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rcu: Remove expedited GP funnel-lock bypass

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Commit #cdacbe1f91 ("rcu: Add fastpath bypassing funnel locking")
turns out to be a pessimization at high load because it forces a tree
full of tasks to wait for an expedited grace period that they probably
do not need.  This commit therefore removes this optimization.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2016-01-30 17:23:19 -08:00
parent 4f41530245
commit e2fd9d3584
4 changed files with 8 additions and 29 deletions
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19
kernel/rcu/tree.c
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@ -3616,25 +3616,6 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
struct rcu_node *rnp0;
struct rcu_node *rnp1 = NULL;
/*
* First try directly acquiring the root lock in order to reduce
* latency in the common case where expedited grace periods are
* rare. We check mutex_is_locked() to avoid pathological levels of
* memory contention on ->exp_funnel_mutex in the heavy-load case.
*/
rnp0 = rcu_get_root(rsp);
if (!mutex_is_locked(&rnp0->exp_funnel_mutex)) {
if (mutex_trylock(&rnp0->exp_funnel_mutex)) {
trace_rcu_exp_funnel_lock(rsp->name, rnp0->level,
rnp0->grplo, rnp0->grphi,
TPS("acq"));
if (sync_exp_work_done(rsp, rnp0, NULL,
&rdp->expedited_workdone0, s))
return NULL;
return rnp0;
}
}
/*
* Each pass through the following loop works its way
* up the rcu_node tree, returning if others have done the