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

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Date: Mon, 29 Jan 2024 14:29:43 +0000
From: Robin Murphy <robin.murphy@....com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Christoph Hellwig <hch@....de>,
 Marek Szyprowski <m.szyprowski@...sung.com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Magnus Karlsson <magnus.karlsson@...el.com>,
 Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
 Alexander Duyck <alexanderduyck@...com>, bpf@...r.kernel.org,
 netdev@...r.kernel.org, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 2/7] dma: avoid expensive redundant calls for
 sync operations

On 2024-01-29 2:07 pm, Alexander Lobakin wrote:
> From: Alexander Lobakin <aleksander.lobakin@...el.com>
> Date: Fri, 26 Jan 2024 17:45:11 +0100
> 
>> From: Robin Murphy <robin.murphy@....com>
>> Date: Fri, 26 Jan 2024 15:48:54 +0000
>>
>>> On 26/01/2024 1:54 pm, Alexander Lobakin wrote:
>>>> From: Eric Dumazet <edumazet@...gle.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.