From 36221e109eb20ac111bc3bf3e8d5639aa457c7e0 Mon Sep 17 00:00:00 2001 From: Julia Cartwright Date: Tue, 15 Dec 2020 15:16:47 +0100 Subject: rcu: Enable rcu_normal_after_boot unconditionally for RT Expedited RCU grace periods send IPIs to all non-idle CPUs, and thus can disrupt time-critical code in real-time applications. However, there is a portion of boot-time processing (presumably before any real-time applications have started) where expedited RCU grace periods are the only option. And so it is that experience with the -rt patchset indicates that PREEMPT_RT systems should always set the rcupdate.rcu_normal_after_boot kernel boot parameter. This commit therefore makes the post-boot application environment safe for real-time applications by making PREEMPT_RT systems disable the rcupdate.rcu_normal_after_boot kernel boot parameter and acting as if this parameter had been set. This means that post-boot calls to synchronize_rcu_expedited() will be treated as if they were instead calls to synchronize_rcu(), thus preventing the IPIs, and thus avoiding disrupting real-time applications. Suggested-by: Luiz Capitulino Acked-by: Paul E. McKenney Signed-off-by: Julia Cartwright Signed-off-by: Sebastian Andrzej Siewior [ paulmck: Update kernel-parameters.txt accordingly. ] Signed-off-by: Paul E. McKenney --- Documentation/admin-guide/kernel-parameters.txt | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'Documentation/admin-guide/kernel-parameters.txt') diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 521255f4ef13..e0008d9caccb 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -4474,6 +4474,13 @@ only normal grace-period primitives. No effect on CONFIG_TINY_RCU kernels. + But note that CONFIG_PREEMPT_RT=y kernels enables + this kernel boot parameter, forcibly setting + it to the value one, that is, converting any + post-boot attempt at an expedited RCU grace + period to instead use normal non-expedited + grace-period processing. + rcupdate.rcu_task_ipi_delay= [KNL] Set time in jiffies during which RCU tasks will avoid sending IPIs, starting with the beginning -- cgit v1.2.3