* [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 <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
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH net-next] net/core: disable NET_RX_BUSY_POLL on PREEMPT_RT
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
0 siblings, 0 replies; 2+ messages in thread
From: patchwork-bot+netdevbpf @ 2021-10-01 22:50 UTC (permalink / raw)
To: Sebastian Andrzej Siewior; +Cc: netdev, davem, kuba, tglx
Hello:
This patch was applied to netdev/net-next.git (refs/heads/master):
On Fri, 1 Oct 2021 16:58:41 +0200 you wrote:
> 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.
>
> [...]
Here is the summary with links:
- [net-next] net/core: disable NET_RX_BUSY_POLL on PREEMPT_RT
https://git.kernel.org/netdev/net-next/c/20ab39d13e2e
You are awesome, thank you!
--
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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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