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rcu: Unconditionally use rcuc threads on PREEMPT_RT
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>
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2 changed files with 7 additions and 1 deletions
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@ -4092,6 +4092,10 @@
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value, meaning that RCU_SOFTIRQ is used by default.
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Specify rcutree.use_softirq=0 to use rcuc kthreads.
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But note that CONFIG_PREEMPT_RT=y kernels disable
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this kernel boot parameter, forcibly setting it
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to zero.
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rcutree.rcu_fanout_exact= [KNL]
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Disable autobalancing of the rcu_node combining
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tree. This is used by rcutorture, and might
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@ -100,8 +100,10 @@ static struct rcu_state rcu_state = {
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static bool dump_tree;
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module_param(dump_tree, bool, 0444);
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/* By default, use RCU_SOFTIRQ instead of rcuc kthreads. */
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static bool use_softirq = true;
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static bool use_softirq = !IS_ENABLED(CONFIG_PREEMPT_RT);
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#ifndef CONFIG_PREEMPT_RT
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module_param(use_softirq, bool, 0444);
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#endif
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/* Control rcu_node-tree auto-balancing at boot time. */
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static bool rcu_fanout_exact;
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module_param(rcu_fanout_exact, bool, 0444);
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