linux-kernel - [tip: core/rcu] rcu: Enable rcu_normal_after

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Date:   Mon, 15 Feb 2021 14:55:45 -0000
From:   "tip-bot2 for Julia Cartwright" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Luiz Capitulino <lcapitulino@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Julia Cartwright <julia@...com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: core/rcu] rcu: Enable rcu_normal_after_boot unconditionally for RT

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     36221e109eb20ac111bc3bf3e8d5639aa457c7e0
Gitweb:        https://git.kernel.org/tip/36221e109eb20ac111bc3bf3e8d5639aa457c7e0
Author:        Julia Cartwright <julia@...com>
AuthorDate:    Tue, 15 Dec 2020 15:16:47 +01:00
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Mon, 04 Jan 2021 13:43:51 -08:00

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 <lcapitulino@...hat.com>
Acked-by: Paul E. McKenney <paulmck@...ux.ibm.com>
Signed-off-by: Julia Cartwright <julia@...com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
[ paulmck: Update kernel-parameters.txt accordingly. ]
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 Documentation/admin-guide/kernel-parameters.txt | 7 +++++++
 kernel/rcu/update.c                             | 4 +++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 521255f..e0008d9 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
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 39334d2..b95ae86 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -56,8 +56,10 @@
 #ifndef CONFIG_TINY_RCU
 module_param(rcu_expedited, int, 0);
 module_param(rcu_normal, int, 0);
-static int rcu_normal_after_boot;
+static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT);
+#ifndef CONFIG_PREEMPT_RT
 module_param(rcu_normal_after_boot, int, 0);
+#endif
 #endif /* #ifndef CONFIG_TINY_RCU */
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC