From: Yunsheng Lin Date: Fri, 20 Oct 2023 09:59:48 +0000 (+0800) Subject: page_pool: unify frag_count handling in page_pool_is_last_frag() X-Git-Tag: accepted/tizen/unified/20240422.153132~4 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=b64bd10d99215cebb935563d04773b9609b2a214;p=platform%2Fkernel%2Flinux-rpi.git page_pool: unify frag_count handling in page_pool_is_last_frag() Currently when page_pool_create() is called with PP_FLAG_PAGE_FRAG flag, page_pool_alloc_pages() is only allowed to be called under the below constraints: 1. page_pool_fragment_page() need to be called to setup page->pp_frag_count immediately. 2. page_pool_defrag_page() often need to be called to drain the page->pp_frag_count when there is no more user will be holding on to that page. Those constraints exist in order to support a page to be split into multi fragments. And those constraints have some overhead because of the cache line dirtying/bouncing and atomic update. Those constraints are unavoidable for case when we need a page to be split into more than one fragment, but there is also case that we want to avoid the above constraints and their overhead when a page can't be split as it can only hold a fragment as requested by user, depending on different use cases: use case 1: allocate page without page splitting. use case 2: allocate page with page splitting. use case 3: allocate page with or without page splitting depending on the fragment size. Currently page pool only provide page_pool_alloc_pages() and page_pool_alloc_frag() API to enable the 1 & 2 separately, so we can not use a combination of 1 & 2 to enable 3, it is not possible yet because of the per page_pool flag PP_FLAG_PAGE_FRAG. So in order to allow allocating unsplit page without the overhead of split page while still allow allocating split page we need to remove the per page_pool flag in page_pool_is_last_frag(), as best as I can think of, it seems there are two methods as below: 1. Add per page flag/bit to indicate a page is split or not, which means we might need to update that flag/bit everytime the page is recycled, dirtying the cache line of 'struct page' for use case 1. 2. Unify the page->pp_frag_count handling for both split and unsplit page by assuming all pages in the page pool is split into a big fragment initially. As page pool already supports use case 1 without dirtying the cache line of 'struct page' whenever a page is recyclable, we need to support the above use case 3 with minimal overhead, especially not adding any noticeable overhead for use case 1, and we are already doing an optimization by not updating pp_frag_count in page_pool_defrag_page() for the last fragment user, this patch chooses to unify the pp_frag_count handling to support the above use case 3. There is no noticeable performance degradation and some justification for unifying the frag_count handling with this patch applied using a micro-benchmark testing in [1]. 1. https://lore.kernel.org/all/bf2591f8-7b3c-4480-bb2c-31dc9da1d6ac@huawei.com/ Signed-off-by: Yunsheng Lin CC: Lorenzo Bianconi CC: Alexander Duyck CC: Liang Chen CC: Alexander Lobakin Link: https://lore.kernel.org/r/20231020095952.11055-2-linyunsheng@huawei.com Signed-off-by: Jakub Kicinski (cherry picked from commit 58d53d8f7da63dd13903bec0a40b3009a841b61b) Signed-off-by: Jaehoon Chung Change-Id: I4020fab404d9c362383a10f8b596a767b1b091a8 --- diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h index 8f64adf..759489c 100644 --- a/include/net/page_pool/helpers.h +++ b/include/net/page_pool/helpers.h @@ -115,28 +115,49 @@ static inline long page_pool_defrag_page(struct page *page, long nr) long ret; /* If nr == pp_frag_count then we have cleared all remaining - * references to the page. No need to actually overwrite it, instead - * we can leave this to be overwritten by the calling function. + * references to the page: + * 1. 'n == 1': no need to actually overwrite it. + * 2. 'n != 1': overwrite it with one, which is the rare case + * for pp_frag_count draining. * - * The main advantage to doing this is that an atomic_read is - * generally a much cheaper operation than an atomic update, - * especially when dealing with a page that may be partitioned - * into only 2 or 3 pieces. + * The main advantage to doing this is that not only we avoid a atomic + * update, as an atomic_read is generally a much cheaper operation than + * an atomic update, especially when dealing with a page that may be + * partitioned into only 2 or 3 pieces; but also unify the pp_frag_count + * handling by ensuring all pages have partitioned into only 1 piece + * initially, and only overwrite it when the page is partitioned into + * more than one piece. */ - if (atomic_long_read(&page->pp_frag_count) == nr) + if (atomic_long_read(&page->pp_frag_count) == nr) { + /* As we have ensured nr is always one for constant case using + * the BUILD_BUG_ON(), only need to handle the non-constant case + * here for pp_frag_count draining, which is a rare case. + */ + BUILD_BUG_ON(__builtin_constant_p(nr) && nr != 1); + if (!__builtin_constant_p(nr)) + atomic_long_set(&page->pp_frag_count, 1); + return 0; + } ret = atomic_long_sub_return(nr, &page->pp_frag_count); WARN_ON(ret < 0); + + /* We are the last user here too, reset pp_frag_count back to 1 to + * ensure all pages have been partitioned into 1 piece initially, + * this should be the rare case when the last two fragment users call + * page_pool_defrag_page() currently. + */ + if (unlikely(!ret)) + atomic_long_set(&page->pp_frag_count, 1); + return ret; } -static inline bool page_pool_is_last_frag(struct page_pool *pool, - struct page *page) +static inline bool page_pool_is_last_frag(struct page *page) { - /* If fragments aren't enabled or count is 0 we were the last user */ - return !(pool->p.flags & PP_FLAG_PAGE_FRAG) || - (page_pool_defrag_page(page, 1) == 0); + /* If page_pool_defrag_page() returns 0, we were the last user */ + return page_pool_defrag_page(page, 1) == 0; } /** @@ -161,7 +182,7 @@ static inline void page_pool_put_page(struct page_pool *pool, * allow registering MEM_TYPE_PAGE_POOL, but shield linker. */ #ifdef CONFIG_PAGE_POOL - if (!page_pool_is_last_frag(pool, page)) + if (!page_pool_is_last_frag(page)) return; page_pool_put_defragged_page(pool, page, dma_sync_size, allow_direct); diff --git a/net/core/page_pool.c b/net/core/page_pool.c index 278332e..59bad96 100644 --- a/net/core/page_pool.c +++ b/net/core/page_pool.c @@ -380,6 +380,14 @@ static void page_pool_set_pp_info(struct page_pool *pool, { page->pp = pool; page->pp_magic |= PP_SIGNATURE; + + /* Ensuring all pages have been split into one fragment initially: + * page_pool_set_pp_info() is only called once for every page when it + * is allocated from the page allocator and page_pool_fragment_page() + * is dirtying the same cache line as the page->pp_magic above, so + * the overhead is negligible. + */ + page_pool_fragment_page(page, 1); if (pool->p.init_callback) pool->p.init_callback(page, pool->p.init_arg); } @@ -676,7 +684,7 @@ void page_pool_put_page_bulk(struct page_pool *pool, void **data, struct page *page = virt_to_head_page(data[i]); /* It is not the last user for the page frag case */ - if (!page_pool_is_last_frag(pool, page)) + if (!page_pool_is_last_frag(page)) continue; page = __page_pool_put_page(pool, page, -1, false);