Messages in this thread | | | Subject | Re: [PATCH net-next 6/9] page_pool: avoid calling no-op externals when possible | From | Yunsheng Lin <> | Date | Fri, 28 Jul 2023 20:39:24 +0800 |
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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?
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