rcu: Mark task as .need_qs less aggressively

If any scheduling-clock interrupt interrupts an RCU-preempt read-side
critical section, the interrupted task's ->rcu_read_unlock_special.b.need_qs
field is set.  This causes the outermost rcu_read_unlock() to incur the
extra overhead of calling into rcu_read_unlock_special().  This commit
reduces that overhead by setting ->rcu_read_unlock_special.b.need_qs only
if the grace period has been in effect for more than one second.

Why one second?  Because this is comfortably smaller than the minimum
RCU CPU stall-warning timeout of three seconds, but long enough that the
.need_qs marking should happen quite rarely.  And if your RCU read-side
critical section has run on-CPU for a full second, it is not unreasonable
to invest some CPU time in ending the grace period quickly.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index dbfe901..0239cf8 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -730,6 +730,7 @@ rcu_preempt_check_blocked_tasks(struct rcu_state *rsp, struct rcu_node *rnp)
  */
 static void rcu_preempt_check_callbacks(void)
 {
+       struct rcu_state *rsp = &rcu_preempt_state;
        struct task_struct *t = current;
 
        if (t->rcu_read_lock_nesting == 0) {
@@ -738,7 +739,9 @@ static void rcu_preempt_check_callbacks(void)
        }
        if (t->rcu_read_lock_nesting > 0 &&
            __this_cpu_read(rcu_data_p->core_needs_qs) &&
-           __this_cpu_read(rcu_data_p->cpu_no_qs.b.norm))
+           __this_cpu_read(rcu_data_p->cpu_no_qs.b.norm) &&
+           !t->rcu_read_unlock_special.b.need_qs &&
+           time_after(jiffies, rsp->gp_start + HZ))
                t->rcu_read_unlock_special.b.need_qs = true;
 }