Subject: [PATCH net-next 1/2] mlx5/core: avoid memory barrier in eq_update_ci()

From: Caleb Sander Mateos <[email-masked]>
Date: Thu, 31 Oct 2024 21:46:39 -0600

The memory barrier in eq_update_ci() after the doorbell write is a
significant hot spot in mlx5_eq_comp_int(). Under heavy TCP load, we see
3% of CPU time spent on the mfence. As explained in [1], this barrier is
only needed to preserve the ordering of writes to the doorbell register.

Use writel() instead of __raw_writel() for the doorbell write to provide
this ordering without the need for a full memory barrier.
memory-barriers.txt guarantees MMIO writes using writel() appear to the
device in the same order they were made. On strongly-ordered
architectures like x86, writel() adds no overhead to __raw_writel();
both translate into a single store instruction. Removing the mb() avoids
the costly mfence instruction.

[1]: https://lore.kernel.org/netdev/CALzJLG8af0SMfA1C8U8r_Fddb_ZQhvEZd6=[email-masked]/

Signed-off-by: Caleb Sander Mateos <[email-masked]>
---
 drivers/net/ethernet/mellanox/mlx5/core/lib/eq.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/lib/eq.h b/drivers/net/ethernet/mellanox/mlx5/core/lib/eq.h
index 4b7f7131c560..f03736711343 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/lib/eq.h
+++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/eq.h
@@ -68,13 +68,12 @@ static inline struct mlx5_eqe *next_eqe_sw(struct mlx5_eq *eq)
 static inline void eq_update_ci(struct mlx5_eq *eq, int arm)
 {
 	__be32 __iomem *addr = eq->doorbell + (arm ? 0 : 2);
 	u32 val = (eq->cons_index & 0xffffff) | (eq->eqn << 24);
 
-	__raw_writel((__force u32)cpu_to_be32(val), addr);
-	/* We still want ordering, just not swabbing, so add a barrier */
-	mb();
+	/* Ensure ordering of consecutive doorbell writes */
+	writel((__force u32)cpu_to_be32(val), addr);
 }
 
 int mlx5_eq_table_init(struct mlx5_core_dev *dev);
 void mlx5_eq_table_cleanup(struct mlx5_core_dev *dev);
 int mlx5_eq_table_create(struct mlx5_core_dev *dev);
-- 
2.45.2