Re: [PATCH net-next v2 2/2] page_pool: optimize the cpu sync operation when DMA mapping

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

On Mon, Aug 23, 2021 at 11:56:48AM +0800, Yunsheng Lin wrote:
> 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".

But we aren't syncing anything right now when we allocate the pages since
this is called with DMA_ATTR_SKIP_CPU_SYNC. We are syncing the allocated
range on the end of the function, if the pool was created and was requested
to take care of the mappings for us.

> 
> 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 am not sure I am following here. We always sync the entire range as well
in the current code as the mapping function is called with max_len.

> 
> I assumed the above is not a issue? only sync more than we need?
> and it won't hurt the performance?

We can sync more than we need, but if it's a non-coherent architecture,
there's a performance penalty. 

Regards
/Ilias
> 
> > 
> > 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
> >>
> > .
> >