netdev - [PATCH v1 net-next 3/4] net: add /proc/sys/net/core/txq_reselection

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Message-ID: <20251013152234.842065-4-edumazet@google.com>
Date: Mon, 13 Oct 2025 15:22:33 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Neal Cardwell <ncardwell@...gle.com>
Cc: Simon Horman <horms@...nel.org>, Kuniyuki Iwashima <kuniyu@...gle.com>, 
	Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH v1 net-next 3/4] net: add /proc/sys/net/core/txq_reselection_ms
 control

Add a new sysctl to control how often a queue reselection
can happen even if a flow has a persistent queue of skbs
in a Qdisc or NIC queue.

A value of zero means the feature is disabled.

Default is 1000 (1 second).

This sysctl is used in the following patch.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reviewed-by: Neal Cardwell <ncardwell@...gle.com>
---
 Documentation/admin-guide/sysctl/net.rst | 17 +++++++++++++++++
 include/net/netns/core.h                 |  1 +
 net/core/net_namespace.c                 |  1 +
 net/core/sysctl_net_core.c               |  7 +++++++
 4 files changed, 26 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/net.rst b/Documentation/admin-guide/sysctl/net.rst
index 2ef50828aff16b01baf32f5ded9b75a6e699b184..40749b3cd356973b9990add5dfdc87bb18711e2d 100644
--- a/Documentation/admin-guide/sysctl/net.rst
+++ b/Documentation/admin-guide/sysctl/net.rst
@@ -406,6 +406,23 @@ to SOCK_TXREHASH_DEFAULT (i. e. not overridden by setsockopt).
 If set to 1 (default), hash rethink is performed on listening socket.
 If set to 0, hash rethink is not performed.
 
+txq_reselection_ms
+------------------
+
+Controls how often (in ms) a busy connected flow can select another tx queue.
+
+A resection is desirable when/if user thread has migrated and XPS
+would select a different queue. Same can occur without XPS
+if the flow hash has changed.
+
+But switching txq can introduce reorders, especially if the
+old queue is under high pressure. Modern TCP stacks deal
+well with reorders if they happen not too often.
+
+To disable this feature, set the value to 0.
+
+Default : 1000
+
 gro_normal_batch
 ----------------
 
diff --git a/include/net/netns/core.h b/include/net/netns/core.h
index 9b36f0ff0c200c9cf89753f2c78a3ee0fe5256b7..cb9c3e4cd7385016de3ac87dac65411d54bd093b 100644
--- a/include/net/netns/core.h
+++ b/include/net/netns/core.h
@@ -13,6 +13,7 @@ struct netns_core {
 	struct ctl_table_header	*sysctl_hdr;
 
 	int	sysctl_somaxconn;
+	int	sysctl_txq_reselection;
 	int	sysctl_optmem_max;
 	u8	sysctl_txrehash;
 	u8	sysctl_tstamp_allow_data;
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index b0e0f22d7b213c522c2b83a5fcbcabb43e72cd11..adcfef55a66f1691cb76d954af32334e532864bb 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -395,6 +395,7 @@ static __net_init void preinit_net_sysctl(struct net *net)
 	net->core.sysctl_optmem_max = 128 * 1024;
 	net->core.sysctl_txrehash = SOCK_TXREHASH_ENABLED;
 	net->core.sysctl_tstamp_allow_data = 1;
+	net->core.sysctl_txq_reselection = msecs_to_jiffies(1000);
 }
 
 /* init code that must occur even if setup_net() is not called. */
diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
index 8cf04b57ade1e0bf61ad4ac219ab4eccf638658a..f79137826d7f9d653e2e22d8f42e23bec08e083c 100644
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -667,6 +667,13 @@ static struct ctl_table netns_core_table[] = {
 		.extra2		= SYSCTL_ONE,
 		.proc_handler	= proc_dou8vec_minmax,
 	},
+	{
+		.procname	= "txq_reselection_ms",
+		.data		= &init_net.core.sysctl_txq_reselection,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_ms_jiffies,
+	},
 	{
 		.procname	= "tstamp_allow_data",
 		.data		= &init_net.core.sysctl_tstamp_allow_data,
-- 
2.51.0.740.g6adb054d12-goog