Messages in this thread | | | Subject | Re: [PATCH net-next v2 2/2] page_pool: optimize the cpu sync operation when DMA mapping | From | Yunsheng Lin <> | Date | Mon, 23 Aug 2021 11:56:48 +0800 |
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On 2021/8/20 17:39, Ilias Apalodimas wrote: > On Fri, Aug 20, 2021 at 02:56:51PM +0800, Yunsheng Lin wrote: >> If the DMA_ATTR_SKIP_CPU_SYNC is not set, cpu syncing is >> also done in dma_map_page_attrs(), so set the attrs according >> to pool->p.flags to avoid calling cpu sync function again. > > Isn't DMA_ATTR_SKIP_CPU_SYNC checked within dma_map_page_attrs() anyway?
Yes, the checking in dma_map_page_attrs() should save us from calling dma_sync_single_for_device() again if we set the attrs according to "pool->p.flags & PP_FLAG_DMA_SYNC_DEV".
As dma_sync_single_for_device() is EXPORT_SYMBOL()'ed, and should be a no-op for dma coherent device, so there may be a function calling overhead for dma coherent device, letting dma_map_page_attrs() handling the sync seems to avoid the stack pushing/poping overhead:
https://elixir.bootlin.com/linux/latest/source/kernel/dma/direct.h#L104
The one thing I am not sure about is that the pool->p.offset and pool->p.max_len are used to decide the sync range before this patch, while the sync range is the same as the map range when doing the sync in dma_map_page_attrs().
I assumed the above is not a issue? only sync more than we need? and it won't hurt the performance?
> > Regards > /Ilias >> >> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com> >> --- >> net/core/page_pool.c | 9 +++++---- >> 1 file changed, 5 insertions(+), 4 deletions(-) >> >> diff --git a/net/core/page_pool.c b/net/core/page_pool.c >> index 1a69784..3df5554 100644 >> --- a/net/core/page_pool.c >> +++ b/net/core/page_pool.c >> @@ -191,8 +191,12 @@ static void page_pool_dma_sync_for_device(struct page_pool *pool, >> >> static bool page_pool_dma_map(struct page_pool *pool, struct page *page) >> { >> + unsigned long attrs = DMA_ATTR_SKIP_CPU_SYNC; >> dma_addr_t dma; >> >> + if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV) >> + attrs = 0; >> + >> /* Setup DMA mapping: use 'struct page' area for storing DMA-addr >> * since dma_addr_t can be either 32 or 64 bits and does not always fit >> * into page private data (i.e 32bit cpu with 64bit DMA caps) >> @@ -200,15 +204,12 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page) >> */ >> dma = dma_map_page_attrs(pool->p.dev, page, 0, >> (PAGE_SIZE << pool->p.order), >> - pool->p.dma_dir, DMA_ATTR_SKIP_CPU_SYNC); >> + pool->p.dma_dir, attrs); >> if (dma_mapping_error(pool->p.dev, dma)) >> return false; >> >> page_pool_set_dma_addr(page, dma); >> >> - if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV) >> - page_pool_dma_sync_for_device(pool, page, pool->p.max_len); >> - >> return true; >> } >> >> -- >> 2.7.4 >> > . >
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