224 lines
8.0 KiB
ReStructuredText
224 lines
8.0 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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=============
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Page Pool API
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=============
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The page_pool allocator is optimized for the XDP mode that uses one frame
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per-page, but it can fallback on the regular page allocator APIs.
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Basic use involves replacing alloc_pages() calls with the
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page_pool_alloc_pages() call. Drivers should use page_pool_dev_alloc_pages()
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replacing dev_alloc_pages().
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API keeps track of inflight pages, in order to let API user know
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when it is safe to free a page_pool object. Thus, API users
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must run page_pool_release_page() when a page is leaving the page_pool or
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call page_pool_put_page() where appropriate in order to maintain correct
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accounting.
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API user must call page_pool_put_page() once on a page, as it
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will either recycle the page, or in case of refcnt > 1, it will
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release the DMA mapping and inflight state accounting.
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Architecture overview
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=====================
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.. code-block:: none
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+------------------+
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| Driver |
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+------------------+
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^
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v
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+--------------------------------------------+
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| request memory |
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+--------------------------------------------+
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^ ^
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| Pool empty | Pool has entries
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v v
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+-----------------------+ +------------------------+
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| alloc (and map) pages | | get page from cache |
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+-----------------------+ +------------------------+
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^ ^
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| cache available | No entries, refill
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| | from ptr-ring
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v v
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+-----------------+ +------------------+
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| Fast cache | | ptr-ring cache |
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+-----------------+ +------------------+
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API interface
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=============
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The number of pools created **must** match the number of hardware queues
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unless hardware restrictions make that impossible. This would otherwise beat the
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purpose of page pool, which is allocate pages fast from cache without locking.
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This lockless guarantee naturally comes from running under a NAPI softirq.
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The protection doesn't strictly have to be NAPI, any guarantee that allocating
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a page will cause no race conditions is enough.
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* page_pool_create(): Create a pool.
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* flags: PP_FLAG_DMA_MAP, PP_FLAG_DMA_SYNC_DEV
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* order: 2^order pages on allocation
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* pool_size: size of the ptr_ring
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* nid: preferred NUMA node for allocation
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* dev: struct device. Used on DMA operations
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* dma_dir: DMA direction
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* max_len: max DMA sync memory size
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* offset: DMA address offset
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* page_pool_put_page(): The outcome of this depends on the page refcnt. If the
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driver bumps the refcnt > 1 this will unmap the page. If the page refcnt is 1
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the allocator owns the page and will try to recycle it in one of the pool
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caches. If PP_FLAG_DMA_SYNC_DEV is set, the page will be synced for_device
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using dma_sync_single_range_for_device().
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* page_pool_put_full_page(): Similar to page_pool_put_page(), but will DMA sync
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for the entire memory area configured in area pool->max_len.
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* page_pool_recycle_direct(): Similar to page_pool_put_full_page() but caller
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must guarantee safe context (e.g NAPI), since it will recycle the page
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directly into the pool fast cache.
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* page_pool_release_page(): Unmap the page (if mapped) and account for it on
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inflight counters.
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* page_pool_dev_alloc_pages(): Get a page from the page allocator or page_pool
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caches.
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* page_pool_get_dma_addr(): Retrieve the stored DMA address.
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* page_pool_get_dma_dir(): Retrieve the stored DMA direction.
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* page_pool_put_page_bulk(): Tries to refill a number of pages into the
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ptr_ring cache holding ptr_ring producer lock. If the ptr_ring is full,
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page_pool_put_page_bulk() will release leftover pages to the page allocator.
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page_pool_put_page_bulk() is suitable to be run inside the driver NAPI tx
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completion loop for the XDP_REDIRECT use case.
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Please note the caller must not use data area after running
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page_pool_put_page_bulk(), as this function overwrites it.
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* page_pool_get_stats(): Retrieve statistics about the page_pool. This API
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is only available if the kernel has been configured with
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``CONFIG_PAGE_POOL_STATS=y``. A pointer to a caller allocated ``struct
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page_pool_stats`` structure is passed to this API which is filled in. The
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caller can then report those stats to the user (perhaps via ethtool,
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debugfs, etc.). See below for an example usage of this API.
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Stats API and structures
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------------------------
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If the kernel is configured with ``CONFIG_PAGE_POOL_STATS=y``, the API
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``page_pool_get_stats()`` and structures described below are available. It
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takes a pointer to a ``struct page_pool`` and a pointer to a ``struct
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page_pool_stats`` allocated by the caller.
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The API will fill in the provided ``struct page_pool_stats`` with
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statistics about the page_pool.
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The stats structure has the following fields::
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struct page_pool_stats {
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struct page_pool_alloc_stats alloc_stats;
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struct page_pool_recycle_stats recycle_stats;
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};
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The ``struct page_pool_alloc_stats`` has the following fields:
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* ``fast``: successful fast path allocations
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* ``slow``: slow path order-0 allocations
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* ``slow_high_order``: slow path high order allocations
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* ``empty``: ptr ring is empty, so a slow path allocation was forced.
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* ``refill``: an allocation which triggered a refill of the cache
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* ``waive``: pages obtained from the ptr ring that cannot be added to
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the cache due to a NUMA mismatch.
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The ``struct page_pool_recycle_stats`` has the following fields:
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* ``cached``: recycling placed page in the page pool cache
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* ``cache_full``: page pool cache was full
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* ``ring``: page placed into the ptr ring
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* ``ring_full``: page released from page pool because the ptr ring was full
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* ``released_refcnt``: page released (and not recycled) because refcnt > 1
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Coding examples
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===============
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Registration
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------------
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.. code-block:: c
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/* Page pool registration */
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struct page_pool_params pp_params = { 0 };
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struct xdp_rxq_info xdp_rxq;
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int err;
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pp_params.order = 0;
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/* internal DMA mapping in page_pool */
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pp_params.flags = PP_FLAG_DMA_MAP;
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pp_params.pool_size = DESC_NUM;
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pp_params.nid = NUMA_NO_NODE;
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pp_params.dev = priv->dev;
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pp_params.dma_dir = xdp_prog ? DMA_BIDIRECTIONAL : DMA_FROM_DEVICE;
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page_pool = page_pool_create(&pp_params);
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err = xdp_rxq_info_reg(&xdp_rxq, ndev, 0);
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if (err)
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goto err_out;
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err = xdp_rxq_info_reg_mem_model(&xdp_rxq, MEM_TYPE_PAGE_POOL, page_pool);
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if (err)
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goto err_out;
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NAPI poller
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-----------
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.. code-block:: c
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/* NAPI Rx poller */
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enum dma_data_direction dma_dir;
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dma_dir = page_pool_get_dma_dir(dring->page_pool);
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while (done < budget) {
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if (some error)
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page_pool_recycle_direct(page_pool, page);
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if (packet_is_xdp) {
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if XDP_DROP:
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page_pool_recycle_direct(page_pool, page);
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} else (packet_is_skb) {
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page_pool_release_page(page_pool, page);
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new_page = page_pool_dev_alloc_pages(page_pool);
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}
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}
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Stats
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-----
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.. code-block:: c
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#ifdef CONFIG_PAGE_POOL_STATS
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/* retrieve stats */
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struct page_pool_stats stats = { 0 };
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if (page_pool_get_stats(page_pool, &stats)) {
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/* perhaps the driver reports statistics with ethool */
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ethtool_print_allocation_stats(&stats.alloc_stats);
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ethtool_print_recycle_stats(&stats.recycle_stats);
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}
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#endif
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Driver unload
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-------------
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.. code-block:: c
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/* Driver unload */
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page_pool_put_full_page(page_pool, page, false);
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xdp_rxq_info_unreg(&xdp_rxq);
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