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

[Robin Murphy]

On 2024-01-29 2:07 pm, Alexander Lobakin wrote:
> 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.

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.