linux-kernel - [PATCH] rcu: Make rcu_normal_after

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Message-Id: <20210805080123.16320-1-juri.lelli@redhat.com>
Date:   Thu,  5 Aug 2021 10:01:23 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     paulmck@...nel.org, josh@...htriplett.org
Cc:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        peterz@...radead.org, bigeasy@...utronix.de, tglx@...utronix.de,
        rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, joel@...lfernandes.org,
        rcu@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>
Subject: [PATCH] rcu: Make rcu_normal_after_boot writable on RT

Certain configurations (e.g., systems that make heavy use of netns)
need to use synchronize_rcu_expedited() to service RCU grace periods
even after boot.

Even though synchronize_rcu_expedited() has been traditionally
considered harmful for RT for the heavy use of IPIs, it is perfectly
usable under certain conditions (e.g. nohz_full).

Make rcupdate.rcu_normal_after_boot= again writeable on RT, but keep
its default value to 1 (enabled) to avoid regressions. Users who need
synchronize_rcu_expedited() will boot with rcupdate.rcu_normal_after_
boot=0 in the kernel cmdline.

Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
---
 kernel/rcu/update.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index c21b38cc25e9..0fdbf937edac 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -57,9 +57,7 @@
 module_param(rcu_expedited, int, 0);
 module_param(rcu_normal, int, 0);
 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
-- 
2.31.1