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From 116e028b98d7a297eccf815f0a734e80154b53c3 Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Sat, 27 May 2017 19:02:06 +0200
Subject: [PATCH 096/191] net/core: disable NET_RX_BUSY_POLL on RT
napi_busy_loop() disables preemption and performs a NAPI poll. We can't acquire
sleeping locks with disabled preemption so we would have to work around this
and add explicit locking for synchronisation against ksoftirqd.
Without explicit synchronisation a low priority process would "own" the NAPI
state (by setting NAPIF_STATE_SCHED) and could be scheduled out (no
preempt_disable() and BH is preemptible on RT).
In case a network packages arrives then the interrupt handler would set
NAPIF_STATE_MISSED and the system would wait until the task owning the NAPI
would be scheduled in again.
Should a task with RT priority busy poll then it would consume the CPU instead
allowing tasks with lower priority to run.
The NET_RX_BUSY_POLL is disabled by default (the system wide sysctls for
poll/read are set to zero) so disable NET_RX_BUSY_POLL on RT to avoid wrong
locking context on RT. Should this feature be considered useful on RT systems
then it could be enabled again with proper locking and synchronisation.
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 8cea808ad9e8..32f101108361 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -286,7 +286,7 @@ config CGROUP_NET_CLASSID
config NET_RX_BUSY_POLL
bool
- default y
+ default y if !PREEMPT_RT
config BQL
bool
--
2.19.1
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