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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>
This commit is contained in:
Paul E. McKenney 2018-05-16 14:41:41 -07:00
parent 6f56f714db
commit 15651201fa
1 changed files with 4 additions and 1 deletions
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5
kernel/rcu/tree_plugin.h
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@ -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;
}