From 48d07c04b4cc1dc1221965312f58fd84926212fe Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Wed, 20 Mar 2019 22:13:33 +0100 Subject: rcu: Enable elimination of Tree-RCU softirq processing Some workloads need to change kthread priority for RCU core processing without affecting other softirq work. This commit therefore introduces the rcutree.use_softirq kernel boot parameter, which moves the RCU core work from softirq to a per-CPU SCHED_OTHER kthread named rcuc. Use of SCHED_OTHER approach avoids the scalability problems that appeared with the earlier attempt to move RCU core processing to from softirq to kthreads. That said, kernels built with RCU_BOOST=y will run the rcuc kthreads at the RCU-boosting priority. Note that rcutree.use_softirq=0 must be specified to move RCU core processing to the rcuc kthreads: rcutree.use_softirq=1 is the default. Reported-by: Thomas Gleixner Tested-by: Mike Galbraith Signed-off-by: Sebastian Andrzej Siewior [ paulmck: Adjust for invoke_rcu_callbacks() only ever being invoked from RCU core processing, in contrast to softirq->rcuc transition in old mainline RCU priority boosting. ] [ paulmck: Avoid wakeups when scheduler might have invoked rcu_read_unlock() while holding rq or pi locks, also possibly fixing a pre-existing latent bug involving raise_softirq()-induced wakeups. ] Signed-off-by: Paul E. McKenney --- Documentation/admin-guide/kernel-parameters.txt | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'Documentation/admin-guide') diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 138f6664b2e2..b96fd15c7316 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -3752,6 +3752,12 @@ the propagation of recent CPU-hotplug changes up the rcu_node combining tree. + rcutree.use_softirq= [KNL] + If set to zero, move all RCU_SOFTIRQ processing to + per-CPU rcuc kthreads. Defaults to a non-zero + value, meaning that RCU_SOFTIRQ is used by default. + Specify rcutree.use_softirq=0 to use rcuc kthreads. + rcutree.rcu_fanout_exact= [KNL] Disable autobalancing of the rcu_node combining tree. This is used by rcutorture, and might -- cgit v1.2.3