netdev - [PATCH RFC net-next] net/core: Enable socket busy polling on -RT

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Message-Id: <20230517110950.78322-1-kurt@linutronix.de>
Date: Wed, 17 May 2023 13:09:50 +0200
From: Kurt Kanzenbach <kurt@...utronix.de>
To: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	netdev@...r.kernel.org,
	Kurt Kanzenbach <kurt@...utronix.de>
Subject: [PATCH RFC net-next] net/core: Enable socket busy polling on -RT

Busy polling is currently not allowed on PREEMPT_RT, because it disables
preemption. It is not possible to acquire sleeping locks with disabled
preemption. For details see commit 20ab39d13e2e ("net/core: disable
NET_RX_BUSY_POLL on PREEMPT_RT").

However, strict cyclic and/or low latency network applications may prefer busy
polling e.g., using AF_XDP instead of interrupt driven communication.

The preempt_disable() is used in order to prevent the poll_lock owner to be
preempted while holding the lock and to ensure progress. To overcome that
problem on -RT, allow busy polling only in case the netconsole (and netpoll) is
not used. No functional change for upstream.

Tested on x86 hardware with v6.1-RT and v6.3-RT on Intel i225 (igc) with
AF_XDP/ZC sockets configured to run in busy polling mode.

Signed-off-by: Kurt Kanzenbach <kurt@...utronix.de>
---
 net/Kconfig    | 2 +-
 net/core/dev.c | 9 ++++++---
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/Kconfig b/net/Kconfig
index 7d39c1773eb4..2fb25b534df5 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -324,7 +324,7 @@ config CGROUP_NET_CLASSID
 
 config NET_RX_BUSY_POLL
 	bool
-	default y if !PREEMPT_RT
+	default y if !PREEMPT_RT || (PREEMPT_RT && !NETCONSOLE)
 
 config BQL
 	bool
diff --git a/net/core/dev.c b/net/core/dev.c
index b3c13e041935..3393c2f3dbe8 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -6197,7 +6197,8 @@ void napi_busy_loop(unsigned int napi_id,
 	if (!napi)
 		goto out;
 
-	preempt_disable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
 	for (;;) {
 		int work = 0;
 
@@ -6239,7 +6240,8 @@ void napi_busy_loop(unsigned int napi_id,
 		if (unlikely(need_resched())) {
 			if (napi_poll)
 				busy_poll_stop(napi, have_poll_lock, prefer_busy_poll, budget);
-			preempt_enable();
+			if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+				preempt_enable();
 			rcu_read_unlock();
 			cond_resched();
 			if (loop_end(loop_end_arg, start_time))
@@ -6250,7 +6252,8 @@ void napi_busy_loop(unsigned int napi_id,
 	}
 	if (napi_poll)
 		busy_poll_stop(napi, have_poll_lock, prefer_busy_poll, budget);
-	preempt_enable();
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
 out:
 	rcu_read_unlock();
 }
-- 
2.30.2