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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 <achiad@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Achiad Shochat 2015-07-23 23:35:59 +03:00 committed by David S. Miller
parent 58d522912a
commit 88a85f99e5
6 changed files with 79 additions and 19 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
drivers/net/ethernet/mellanox/mlx5/core/en.h
View File

@ -60,6 +60,7 @@
#define MLX5E_TX_CQ_POLL_BUDGET 128
#define MLX5E_UPDATE_STATS_INTERVAL 200 /* msecs */
#define MLX5E_SQ_BF_BUDGET 16
static const char vport_strings[][ETH_GSTRING_LEN] = {
/* vport statistics */
@ -268,7 +269,9 @@ struct mlx5e_sq {
/* dirtied @xmit */
u16 pc ____cacheline_aligned_in_smp;
u32 dma_fifo_pc;
u32 bf_offset;
u16 bf_offset;
u16 prev_cc;
u8 bf_budget;
struct mlx5e_sq_stats stats;
struct mlx5e_cq cq;
@ -281,9 +284,10 @@ struct mlx5e_sq {
struct mlx5_wq_cyc wq;
u32 dma_fifo_mask;
void __iomem *uar_map;
void __iomem *uar_bf_map;
struct netdev_queue *txq;
u32 sqn;
u32 bf_buf_size;
u16 bf_buf_size;
u16 max_inline;
u16 edge;
struct device *pdev;
@ -493,8 +497,10 @@ int mlx5e_update_priv_params(struct mlx5e_priv *priv,
struct mlx5e_params *new_params);
static inline void mlx5e_tx_notify_hw(struct mlx5e_sq *sq,
struct mlx5e_tx_wqe *wqe)
struct mlx5e_tx_wqe *wqe, int bf_sz)
{
u16 ofst = MLX5_BF_OFFSET + sq->bf_offset;
/* ensure wqe is visible to device before updating doorbell record */
dma_wmb();
@ -505,9 +511,15 @@ static inline void mlx5e_tx_notify_hw(struct mlx5e_sq *sq,
*/
wmb();
mlx5_write64((__be32 *)&wqe->ctrl,
sq->uar_map + MLX5_BF_OFFSET + sq->bf_offset,
NULL);
if (bf_sz) {
__iowrite64_copy(sq->uar_bf_map + ofst, &wqe->ctrl, bf_sz);
/* flush the write-combining mapped buffer */
wmb();
} else {
mlx5_write64((__be32 *)&wqe->ctrl, sq->uar_map + ofst, NULL);
}
sq->bf_offset ^= sq->bf_buf_size;
}

12
drivers/net/ethernet/mellanox/mlx5/core/en_main.c
View File

@ -514,6 +514,7 @@ static int mlx5e_create_sq(struct mlx5e_channel *c,
sq->wq.db = &sq->wq.db[MLX5_SND_DBR];
sq->uar_map = sq->uar.map;
sq->uar_bf_map = sq->uar.bf_map;
sq->bf_buf_size = (1 << MLX5_CAP_GEN(mdev, log_bf_reg_size)) / 2;
sq->max_inline = param->max_inline;
@ -524,11 +525,12 @@ static int mlx5e_create_sq(struct mlx5e_channel *c,
txq_ix = c->ix + tc * priv->params.num_channels;
sq->txq = netdev_get_tx_queue(priv->netdev, txq_ix);
sq->pdev = c->pdev;
sq->mkey_be = c->mkey_be;
sq->channel = c;
sq->tc = tc;
sq->edge = (sq->wq.sz_m1 + 1) - MLX5_SEND_WQE_MAX_WQEBBS;
sq->pdev = c->pdev;
sq->mkey_be = c->mkey_be;
sq->channel = c;
sq->tc = tc;
sq->edge = (sq->wq.sz_m1 + 1) - MLX5_SEND_WQE_MAX_WQEBBS;
sq->bf_budget = MLX5E_SQ_BF_BUDGET;
priv->txq_to_sq_map[txq_ix] = sq;
return 0;

26
drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
View File

@ -57,7 +57,7 @@ void mlx5e_send_nop(struct mlx5e_sq *sq, bool notify_hw)
if (notify_hw) {
cseg->fm_ce_se = MLX5_WQE_CTRL_CQ_UPDATE;
mlx5e_tx_notify_hw(sq, wqe);
mlx5e_tx_notify_hw(sq, wqe, 0);
}
}
@ -110,7 +110,7 @@ u16 mlx5e_select_queue(struct net_device *dev, struct sk_buff *skb,
}
static inline u16 mlx5e_get_inline_hdr_size(struct mlx5e_sq *sq,
struct sk_buff *skb)
struct sk_buff *skb, bool bf)
{
/* Some NIC TX decisions, e.g loopback, are based on the packet
* headers and occur before the data gather.
@ -118,7 +118,7 @@ static inline u16 mlx5e_get_inline_hdr_size(struct mlx5e_sq *sq,
*/
#define MLX5E_MIN_INLINE (ETH_HLEN + 2/*vlan tag*/)
if (skb_headlen(skb) <= sq->max_inline)
if (bf && (skb_headlen(skb) <= sq->max_inline))
return skb_headlen(skb);
return MLX5E_MIN_INLINE;
@ -137,6 +137,7 @@ static netdev_tx_t mlx5e_sq_xmit(struct mlx5e_sq *sq, struct sk_buff *skb)
u8 opcode = MLX5_OPCODE_SEND;
dma_addr_t dma_addr = 0;
bool bf = false;
u16 headlen;
u16 ds_cnt;
u16 ihs;
@ -149,6 +150,11 @@ static netdev_tx_t mlx5e_sq_xmit(struct mlx5e_sq *sq, struct sk_buff *skb)
else
sq->stats.csum_offload_none++;
if (sq->cc != sq->prev_cc) {
sq->prev_cc = sq->cc;
sq->bf_budget = (sq->cc == sq->pc) ? MLX5E_SQ_BF_BUDGET : 0;
}
if (skb_is_gso(skb)) {
u32 payload_len;
@ -161,7 +167,10 @@ static netdev_tx_t mlx5e_sq_xmit(struct mlx5e_sq *sq, struct sk_buff *skb)
sq->stats.tso_packets++;
sq->stats.tso_bytes += payload_len;
} else {
ihs = mlx5e_get_inline_hdr_size(sq, skb);
bf = sq->bf_budget &&
!skb->xmit_more &&
!skb_shinfo(skb)->nr_frags;
ihs = mlx5e_get_inline_hdr_size(sq, skb, bf);
MLX5E_TX_SKB_CB(skb)->num_bytes = max_t(unsigned int, skb->len,
ETH_ZLEN);
}
@ -233,14 +242,21 @@ static netdev_tx_t mlx5e_sq_xmit(struct mlx5e_sq *sq, struct sk_buff *skb)
}
if (!skb->xmit_more || netif_xmit_stopped(sq->txq)) {
int bf_sz = 0;
if (bf && sq->uar_bf_map)
bf_sz = MLX5E_TX_SKB_CB(skb)->num_wqebbs << 3;
cseg->fm_ce_se = MLX5_WQE_CTRL_CQ_UPDATE;
mlx5e_tx_notify_hw(sq, wqe);
mlx5e_tx_notify_hw(sq, wqe, bf_sz);
}
/* fill sq edge with nops to avoid wqe wrap around */
while ((sq->pc & wq->sz_m1) > sq->edge)
mlx5e_send_nop(sq, false);
sq->bf_budget = bf ? sq->bf_budget - 1 : 0;
sq->stats.packets++;
return NETDEV_TX_OK;

26
drivers/net/ethernet/mellanox/mlx5/core/main.c
View File

@ -654,6 +654,22 @@ static int mlx5_core_set_issi(struct mlx5_core_dev *dev)
}
#endif
static int map_bf_area(struct mlx5_core_dev *dev)
{
resource_size_t bf_start = pci_resource_start(dev->pdev, 0);
resource_size_t bf_len = pci_resource_len(dev->pdev, 0);
dev->priv.bf_mapping = io_mapping_create_wc(bf_start, bf_len);
return dev->priv.bf_mapping ? 0 : -ENOMEM;
}
static void unmap_bf_area(struct mlx5_core_dev *dev)
{
if (dev->priv.bf_mapping)
io_mapping_free(dev->priv.bf_mapping);
}
static int mlx5_dev_init(struct mlx5_core_dev *dev, struct pci_dev *pdev)
{
struct mlx5_priv *priv = &dev->priv;
@ -808,10 +824,13 @@ static int mlx5_dev_init(struct mlx5_core_dev *dev, struct pci_dev *pdev)
goto err_stop_eqs;
}
if (map_bf_area(dev))
dev_err(&pdev->dev, "Failed to map blue flame area\n");
err = mlx5_irq_set_affinity_hints(dev);
if (err) {
dev_err(&pdev->dev, "Failed to alloc affinity hint cpumask\n");
goto err_free_comp_eqs;
goto err_unmap_bf_area;
}
MLX5_INIT_DOORBELL_LOCK(&priv->cq_uar_lock);
@ -823,7 +842,9 @@ static int mlx5_dev_init(struct mlx5_core_dev *dev, struct pci_dev *pdev)
return 0;
err_free_comp_eqs:
err_unmap_bf_area:
unmap_bf_area(dev);
free_comp_eqs(dev);
err_stop_eqs:
@ -881,6 +902,7 @@ static void mlx5_dev_cleanup(struct mlx5_core_dev *dev)
mlx5_cleanup_qp_table(dev);
mlx5_cleanup_cq_table(dev);
mlx5_irq_clear_affinity_hints(dev);
unmap_bf_area(dev);
free_comp_eqs(dev);
mlx5_stop_eqs(dev);
mlx5_free_uuars(dev, &priv->uuari);

6
drivers/net/ethernet/mellanox/mlx5/core/uar.c
View File

@ -32,6 +32,7 @@
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/io-mapping.h>
#include <linux/mlx5/driver.h>
#include <linux/mlx5/cmd.h>
#include "mlx5_core.h"
@ -246,6 +247,10 @@ int mlx5_alloc_map_uar(struct mlx5_core_dev *mdev, struct mlx5_uar *uar)
goto err_free_uar;
}
if (mdev->priv.bf_mapping)
uar->bf_map = io_mapping_map_wc(mdev->priv.bf_mapping,
uar->index << PAGE_SHIFT);
return 0;
err_free_uar:
@ -257,6 +262,7 @@ EXPORT_SYMBOL(mlx5_alloc_map_uar);
void mlx5_unmap_free_uar(struct mlx5_core_dev *mdev, struct mlx5_uar *uar)
{
io_mapping_unmap(uar->bf_map);
iounmap(uar->map);
mlx5_cmd_free_uar(mdev, uar->index);
}

4
include/linux/mlx5/driver.h
View File

@ -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;