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

[Robin Murphy <robin.murphy@arm.com>]

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

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.

Thanks,
Robin.

> On iavf, the UDP trafficgen with XDP_DROP in skb mode test shows
> +3-5% increase for direct DMA.
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Co-developed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
> ---
>   include/linux/device.h      |  5 +++++
>   include/linux/dma-map-ops.h | 17 +++++++++++++++++
>   include/linux/dma-mapping.h | 12 ++++++++++--
>   drivers/base/dd.c           |  2 ++
>   kernel/dma/mapping.c        | 34 +++++++++++++++++++++++++++++++---
>   kernel/dma/swiotlb.c        | 14 ++++++++++++++
>   6 files changed, 79 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 97c4b046c09d..f23e6a32bea0 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -686,6 +686,8 @@ struct device_physical_location {
>    *		other devices probe successfully.
>    * @dma_coherent: this particular device is dma coherent, even if the
>    *		architecture supports non-coherent devices.
> + * @dma_skip_sync: DMA sync operations can be skipped for coherent non-SWIOTLB
> + *		buffers.
>    * @dma_ops_bypass: If set to %true then the dma_ops are bypassed for the
>    *		streaming DMA operations (->map_* / ->unmap_* / ->sync_*),
>    *		and optionall (if the coherent mask is large enough) also
> @@ -800,6 +802,9 @@ struct device {
>       defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL)
>   	bool			dma_coherent:1;
>   #endif
> +#ifdef CONFIG_DMA_NEED_SYNC
> +	bool			dma_skip_sync:1;
> +#endif
>   #ifdef CONFIG_DMA_OPS_BYPASS
>   	bool			dma_ops_bypass : 1;
>   #endif
> diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
> index 4abc60f04209..937c295e9da8 100644
> --- a/include/linux/dma-map-ops.h
> +++ b/include/linux/dma-map-ops.h
> @@ -78,6 +78,7 @@ struct dma_map_ops {
>   			int nents, enum dma_data_direction dir);
>   	void (*cache_sync)(struct device *dev, void *vaddr, size_t size,
>   			enum dma_data_direction direction);
> +	bool (*can_skip_sync)(struct device *dev);
>   	int (*dma_supported)(struct device *dev, u64 mask);
>   	u64 (*get_required_mask)(struct device *dev);
>   	size_t (*max_mapping_size)(struct device *dev);
> @@ -111,6 +112,22 @@ static inline void set_dma_ops(struct device *dev,
>   }
>   #endif /* CONFIG_DMA_OPS */
>   
> +#ifdef CONFIG_DMA_NEED_SYNC
> +
> +static inline void dma_set_skip_sync(struct device *dev, bool skip)
> +{
> +	dev->dma_skip_sync = skip;
> +}
> +
> +void dma_setup_skip_sync(struct device *dev);
> +
> +#else /* !CONFIG_DMA_NEED_SYNC */
> +
> +#define dma_set_skip_sync(dev, skip)		do { } while (0)
> +#define dma_setup_skip_sync(dev)		do { } while (0)
> +
> +#endif /* !CONFIG_DMA_NEED_SYNC */
> +
>   #ifdef CONFIG_DMA_CMA
>   extern struct cma *dma_contiguous_default_area;
>   
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 9dd7e1578bf6..bc9f67e0c139 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -365,9 +365,17 @@ __dma_sync_single_range_for_device(struct device *dev, dma_addr_t addr,
>   
>   #ifdef CONFIG_DMA_NEED_SYNC
>   
> -#define dma_skip_sync(dev)			false
> +static inline bool dma_skip_sync(const struct device *dev)
> +{
> +	return dev->dma_skip_sync;
> +}
> +
> +bool __dma_need_sync(struct device *dev, dma_addr_t dma_addr);
>   
> -bool dma_need_sync(struct device *dev, dma_addr_t dma_addr);
> +static inline bool dma_need_sync(struct device *dev, dma_addr_t dma_addr)
> +{
> +	return dma_skip_sync(dev) ? false : __dma_need_sync(dev, dma_addr);
> +}
>   
>   #else /* !CONFIG_DMA_NEED_SYNC */
>   
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 85152537dbf1..67ad3e1d51f6 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -642,6 +642,8 @@ static int really_probe(struct device *dev, struct device_driver *drv)
>   			goto pinctrl_bind_failed;
>   	}
>   
> +	dma_setup_skip_sync(dev);
> +
>   	ret = driver_sysfs_add(dev);
>   	if (ret) {
>   		pr_err("%s: driver_sysfs_add(%s) failed\n",
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index a30f37f9d4db..8fa464b3954e 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -842,15 +842,43 @@ size_t dma_opt_mapping_size(struct device *dev)
>   EXPORT_SYMBOL_GPL(dma_opt_mapping_size);
>   
>   #ifdef CONFIG_DMA_NEED_SYNC
> -bool dma_need_sync(struct device *dev, dma_addr_t dma_addr)
> +bool __dma_need_sync(struct device *dev, dma_addr_t dma_addr)
>   {
>   	const struct dma_map_ops *ops = get_dma_ops(dev);
>   
>   	if (dma_map_direct(dev, ops))
> +		/*
> +		 * dma_skip_sync could've been set to false on first SWIOTLB
> +		 * buffer mapping, but @dma_addr is not necessary an SWIOTLB
> +		 * buffer. In this case, fall back to more granular check.
> +		 */
>   		return dma_direct_need_sync(dev, dma_addr);
> -	return ops->sync_single_for_cpu || ops->sync_single_for_device;
> +
> +	return true;
> +}
> +EXPORT_SYMBOL_GPL(__dma_need_sync);
> +
> +void dma_setup_skip_sync(struct device *dev)
> +{
> +	const struct dma_map_ops *ops = get_dma_ops(dev);
> +	bool skip;
> +
> +	if (dma_map_direct(dev, ops))
> +		/*
> +		 * dma_skip_sync will be set to false on first SWIOTLB buffer
> +		 * mapping, if any. During the device initialization, it's
> +		 * enough to check only for DMA coherence.
> +		 */
> +		skip = dev_is_dma_coherent(dev);
> +	else if (!ops->sync_single_for_device && !ops->sync_single_for_cpu)
> +		skip = true;
> +	else if (ops->can_skip_sync)
> +		skip = ops->can_skip_sync(dev);
> +	else
> +		skip = false;
> +
> +	dma_set_skip_sync(dev, skip);
>   }
> -EXPORT_SYMBOL_GPL(dma_need_sync);
>   #endif /* CONFIG_DMA_NEED_SYNC */
>   
>   unsigned long dma_get_merge_boundary(struct device *dev)
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index b079a9a8e087..b62ea0a4f106 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1286,6 +1286,16 @@ static unsigned long mem_used(struct io_tlb_mem *mem)
>   
>   #endif /* CONFIG_DEBUG_FS */
>   
> +static inline void swiotlb_disable_dma_skip_sync(struct device *dev)
> +{
> +	/*
> +	 * If dma_skip_sync was set, reset it to false on first SWIOTLB buffer
> +	 * mapping/allocation to always sync SWIOTLB buffers.
> +	 */
> +	if (unlikely(dma_skip_sync(dev)))
> +		dma_set_skip_sync(dev, false);
> +}
> +
>   phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
>   		size_t mapping_size, size_t alloc_size,
>   		unsigned int alloc_align_mask, enum dma_data_direction dir,
> @@ -1323,6 +1333,8 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
>   		return (phys_addr_t)DMA_MAPPING_ERROR;
>   	}
>   
> +	swiotlb_disable_dma_skip_sync(dev);
> +
>   	/*
>   	 * Save away the mapping from the original address to the DMA address.
>   	 * This is needed when we sync the memory.  Then we sync the buffer if
> @@ -1640,6 +1652,8 @@ struct page *swiotlb_alloc(struct device *dev, size_t size)
>   	if (index == -1)
>   		return NULL;
>   
> +	swiotlb_disable_dma_skip_sync(dev);
> +
>   	tlb_addr = slot_addr(pool->start, index);
>   
>   	return pfn_to_page(PFN_DOWN(tlb_addr));