Re: [PATCH net-next 2/7] dma: avoid expensive redundant calls for sync operations

[Alexander Lobakin]

From: Robin Murphy <robin.murphy@xxxxxxx>
Date: Fri, 26 Jan 2024 15:48:54 +0000

> On 26/01/2024 1:54 pm, Alexander Lobakin wrote:
>> From: Eric Dumazet <edumazet@xxxxxxxxxx>
>>
>> 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!

> 
> Similarly I don't think a new op is necessary now that we have
> dma_map_ops.flags. A simple static flag to indicate that sync may be> skipped under the same conditions as implied for dma-direct - i.e.
> dev_is_dma_coherent(dev) && !dev->dma_use_io_tlb - seems like it ought
> to suffice.

In my initial implementation, I used a new dma_map_ops flag, but then I
realized different DMA ops may require or not require syncing under
different conditions, not only dev_is_dma_coherent().
Or am I wrong and they would always be the same?

> 
> Thanks,
> Robin.

Thanks,
Olek