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rcu: Improve non-root rcu_cbs_completed() accuracy

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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>
This commit is contained in:
Paul E. McKenney 2018-04-10 21:17:56 -07:00
parent 60cc43fc88
commit 9036c2ffd5
1 changed files with 15 additions and 0 deletions
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15
kernel/rcu/tree.c
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@ -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.