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* [PATCH net-next] net/core: disable NET_RX_BUSY_POLL on PREEMPT_RT
@ 2021-10-01 14:58 Sebastian Andrzej Siewior
  2021-10-01 22:50 ` patchwork-bot+netdevbpf
  0 siblings, 1 reply; 2+ messages in thread
From: Sebastian Andrzej Siewior @ 2021-10-01 14:58 UTC (permalink / raw)
  To: netdev
  Cc: David S. Miller, Jakub Kicinski, Thomas Gleixner,
	Sebastian Andrzej Siewior

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 <>
 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
-	default y
+	default y if !PREEMPT_RT
 config BQL

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2021-10-01 14:58 [PATCH net-next] net/core: disable NET_RX_BUSY_POLL on PREEMPT_RT Sebastian Andrzej Siewior
2021-10-01 22:50 ` patchwork-bot+netdevbpf

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