[PATCH] net/mlx5e: parameter to disable symmetric hash

[PATCH] net/mlx5e: parameter to disable symmetric hash

[Alexandra Kossovsky]

Some AF_XDP applications assume standard Topelitz hash when spreading
traffic accross queues via RSS.  MLX5 driver always set "symmetric"
bit, which results in unexpected queues for any particular connection.

With this patch is is possible to disable that symmetric bit via
use_symmetric_hash module parameter, and use the standard Toeplitz hash
with well-known predictable result, same as for other NICs.

Signed-off-by: Alexandra N. Kossovsky <alexandra.kossovsky@oktetlabs.ru>
---
 drivers/net/ethernet/mellanox/mlx5/core/en/tir.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/tir.c b/drivers/net/ethernet/mellanox/mlx5/core/en/tir.c
index d4239e3b3c88..63fb2608c0b0 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en/tir.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en/tir.c
@@ -5,6 +5,10 @@
 #include "params.h"
 #include <linux/mlx5/transobj.h>
 
+static bool use_symmetric_hash = true;
+module_param_named(use_symmetric_hash, use_symmetric_hash, bool, 0644);
+MODULE_PARM_DESC(use_symmetric_hash, "Use symmetric Toeplitz hash for RSS.");
+
 #define MLX5E_PARAMS_DEFAULT_LRO_WQE_SZ (64 * 1024)
 
 /* max() doesn't work inside square brackets. */
@@ -121,7 +125,8 @@ void mlx5e_tir_builder_build_rss(struct mlx5e_tir_builder *builder,
 		const size_t len = MLX5_FLD_SZ_BYTES(tirc, rx_hash_toeplitz_key);
 		void *rss_key = MLX5_ADDR_OF(tirc, tirc, rx_hash_toeplitz_key);
 
-		MLX5_SET(tirc, tirc, rx_hash_symmetric, 1);
+		if (use_symmetric_hash)
+			MLX5_SET(tirc, tirc, rx_hash_symmetric, 1);
 		memcpy(rss_key, rss_hash->toeplitz_hash_key, len);
 	}
 
-- 
2.30.2


-- 
Alexandra N. Kossovsky
OKTET Labs (http://www.oktetlabs.ru/)

Re: [PATCH] net/mlx5e: parameter to disable symmetric hash

[Jakub Kicinski]

On Wed, 14 Dec 2022 17:35:42 +0300 Alexandra Kossovsky wrote:
> Some AF_XDP applications assume standard Topelitz hash when spreading
> traffic accross queues via RSS.  MLX5 driver always set "symmetric"
> bit, which results in unexpected queues for any particular connection.
> 
> With this patch is is possible to disable that symmetric bit via
> use_symmetric_hash module parameter, and use the standard Toeplitz hash
> with well-known predictable result, same as for other NICs.

This module param, OTOH, not okay..

[ https://lore.kernel.org/all/20221214085106.42a88df1@kernel.org/ ]