rcutorture: Don't count CPU-stalled time against priority boosting
It will frequently be the case that rcu_torture_boost() will get a
->start_gp_poll() cookie that needs almost all of the current grace period
plus an additional grace period to elapse before ->poll_gp_state() will
return true. It is quite possible that the current grace period will have
(say) two seconds of stall by a CPU failing to pass through a quiescent
state, followed by 300 milliseconds of delay due to a preempted reader.
The next grace period might suffer only one second of stall by a CPU,
followed by another 300 milliseconds of delay due to a preempted reader.
This is an example of RCU priority boosting doing its job, but the full
elapsed time of 3.6 seconds exceeds the 3.5-second limit. In addition,
there is no CPU stall in force at the 3.5-second mark, so this would
nevertheless currently be counted as an RCU priority boosting failure.
This commit therefore avoids this sort of false positive by resetting
the gp_state_time timestamp any time that the current grace period is
being blocked by a CPU. This results in extremely frequent calls to
the ->check_boost_failed() function, so this commit provides a lockless
fastpath that is selected by supplying a NULL CPU-number pointer.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index af92d9f..8bde1b5 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -723,6 +723,10 @@ static void check_cpu_stall(struct rcu_data *rdp)
* count this as an RCU priority boosting failure. A return of true says
* RCU priority boosting is to blame, and false says otherwise. If false
* is returned, the first of the CPUs to blame is stored through cpup.
+ *
+ * If cpup is NULL, then a lockless quick check is carried out, suitable
+ * for high-rate usage. On the other hand, if cpup is non-NULL, each
+ * rcu_node structure's ->lock is acquired, ruling out high-rate usage.
*/
bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
{
@@ -731,6 +735,12 @@ bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
struct rcu_node *rnp;
rcu_for_each_leaf_node(rnp) {
+ if (!cpup) {
+ if (READ_ONCE(rnp->qsmask))
+ return false;
+ else
+ continue;
+ }
raw_spin_lock_irqsave_rcu_node(rnp, flags);
if (!rnp->qsmask) {
// No CPUs without quiescent states for this rnp.
