Re: [PATCH net-next 6/9] page_pool: avoid calling no-op externals when possible

[Yunsheng Lin]

On 2023/7/27 22:43, Alexander Lobakin wrote:
> 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_page()
>   page_pool_put_defragged_page()                  <- external
>     __page_pool_put_page()
>       page_pool_dma_sync_for_device()             <- non-inline
>         dma_sync_single_range_for_device()
>           dma_sync_single_for_device()            <- external
>             dma_direct_sync_single_for_device()
>               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 the new PP DMA sync operation type, which will mean "do DMA syncs
> for the 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
> the pool has no pages requiring DMA syncs, this cuts off a good piece
> of calls and checks. When at least one page required it, the pool
> conservatively falls back to "always call sync functions", no per-page
> verdicts. It's a fairly rare case anyway that only a few pages would
> require syncing.
> 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
> used.
> 

It seems other subsystem may have the similar problem as page_pool,
is it possible to implement this kind of trick in the dma subsystem
instead of every subsystem inventing their own trick?