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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
Download Patch File Download Diff File Expand all files Collapse all files

7
Documentation/admin-guide/kernel-parameters.txt
View File

@@ -4474,6 +4474,13 @@
only normal grace-period primitives. No effect only normal grace-period primitives. No effect
on CONFIG_TINY_RCU kernels. 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] rcupdate.rcu_task_ipi_delay= [KNL]
Set time in jiffies during which RCU tasks will Set time in jiffies during which RCU tasks will
avoid sending IPIs, starting with the beginning avoid sending IPIs, starting with the beginning

4
kernel/rcu/update.c
View File

@@ -56,8 +56,10 @@
#ifndef CONFIG_TINY_RCU #ifndef CONFIG_TINY_RCU
module_param(rcu_expedited, int, 0); module_param(rcu_expedited, int, 0);
module_param(rcu_normal, 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); module_param(rcu_normal_after_boot, int, 0);
#endif
#endif /* #ifndef CONFIG_TINY_RCU */ #endif /* #ifndef CONFIG_TINY_RCU */
#ifdef CONFIG_DEBUG_LOCK_ALLOC #ifdef CONFIG_DEBUG_LOCK_ALLOC