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rcu: Don't funnel-lock above leaf node if GP in progress

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The old grace-period start code would acquire only the leaf's rcu_node
structure's ->lock if that structure believed that a grace period was
in progress.  The new code advances to the leaf's parent in this case,
needlessly acquiring then leaf's parent's ->lock.  This commit therefore
checks the grace-period state after marking the leaf with the need for
the specified grace period, and if the leaf believes that a grace period
is in progress, takes an early exit.

Reported-by: Joel Fernandes <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
[ paulmck: Add "Startedleaf" tracing as suggested by Joel Fernandes. ]
This commit is contained in:
Paul E. McKenney 2018-05-12 07:42:20 -07:00
commit a2165e4168
2 changed files with 11 additions and 2 deletions
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9
kernel/rcu/tree.c
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@ -1590,6 +1590,15 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
goto unlock_out;
}
rnp_root->gp_seq_needed = c;
if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
/*
* We just marked the leaf, and a grace period
* is in progress, which means that rcu_gp_cleanup()
* will see the marking. Bail to reduce contention.
*/
trace_rcu_this_gp(rnp, rdp, c, TPS("Startedleaf"));
goto unlock_out;
}
if (rnp_root != rnp && rnp_root->parent != NULL)
raw_spin_unlock_rcu_node(rnp_root);
if (!rnp_root->parent)