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Date: Tue, 11 Feb 2020 09:22:21 -0800
From: Randy Dunlap <rdunlap@...radead.org>
To: Ilias Apalodimas <ilias.apalodimas@...aro.org>, brouer@...hat.com,
lorenzo@...nel.org, davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH, net-next] net: page_pool: Add documentation on page_pool
API
Hi,
Here are some doc comments for you...
On 2/11/20 7:42 AM, Ilias Apalodimas wrote:
> Add documentation explaining the basic functionality and design
> principles of the API
>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>
> ---
> Documentation/networking/page_pool.rst | 153 +++++++++++++++++++++++++
> 1 file changed, 153 insertions(+)
> create mode 100644 Documentation/networking/page_pool.rst
>
> diff --git a/Documentation/networking/page_pool.rst b/Documentation/networking/page_pool.rst
> new file mode 100644
> index 000000000000..e143339e4c80
> --- /dev/null
> +++ b/Documentation/networking/page_pool.rst
> @@ -0,0 +1,153 @@
> +=============
> +Page Pool API
> +=============
> +
> +The page_pool allocator is optimized for the XDP mode that uses one frame
> +per-page, but it can fallback on the regular page allocator APIs.
> +
> +Basic use involve replacing alloc_pages() calls with the
> +page_pool_alloc_pages() call. Drivers should use page_pool_dev_alloc_pages()
> +replacing dev_alloc_pages().
> +
> +API keeps track of in-flight pages, in-order to let API user know
in order
> +when it is safe to free a page_pool object. Thus, API users
> +must run page_pool_release_page() when a page is leaving the page_pool or
> +call page_pool_put_page() where appropriate in order to maintain correct
> +accounting.
> +
> +API user must call page_pool_put_page() once on a page, as it
> +will either recycle the page, or in case of refcnt > 1, it will
> +release the DMA mapping and in-flight state accounting.
> +
> +Architecture overview
> +=====================
> +
> +.. code-block:: none
> +
> + +------------------+
> + | Driver |
> + +------------------+
> + ^
> + |
> + |
> + |
> + v
> + +--------------------------------------------+
> + | request memory |
> + +--------------------------------------------+
> + ^ ^
> + | |
> + | Pool empty | Pool has entries
> + | |
> + v v
> + +-----------------------+ +------------------------+
> + | alloc (and map) pages | | get page from cache |
> + +-----------------------+ +------------------------+
> + ^ ^
> + | |
> + | cache available | No entries, refill
> + | | from ptr-ring
> + | |
> + v v
> + +-----------------+ +------------------+
> + | Fast cache | | ptr-ring cache |
> + +-----------------+ +------------------+
> +
> +API interface
> +=============
> +The number of pools created **must** match the number of hardware queues
> +unless hardware restrictions make that impossible. This would otherwise beat the
> +purpose of page pool, which is allocate pages fast from cache without locking.
> +This lockless guarantee naturally comes from running under a NAPI softirq.
> +The protection doesn't strictly has to be NAPI, any guarantee that allocating a
have to be NAPI;
> +page will cause no race-conditions is enough.
race conditions
> +
> +* page_pool_create(): Create a pool.
> + * flags: PP_FLAG_DMA_MAP, PP_FLAG_DMA_SYNC_DEV
what about 'order'? is it optional?
> + * pool_size: size of the ptr_ring
> + * nid: preferred NUMA node for allocation
> + * dev: struct device. Used on DMA operations
> + * dma_dir: DMA direction
> + * max_len: max DMA sync memory size
> + * offset: DMA address offset
> +
> +* page_pool_put_page(): The outcome of this depends on the page refcnt. If the
> + driver uses refcnt > 1 this will unmap the page. If the pool object is
> + responsible for DMA operations and account for the in-flight counting.
Hm, above is not a sentence and it ends with a space character.
Several lines end with a space character. :(
> + If the refcnt is 1, the allocator owns the page and will try to recycle and
> + sync it to be re-used by the device using dma_sync_single_range_for_device().
> +
> +* page_pool_release_page(): Unmap the page (if mapped) and account for it on
> + inflight counters.
inflight is spelled as in-flight earlier. Just choose one way, please.
> +
> +* page_pool_dev_alloc_pages(): Get a page from the page allocator or page_pool
> + caches.
> +
> +* page_pool_get_dma_addr(): Retrieve the stored DMA address.
> +
> +* page_pool_get_dma_dir(): Retrieve the stored DMA direction.
> +
> +* page_pool_recycle_direct(): Recycle the page immediately. Must be used under
> + NAPI context
> +
> +Coding examples
> +===============
> +
> +Registration
> +------------
> +
> +.. code-block:: c
> +
> + /* Page pool registration */
> + struct page_pool_params pp_params = { 0 };
> + struct xdp_rxq_info xdp_rxq;
> + int err;
> +
> + pp_params.order = 0;
> + /* internal DMA mapping in page_pool */
> + pp_params.flags = PP_FLAG_DMA_MAP;
> + pp_params.pool_size = DESC_NUM;
> + pp_params.nid = NUMA_NO_NODE;
> + pp_params.dev = priv->dev;
> + pp_params.dma_dir = xdp_prog ? DMA_BIDIRECTIONAL : DMA_FROM_DEVICE;
> + page_pool = page_pool_create(&pp_params);
> +
> + err = xdp_rxq_info_reg(&xdp_rxq, ndev, 0);
> + if (err)
> + goto err_out;
> +
> + err = xdp_rxq_info_reg_mem_model(&xdp_rxq, MEM_TYPE_PAGE_POOL, page_pool);
> + if (err)
> + goto err_out;
> +
> +NAPI poller
> +-----------
> +
> +
> +.. code-block:: c
> +
> + /* NAPI Rx poller */
> + enum dma_data_direction dma_dir;
> +
> + dma_dir = page_pool_get_dma_dir(dring->page_pool);
> + while (done < budget) {
> + if (some error)
> + page_pool_recycle_direct(page_pool, page);
> + if (packet_is_xdp) {
> + if XDP_DROP:
> + page_pool_recycle_direct(page_pool, page);
> + } else (packet_is_skb) {
> + page_pool_release_page(page_pool, page);
> + new_page = page_pool_dev_alloc_pages(page_pool);
> + }
> + }
> +
> +Driver unload
> +-------------
> +
> +.. code-block:: c
> +
> + /* Driver unload */
> + page_pool_put_page(page_pool, page, false);
> + xdp_rxq_info_unreg(&xdp_rxq);
> + page_pool_destroy(page_pool);
>
thanks.
--
~Randy
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