From 88a85f99e51fb2373259ab83c8bb130a9bbf3804 Mon Sep 17 00:00:00 2001 From: Achiad Shochat Date: Thu, 23 Jul 2015 23:35:59 +0300 Subject: net/mlx5e: TX latency optimization to save DMA reads A regular TX WQE execution involves two or more DMA reads - one to fetch the WQE, and another one per WQE gather entry. These DMA reads obviously increase the TX latency. There are two mlx5 mechanisms to bypass these DMA reads: 1) Inline WQE 2) Blue Flame (BF) An inline WQE contains a whole packet, thus saves the DMA read/s of the regular WQE gather entry/s. Inline WQE support was already added in the previous commit. A BF WQE is written directly to the device I/O mapped memory, thus enables saving the DMA read that fetches the WQE. The BF WQE I/O write must be in cache line granularity, thus uses the CPU write combining mechanism. A BF WQE I/O write acts also as a TX doorbell for notifying the device of new TX WQEs. A BF WQE is written to the same I/O mapped address as the regular TX doorbell, thus this address is being mapped twice - once by ioremap() and once by io_mapping_map_wc(). While both mechanisms reduce the TX latency, they both consume more CPU cycles than a regular WQE: - A BF WQE must still be written to host memory, in addition to being written directly to the device I/O mapped memory. - An inline WQE involves copying the SKB data into it. To handle this tradeoff, we introduce here a heuristic algorithm that strives to avoid using these two mechanisms in case the TX queue is being back-pressured by the device, and limit their usage rate otherwise. An inline WQE will always be "Blue Flamed" (written directly to the device I/O mapped memory) while a BF WQE may not be inlined (may contain gather entries). Preliminary testing using netperf UDP_RR shows that the latency goes down from 17.5us to 16.9us, while the message rate (tested with pktgen) stays the same. Signed-off-by: Achiad Shochat Signed-off-by: Amir Vadai Signed-off-by: David S. Miller --- include/linux/mlx5/driver.h | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'include/linux') diff --git a/include/linux/mlx5/driver.h b/include/linux/mlx5/driver.h index 1c0d5d062d7c..5fe0cae1a515 100644 --- a/include/linux/mlx5/driver.h +++ b/include/linux/mlx5/driver.h @@ -380,7 +380,7 @@ struct mlx5_uar { u32 index; struct list_head bf_list; unsigned free_bf_bmap; - void __iomem *wc_map; + void __iomem *bf_map; void __iomem *map; }; @@ -435,6 +435,8 @@ struct mlx5_priv { struct mlx5_uuar_info uuari; MLX5_DECLARE_DOORBELL_LOCK(cq_uar_lock); + struct io_mapping *bf_mapping; + /* pages stuff */ struct workqueue_struct *pg_wq; struct rb_root page_root; -- cgit v1.2.3