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

[Petr Tesařík]

On Mon, 29 Jan 2024 15:36:35 +0100
Alexander Lobakin <aleksander.lobakin@xxxxxxxxx> wrote:

> From: Petr Tesařík <petr@xxxxxxxxxxx>
> Date: Fri, 26 Jan 2024 19:48:19 +0100
> 
> > On Fri, 26 Jan 2024 17:21:24 +0000
> > Robin Murphy <robin.murphy@xxxxxxx> wrote:
> >   
> >> On 26/01/2024 4:45 pm, Alexander Lobakin wrote:  
> >>> 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?    
> >>
> >> I think it's safe to assume that, as with P2P support, this will only 
> >> matter for dma-direct and iommu-dma for the foreseeable future, and 
> >> those do currently share the same conditions as above. Thus we may as 
> >> well keep things simple for now, and if anything ever does have cause to 
> >> change, it can be the future's problem to keep this mechanism working as 
> >> intended.  
> > 
> > Can we have a comment that states this assumption along with the flag?
> > Because when it breaks, it will keep someone cursing for days why DMA
> > sometimes fails on their device before they find out it's not synced.  
> 
> BTW, dma_skip_sync is set right before driver->probe(), so that if any
> problematic device appears, it could easily be fixed by adding one line
> to its probe callback.

Ah, perfect!

Petr T