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

[Alexander Lobakin]

From: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
Date: Fri, 26 Jan 2024 17:45:11 +0100

> 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!

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.

> 
>>
>> 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

Thanks,
Olek