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

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Date: Tue, 23 May 2023 13:15:18 +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: netdev@...r.kernel.org,
	Kurt Kanzenbach <kurt@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH net-next] net/core: Enable socket busy polling on -RT

Busy polling is currently not allowed on PREEMPT_RT, because it disables
preemption while invoking the NAPI callback. 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_owner and NAPI owner
to be preempted while owning the resource to ensure progress. Netpoll performs
busy polling in order to acquire the lock. NAPI is locked by setting the
NAPIF_STATE_SCHED flag. There is no busy polling if the flag is set and the
"owner" is preempted. Worst case is that the task owning NAPI gets preempted and
NAPI processing stalls.  This is can be prevented by properly prioritising the
tasks within the system.

Allow RX_BUSY_POLL on PREEMPT_RT if NETPOLL is disabled. Don't disable
preemption on PREEMPT_RT within the busy poll loop.

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.

Suggested-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Kurt Kanzenbach <kurt@...utronix.de>
---

Changes since RFC:

 * Commit message

Previous version:

 * https://lore.kernel.org/all/20230517110950.78322-1-kurt@linutronix.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