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rcu: Prevent rcu_barrier() from starting needless grace periods

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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@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2017-04-10 15:40:35 -07:00
parent c0ee4500ff
commit f92c734f02
2 changed files with 9 additions and 2 deletions
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1
include/trace/events/rcu.h
View File

@ -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.

10
kernel/rcu/tree.c
View File

@ -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);
}
}
/*