Commit 9036c2ff authored Apr 10, 2018 by Paul E. McKenney

rcu: Improve non-root rcu_cbs_completed() accuracy

When rcu_cbs_completed() is invoked on a non-root rcu_node structure,
it unconditionally assumes that two grace periods must complete before
the callbacks at hand can be invoked. This is overly conservative because
if that non-root rcu_node structure believes that no grace period is in
progress, and if the corresponding rcu_state structure's ->gpnum field
has not yet been incremented, then these callbacks may safely be invoked
after only one grace period has completed.

This change is required to permit grace-period start requests to use
funnel locking, which is in turn permitted to reduce root rcu_node ->lock
contention, which has been observed by Nick Piggin. Furthermore, such
contention will likely be increased by the merging of RCU-bh, RCU-preempt,
and RCU-sched, so it makes sense to take steps to decrease it.

This commit therefore improves the accuracy of rcu_cbs_completed() when
invoked on a non-root rcu_node structure as described above.

Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>

parent 60cc43fc

kernel/rcu/tree.c

+15 −0

Original line number	Diff line number	Diff line
		@@ -1641,6 +1641,21 @@ static unsigned long rcu_cbs_completed(struct rcu_state *rsp,
		if (rcu_get_root(rsp) == rnp && rnp->gpnum == rnp->completed)
		return rnp->completed + 1;

		/*
		* If the current rcu_node structure believes that RCU is
		* idle, and if the rcu_state structure does not yet reflect
		* the start of a new grace period, then the next grace period
		* will suffice. The memory barrier is needed to accurately
		* sample the rsp->gpnum, and pairs with the second lock
		* acquisition in rcu_gp_init(), which is augmented with
		* smp_mb__after_unlock_lock() for this purpose.
		*/
		if (rnp->gpnum == rnp->completed) {
		smp_mb(); /* See above block comment. */
		if (READ_ONCE(rsp->gpnum) == rnp->completed)
		return rnp->completed + 1;
		}

		/*
		* Otherwise, wait for a possible partial grace period and
		* then the subsequent full grace period.

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