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rcu: Enable elimination of Tree-RCU softirq processing

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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 <tglx@linutronix.de>
Tested-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
[ 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 <paulmck@linux.ibm.com>
This commit is contained in:
Sebastian Andrzej Siewior
2019-03-20 22:13:33 +01:00
committed by Paul E. McKenney
parent a188339ca5
commit 48d07c04b4
4 changed files with 147 additions and 135 deletions
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6
Documentation/admin-guide/kernel-parameters.txt
View File

@@ -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