[PATCH net-next v2 07/12] net: page_pool: avoid calling no-op externals when possible

From: Alexander Lobakin
Date: Thu May 25 2023 - 09:00:59 EST

Turned out page_pool_put{,_full}_page() can burn quite a bunch of cycles
even when on DMA-coherent platforms (like x86) with no active IOMMU or
swiotlb, just for the call ladder.
Indeed, it's

page_pool_put_defragged_page() <- external
page_pool_dma_sync_for_device() <- non-inline
dma_sync_single_for_device() <- external
dev_is_dma_coherent() <- exit

For the inline functions, no guarantees the compiler won't uninline them
(they're clearly not one-liners and sometimes compilers uninline even
2 + 2). The first external call is necessary, but the rest 2+ are done
for nothing each time, plus a bunch of checks here and there.
Since Page Pool mappings are long-term and for one "device + addr" pair
dma_need_sync() will always return the same value (basically, whether it
belongs to an swiotlb pool), addresses can be tested once right after
they're obtained and the result can be reused until the page is unmapped.
Define new PP flag, which will mean "do DMA syncs for device, but only
when needed" and turn it on by default when the driver asks to sync
pages. When a page is mapped, check whether it needs syncs and if so,
replace that "sync when needed" back to "always do syncs" globally for
the whole pool (better safe than sorry). As long as a pool has no pages
requiring DMA syncs, this cuts off a good piece of calls and checks.
On my x86_64, this gives from 2% to 5% performance benefit with no
negative impact for cases when IOMMU is on and the shortcut can't be

Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
include/net/page_pool.h | 3 +++
net/core/page_pool.c | 10 ++++++++++
2 files changed, 13 insertions(+)

diff --git a/include/net/page_pool.h b/include/net/page_pool.h
index 821c75bba125..08e9571d2545 100644
--- a/include/net/page_pool.h
+++ b/include/net/page_pool.h
@@ -46,6 +46,9 @@
* device driver responsibility
#define PP_FLAG_PAGE_FRAG BIT(2) /* for page frag feature */
+#define PP_FLAG_DMA_MAYBE_SYNC BIT(3) /* Internal, should not be used in
+ * drivers
+ */
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index e212e9d7edcb..57f323dee6c4 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -175,6 +175,10 @@ static int page_pool_init(struct page_pool *pool,
/* pool->p.offset has to be set according to the address
* offset used by the DMA engine to start copying rx data
+ /* Try to avoid calling no-op syncs */
+ pool->p.flags |= PP_FLAG_DMA_MAYBE_SYNC;
+ pool->p.flags &= ~PP_FLAG_DMA_SYNC_DEV;

@@ -323,6 +327,12 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)

page_pool_set_dma_addr(page, dma);

+ if ((pool->p.flags & PP_FLAG_DMA_MAYBE_SYNC) &&
+ dma_need_sync(pool->p.dev, dma)) {
+ pool->p.flags |= PP_FLAG_DMA_SYNC_DEV;
+ pool->p.flags &= ~PP_FLAG_DMA_MAYBE_SYNC;
+ }
if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
page_pool_dma_sync_for_device(pool, page, pool->p.max_len);