return_skb_page);