Commit-ID: f92c734f02cbf10e40569facff82059ae9b61920
Gitweb: http://git.kernel.org/tip/f92c734f02cbf10e40569facff82059ae9b61920
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
AuthorDate: Mon, 10 Apr 2017 15:40:35 -0700
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Thu, 8 Jun 2017 08:25:22 -0700
rcu: Prevent rcu_barrier() from starting needless grace periods
Currently rcu_barrier() uses call_rcu() to enqueue new callbacks
on each CPU with a non-empty callback list. This works, but means
that rcu_barrier() forces grace periods that are not otherwise needed.
The key point is that rcu_barrier() never needs to wait for a grace
period, but instead only for all pre-existing callbacks to be invoked.
This means that rcu_barrier()'s new callbacks should be placed in
the callback-list segment containing the last pre-existing callback.
This commit makes this change using the new rcu_segcblist_entrain()
function.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
include/trace/events/rcu.h | 1 +
kernel/rcu/tree.c | 10 ++++++++--
2 files changed, 9 insertions(+), 2 deletions(-)
diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index e3facb3..91dc089 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -742,6 +742,7 @@ TRACE_EVENT(rcu_torture_read,
* "OnlineQ": _rcu_barrier() found online CPU with callbacks.
* "OnlineNQ": _rcu_barrier() found online CPU, no callbacks.
* "IRQ": An rcu_barrier_callback() callback posted on remote CPU.
+ * "IRQNQ": An rcu_barrier_callback() callback found no callbacks.
* "CB": An rcu_barrier_callback() invoked a callback, not the last.
* "LastCB": An rcu_barrier_callback() invoked the last callback.
* "Inc2": _rcu_barrier() piggyback check counter incremented.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e354e47..657056c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3578,8 +3578,14 @@ static void rcu_barrier_func(void *type)
struct rcu_data *rdp = raw_cpu_ptr(rsp->rda);
_rcu_barrier_trace(rsp, "IRQ", -1, rsp->barrier_sequence);
- atomic_inc(&rsp->barrier_cpu_count);
- rsp->call(&rdp->barrier_head, rcu_barrier_callback);
+ rdp->barrier_head.func = rcu_barrier_callback;
+ debug_rcu_head_queue(&rdp->barrier_head);
+ if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head, 0)) {
+ atomic_inc(&rsp->barrier_cpu_count);
+ } else {
+ debug_rcu_head_unqueue(&rdp->barrier_head);
+ _rcu_barrier_trace(rsp, "IRQNQ", -1, rsp->barrier_sequence);
+ }
}
/*
--
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