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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: [PATCH net-next] net/core: disable NET_RX_BUSY_POLL on PREEMPT_RT
Date: Fri,  1 Oct 2021 16:58:41 +0200	[thread overview]
Message-ID: <20211001145841.2308454-1-bigeasy@linutronix.de> (raw)

napi_busy_loop() disables preemption and performs a NAPI poll. We can't acquire
sleeping locks with disabled preemption which would be required while
__napi_poll() invokes the callback of the driver.

A threaded interrupt performing the NAPI-poll can be preempted on PREEMPT_RT.
A RT thread on another CPU may observe NAPIF_STATE_SCHED bit set and busy-spin
until it is cleared or its spin time runs out. Given it is the task with the
highest priority it will never observe the NEED_RESCHED bit set.
In this case the time is better spent by simply sleeping.

The NET_RX_BUSY_POLL is disabled by default (the system wide sysctls for
poll/read are set to zero). Disabling NET_RX_BUSY_POLL on PREEMPT_RT to avoid
wrong locking context in case it is used.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 net/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/Kconfig b/net/Kconfig
index fb13460c6dab3..074472dfa94ae 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -294,7 +294,7 @@ config CGROUP_NET_CLASSID
 
 config NET_RX_BUSY_POLL
 	bool
-	default y
+	default y if !PREEMPT_RT
 
 config BQL
 	bool
-- 
2.33.0


             reply	other threads:[~2021-10-01 14:58 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-01 14:58 Sebastian Andrzej Siewior [this message]
2021-10-01 22:50 ` patchwork-bot+netdevbpf

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