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rcu: Unconditionally use rcuc threads on PREEMPT_RT

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PREEMPT_RT systems have long used the rcutree.use_softirq kernel
boot parameter to avoid use of RCU_SOFTIRQ handlers, which can disrupt
real-time applications by invoking callbacks during return from interrupts
that arrived while executing time-critical code.  This kernel boot
parameter instead runs RCU core processing in an 'rcuc' kthread, thus
allowing the scheduler to do its job of avoiding disrupting time-critical
code.

This commit therefore disables the rcutree.use_softirq kernel boot
parameter on PREEMPT_RT systems, thus forcing such systems to do RCU
core processing in 'rcuc' kthreads.  This approach has long been in
use by users of the -rt patchset, and there have been no complaints.
There is therefore no way for the system administrator to override this
choice, at least without modifying and rebuilding the kernel.

Signed-off-by: Scott Wood <swood@redhat.com>
[bigeasy: Reword commit message]
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:
Scott Wood 2020-12-15 15:16:46 +01:00 committed by Paul E. McKenney
parent 2341bc4a03
commit 8b9a0ecc7e
2 changed files with 7 additions and 1 deletions
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4
Documentation/admin-guide/kernel-parameters.txt
View file

@ -4092,6 +4092,10 @@
value, meaning that RCU_SOFTIRQ is used by default.
Specify rcutree.use_softirq=0 to use rcuc kthreads.
But note that CONFIG_PREEMPT_RT=y kernels disable
this kernel boot parameter, forcibly setting it
to zero.
rcutree.rcu_fanout_exact= [KNL]
Disable autobalancing of the rcu_node combining
tree. This is used by rcutorture, and might

4
kernel/rcu/tree.c
View file

@ -100,8 +100,10 @@ static struct rcu_state rcu_state = {
static bool dump_tree;
module_param(dump_tree, bool, 0444);
/* By default, use RCU_SOFTIRQ instead of rcuc kthreads. */
static bool use_softirq = true;
static bool use_softirq = !IS_ENABLED(CONFIG_PREEMPT_RT);
#ifndef CONFIG_PREEMPT_RT
module_param(use_softirq, bool, 0444);
#endif
/* Control rcu_node-tree auto-balancing at boot time. */
static bool rcu_fanout_exact;
module_param(rcu_fanout_exact, bool, 0444);