rcu: Prevent useless FQS scan after all CPUs have checked in

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The force_qs_rnp() function checks for ->qsmask being all zero, that is,
all CPUs for the current rcu_node structure having already passed through
quiescent states.  But with RCU-preempt, this is not sufficient to report
quiescent states further up the tree, so there are further checks that
can initiate RCU priority boosting and also for races with CPU-hotplug
operations.  However, if neither of these further checks apply, the code
proceeds to carry out a useless scan of an all-zero ->qsmask.

This commit therefore adds code to release the current rcu_node
structure's lock and continue on to the next rcu_node structure, thereby
avoiding this useless scan.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

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Paul E. McKenney 2018-05-02 11:07:02 -07:00

parent 91f63ced7d

commit 928164351e

1 changed files with 2 additions and 0 deletions

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kernel/rcu/tree.c

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