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rcu: Enable rcu_normal_after_boot unconditionally for RT

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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@redhat.com>
Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>
Signed-off-by: Julia Cartwright <julia@ni.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
[ paulmck: Update kernel-parameters.txt accordingly. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Julia Cartwright 2020-12-15 15:16:47 +01:00 committed by Paul E. McKenney
parent 8b9a0ecc7e
commit 36221e109e
2 changed files with 10 additions and 1 deletions
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7
Documentation/admin-guide/kernel-parameters.txt
View File

@ -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

4
kernel/rcu/update.c
View File

@ -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