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Date: Wed, 16 Nov 2022 12:10:34 +0800
From: Ethan Zhao <haifeng.zhao@...ux.intel.com>
To: Eric Dumazet <edumazet@...gle.com>, Joerg Roedel <joro@...tes.org>,
Robin Murphy <robin.murphy@....com>,
Will Deacon <will@...nel.org>
Cc: linux-kernel <linux-kernel@...r.kernel.org>,
netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>,
iommu@...ts.linux.dev
Subject: Re: [PATCH v2 -next] iommu/dma: avoid expensive indirect calls for
sync operations
Hi,
On 2022/11/16 2:28, Eric Dumazet wrote:
> 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, as shown in [1]
>
> Even if/when CONFIG_RETPOLINE is not set, there
> is a cost of about 3%.
>
> This patch adds dev->skip_dma_sync boolean that can be opted-in.
>
> For instance iommu_setup_dma_ops() can set this boolean to true
> if CONFIG_DMA_API_DEBUG is not set, and dev_is_dma_coherent(dev).
>
> Then later, if/when swiotlb is used for the first time, the flag
> is turned off, from swiotlb_tbl_map_single()
>
> We might in the future inline again these helpers, like:
>
> static void inline
> dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr,
> size_t size, enum dma_data_direction dir)
> {
> if (!dev_skip_dma_sync(dev))
> __dma_sync_single_for_cpu(dev, addr, size, dir);
> }
>
> perf profile before the patch:
>
> 18.53% [kernel] [k] gq_rx_skb
> 14.77% [kernel] [k] napi_reuse_skb
> 8.95% [kernel] [k] skb_release_data
> 5.42% [kernel] [k] dev_gro_receive
> 5.37% [kernel] [k] memcpy
> <*> 5.26% [kernel] [k] iommu_dma_sync_sg_for_cpu
> 4.78% [kernel] [k] tcp_gro_receive
> <*> 4.42% [kernel] [k] iommu_dma_sync_sg_for_device
> 4.12% [kernel] [k] ipv6_gro_receive
> 3.65% [kernel] [k] gq_pool_get
> 3.25% [kernel] [k] skb_gro_receive
> 2.07% [kernel] [k] napi_gro_frags
> 1.98% [kernel] [k] tcp6_gro_receive
> 1.27% [kernel] [k] gq_rx_prep_buffers
> 1.18% [kernel] [k] gq_rx_napi_handler
> 0.99% [kernel] [k] csum_partial
> 0.74% [kernel] [k] csum_ipv6_magic
> 0.72% [kernel] [k] free_pcp_prepare
> 0.60% [kernel] [k] __napi_poll
> 0.58% [kernel] [k] net_rx_action
> 0.56% [kernel] [k] read_tsc
> <*> 0.50% [kernel] [k] __x86_indirect_thunk_r11
> 0.45% [kernel] [k] memset
>
> After patch, lines with <*> no longer show up, and overall
> cpu usage looks much better (~60% instead of ~72%)
>
> 25.56% [kernel] [k] gq_rx_skb
> 9.90% [kernel] [k] napi_reuse_skb
> 7.39% [kernel] [k] dev_gro_receive
> 6.78% [kernel] [k] memcpy
> 6.53% [kernel] [k] skb_release_data
> 6.39% [kernel] [k] tcp_gro_receive
> 5.71% [kernel] [k] ipv6_gro_receive
> 4.35% [kernel] [k] napi_gro_frags
> 4.34% [kernel] [k] skb_gro_receive
> 3.50% [kernel] [k] gq_pool_get
> 3.08% [kernel] [k] gq_rx_napi_handler
> 2.35% [kernel] [k] tcp6_gro_receive
> 2.06% [kernel] [k] gq_rx_prep_buffers
> 1.32% [kernel] [k] csum_partial
> 0.93% [kernel] [k] csum_ipv6_magic
> 0.65% [kernel] [k] net_rx_action
>
> Many thanks to Robin Murphy for his feedback and ideas to make this patch
> much better !
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Joerg Roedel <joro@...tes.org>
> Cc: Will Deacon <will@...nel.org>
> Cc: iommu@...ts.linux.dev
> ---
> drivers/iommu/dma-iommu.c | 2 ++
> include/linux/device.h | 1 +
> include/linux/dma-map-ops.h | 5 +++++
> kernel/dma/mapping.c | 20 ++++++++++++++++----
> kernel/dma/swiotlb.c | 3 +++
> 5 files changed, 27 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 9297b741f5e80e2408e864fc3f779410d6b04d49..bd3f4d3d646cc57c7588f22d49ea32ac693e38ff 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -1587,6 +1587,8 @@ void iommu_setup_dma_ops(struct device *dev, u64 dma_base, u64 dma_limit)
> if (iommu_dma_init_domain(domain, dma_base, dma_limit, dev))
> goto out_err;
> dev->dma_ops = &iommu_dma_ops;
> + if (!IS_ENABLED(CONFIG_DMA_API_DEBUG) && dev_is_dma_coherent(dev))
> + dev->skip_dma_sync = true;
> }
>
> return;
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 424b55df02727b5742070f72374fd65f5dd68151..2fbb2cc18e44e21eba5f43557ee16d0dc92ef2ef 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -647,6 +647,7 @@ struct device {
> defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL)
> bool dma_coherent:1;
> #endif
> + bool skip_dma_sync:1;
> #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 d678afeb8a13a3a54380a959d14f79bca9c23d8e..4691081f71c51da5468cf6703570ebc7a64d40c5 100644
> --- a/include/linux/dma-map-ops.h
> +++ b/include/linux/dma-map-ops.h
> @@ -275,6 +275,11 @@ static inline bool dev_is_dma_coherent(struct device *dev)
> }
> #endif /* CONFIG_ARCH_HAS_DMA_COHERENCE_H */
>
> +static inline bool dev_skip_dma_sync(struct device *dev)
> +{
> + return dev->skip_dma_sync;
> +}
> +
> void *arch_dma_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
> gfp_t gfp, unsigned long attrs);
> void arch_dma_free(struct device *dev, size_t size, void *cpu_addr,
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index 33437d6206445812b6d4d5b33c77235d18074dec..5d5d286ffae7fa6b7ff1aef46bdc59e7e31a8038 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -328,9 +328,12 @@ EXPORT_SYMBOL(dma_unmap_resource);
> void dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr, size_t size,
> enum dma_data_direction dir)
> {
> - const struct dma_map_ops *ops = get_dma_ops(dev);
> + const struct dma_map_ops *ops;
>
> BUG_ON(!valid_dma_direction(dir));
> + if (dev_skip_dma_sync(dev))
> + return;
May I know why this funciton that is called by all coherent or
non-coherent dev
got a burden to decide to bail out or not ? Is it really the better
point to check
it ?
Thanks,
Ethan
> + ops = get_dma_ops(dev);;
> if (dma_map_direct(dev, ops))
> dma_direct_sync_single_for_cpu(dev, addr, size, dir);
> else if (ops->sync_single_for_cpu)
> @@ -342,9 +345,12 @@ EXPORT_SYMBOL(dma_sync_single_for_cpu);
> void dma_sync_single_for_device(struct device *dev, dma_addr_t addr,
> size_t size, enum dma_data_direction dir)
> {
> - const struct dma_map_ops *ops = get_dma_ops(dev);
> + const struct dma_map_ops *ops;
>
> BUG_ON(!valid_dma_direction(dir));
> + if (dev_skip_dma_sync(dev))
> + return;
> + ops = get_dma_ops(dev);;
> if (dma_map_direct(dev, ops))
> dma_direct_sync_single_for_device(dev, addr, size, dir);
> else if (ops->sync_single_for_device)
> @@ -356,9 +362,12 @@ EXPORT_SYMBOL(dma_sync_single_for_device);
> void dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
> int nelems, enum dma_data_direction dir)
> {
> - const struct dma_map_ops *ops = get_dma_ops(dev);
> + const struct dma_map_ops *ops;
>
> BUG_ON(!valid_dma_direction(dir));
> + if (dev_skip_dma_sync(dev))
> + return;
> + ops = get_dma_ops(dev);;
> if (dma_map_direct(dev, ops))
> dma_direct_sync_sg_for_cpu(dev, sg, nelems, dir);
> else if (ops->sync_sg_for_cpu)
> @@ -370,9 +379,12 @@ EXPORT_SYMBOL(dma_sync_sg_for_cpu);
> void dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
> int nelems, enum dma_data_direction dir)
> {
> - const struct dma_map_ops *ops = get_dma_ops(dev);
> + const struct dma_map_ops *ops;
>
> BUG_ON(!valid_dma_direction(dir));
> + if (dev_skip_dma_sync(dev))
> + return;
> + ops = get_dma_ops(dev);;
> if (dma_map_direct(dev, ops))
> dma_direct_sync_sg_for_device(dev, sg, nelems, dir);
> else if (ops->sync_sg_for_device)
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 339a990554e7fed98dd337efe4fb759a98161cdb..03ebd9803db1a457600f1fac8a18fb3dde724a6f 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -734,6 +734,9 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
> int index;
> phys_addr_t tlb_addr;
>
> + if (unlikely(dev->skip_dma_sync))
> + dev->skip_dma_sync = false;
> +
> if (!mem || !mem->nslabs) {
> dev_warn_ratelimited(dev,
> "Can not allocate SWIOTLB buffer earlier and can't now provide you with the DMA bounce buffer");
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