git: 29fcb8556681 - main - iflib: add a new global tunable "prefer_mpring"

The branch main has been updated by gallatin:

URL: https://cgit.FreeBSD.org/src/commit/?id=29fcb8556681a8920d6b61ade35e4d1a238ea443

commit 29fcb8556681a8920d6b61ade35e4d1a238ea443
Author:     Andrew Gallatin <gallatin@FreeBSD.org>
AuthorDate: 2026-09-16 16:08:17 +0000
Commit:     Andrew Gallatin <gallatin@FreeBSD.org>
CommitDate: 2026-09-16 17:11:19 +0000

    iflib: add a new global tunable "prefer_mpring"
    
    net.iflib.prefer_mpring can be used to control whether or
    not all iflib driver instances default to mp_ring or simple_tx.
    
    This is intended to be temporary, to allow easy testing (now) of
    simple_tx, and to allow an easy fallback to the legacy path
    (later) once the default is switched to simple_tx
---
 share/man/man4/iflib.4 | 17 +++++++++++++++--
 sys/net/iflib.c        |  5 +++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/share/man/man4/iflib.4 b/share/man/man4/iflib.4
index ca1b7918412e..2a752871d9a8 100644
--- a/share/man/man4/iflib.4
+++ b/share/man/man4/iflib.4
@@ -1,4 +1,4 @@
-.Dd September 14, 2026
+.Dd September 16, 2026
 .Dt IFLIB 4
 .Os
 .Sh NAME
@@ -79,7 +79,9 @@ resource contention is not a problem (well behaved applications on
 systems where there are as many NIC queues as CPU cores).
 Note that when this is enabled, the tx_abdicate sysctl is no longer
 applicable and is ignored.
-Defaults to zero.
+By default, this is the inverse of
+.Va net.iflib.prefer_mpring .
+An explicitly configured value takes precedence over that global default.
 .El
 .Pp
 These
@@ -144,6 +146,17 @@ Some NICs allow processing completed transmit descriptors in batches.
 Doing so usually increases the transmit throughput by reducing the number of
 transmit interrupts.
 Setting this to a non-zero value will disable the use of this feature.
+.It Va net.iflib.prefer_mpring
+When set, use the
+.Va mp_ring
+transmit path by default instead of the simple transmit path.
+This is a read-only tunable that must be set before
+.Nm
+is loaded.
+An explicitly configured per-device
+.Va simple_tx
+value takes precedence.
+The default is one.
 .It Va net.iflib.simple_drain_quota
 Sets the maximum number of packets that the transmit task may remove from a
 simple transmit deferral ring in one drain operation.
diff --git a/sys/net/iflib.c b/sys/net/iflib.c
index 2257e64a5a8e..1c43792c9857 100644
--- a/sys/net/iflib.c
+++ b/sys/net/iflib.c
@@ -704,6 +704,10 @@ static int iflib_simple_txbr_size = IFLIB_SIMPLE_TXBR_SIZE;
 SYSCTL_INT(_net_iflib, OID_AUTO, simple_txbr_size, CTLFLAG_RDTUN,
     &iflib_simple_txbr_size, 0,
     "number of entries in the simple tx deferral ring");
+static bool iflib_prefer_mpring = true;
+SYSCTL_BOOL(_net_iflib, OID_AUTO, prefer_mpring, CTLFLAG_RDTUN,
+    &iflib_prefer_mpring, 0,
+    "prefer the mp_ring transmit path by default");
 static u_int iflib_simple_drain_quota = 8;
 SYSCTL_UINT(_net_iflib, OID_AUTO, simple_drain_quota, CTLFLAG_RWTUN,
     &iflib_simple_drain_quota, 0,
@@ -7641,6 +7645,7 @@ iflib_add_device_sysctl_pre(if_ctx_t ctx)
 	    "tx_watchdog_events", CTLFLAG_RD, &ctx->ifc_tx_watchdog_events, 0,
 	    "TX watchdog resets initiated by iflib");
 
+	ctx->ifc_sysctl_simple_tx = !iflib_prefer_mpring;
 	SYSCTL_ADD_BOOL(&ctx->ifc_sysctl_ctx, oid_list, OID_AUTO, "simple_tx",
 	    CTLFLAG_RDTUN, &ctx->ifc_sysctl_simple_tx, 0,
 	    "use simple tx ring");