@@ -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);
@@ -376,6 +376,14 @@ 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);
}
@@ -672,7 +680,7 @@ 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);
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 <linyunsheng@huawei.com> CC: Lorenzo Bianconi <lorenzo@kernel.org> CC: Alexander Duyck <alexander.duyck@gmail.com> CC: Liang Chen <liangchen.linux@gmail.com> CC: Alexander Lobakin <aleksander.lobakin@intel.com> --- include/net/page_pool/helpers.h | 47 ++++++++++++++++++++++++--------- net/core/page_pool.c | 10 ++++++- 2 files changed, 43 insertions(+), 14 deletions(-)