Re: [PATCH net-next 2/7] dma: avoid expensive redundant calls for sync operations
From: Robin Murphy <robin.murphy@arm.com>
Date: 2024-01-29 14:29:48
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On 2024-01-29 2:07 pm, Alexander Lobakin wrote:
From: Alexander Lobakin <aleksander.lobakin@intel.com> Date: Fri, 26 Jan 2024 17:45:11 +0100quoted
From: Robin Murphy <robin.murphy@arm.com> Date: Fri, 26 Jan 2024 15:48:54 +0000quoted
On 26/01/2024 1:54 pm, Alexander Lobakin wrote:quoted
From: Eric Dumazet <edumazet@google.com> Quite often, NIC devices do not need dma_sync operations on x86_64 at least. Indeed, when dev_is_dma_coherent(dev) is true and dev_use_swiotlb(dev) is false, iommu_dma_sync_single_for_cpu() and friends do nothing. However, indirectly calling them when CONFIG_RETPOLINE=y consumes about 10% of cycles on a cpu receiving packets from softirq at ~100Gbit rate. Even if/when CONFIG_RETPOLINE is not set, there is a cost of about 3%. Add dev->skip_dma_sync boolean which is set during the device initialization depending on the setup: dev_is_dma_coherent() for direct DMA, !(sync_single_for_device || sync_single_for_cpu) or positive result from the new callback, dma_map_ops::can_skip_sync for non-NULL DMA ops. Then later, if/when swiotlb is used for the first time, the flag is turned off, from swiotlb_tbl_map_single().I think you could probably just promote the dma_uses_io_tlb flag from SWIOTLB_DYNAMIC to a general SWIOTLB thing to serve this purpose now.Nice catch!BTW, this implies such hotpath check: if (dev->dma_skip_sync && !READ_ONCE(dev->dma_uses_io_tlb)) // ... This seems less effective than just resetting dma_skip_sync on first allocation.
Well, my point is not to have a dma_skip_sync at all; I'm suggesting the check would be: if (dev_is_dma_coherent(dev) && dev_uses_io_tlb(dev)) ... where on the platform which cares about this most, that first condition is a compile-time constant (and as implied, the second would want to be similarly wrapped for !SWIOTLB configs). Thanks, Robin.