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rcu: reduce the number of spurious RCU_SOFTIRQ invocations

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Lai Jiangshan noted that up to 10% of the RCU_SOFTIRQ are spurious, and
traced this down to the fact that the current grace-period machinery
will uselessly raise RCU_SOFTIRQ when a given CPU needs to go through
a quiescent state, but has not yet done so.  In this situation, there
might well be nothing that RCU_SOFTIRQ can do, and the overhead can be
worth worrying about in the ksoftirqd case.  This patch therefore avoids
raising RCU_SOFTIRQ in this situation.

Changes since v1 (http://lkml.org/lkml/2010/3/30/122 from Lai Jiangshan):

o	Omit the rcu_qs_pending() prechecks, as they aren't that
	much less expensive than the quiescent-state checks.

o	Merge with the set_need_resched() patch that reduces IPIs.

o	Add the new n_rp_report_qs field to the rcu_pending tracing output.

o	Update the tracing documentation accordingly.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney
2010-04-14 17:39:26 -07:00
parent 4a90a0681c
commit d21670acab
4 changed files with 29 additions and 22 deletions
Download Patch File Download Diff File Expand all files Collapse all files

11
kernel/rcutree.c
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@@ -1161,8 +1161,6 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
*/
void rcu_check_callbacks(int cpu, int user)
{
if (!rcu_pending(cpu))
return; /* if nothing for RCU to do. */
if (user ||
(idle_cpu(cpu) && rcu_scheduler_active &&
!in_softirq() && hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
@@ -1194,7 +1192,8 @@ void rcu_check_callbacks(int cpu, int user)
rcu_bh_qs(cpu);
}
rcu_preempt_check_callbacks(cpu);
raise_softirq(RCU_SOFTIRQ);
if (rcu_pending(cpu))
raise_softirq(RCU_SOFTIRQ);
}
#ifdef CONFIG_SMP
@@ -1534,18 +1533,20 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
check_cpu_stall(rsp, rdp);
/* Is the RCU core waiting for a quiescent state from this CPU? */
if (rdp->qs_pending) {
if (rdp->qs_pending && !rdp->passed_quiesc) {
/*
* If force_quiescent_state() coming soon and this CPU
* needs a quiescent state, and this is either RCU-sched
* or RCU-bh, force a local reschedule.
*/
rdp->n_rp_qs_pending++;
if (!rdp->preemptable &&
ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs) - 1,
jiffies))
set_need_resched();
rdp->n_rp_qs_pending++;
} else if (rdp->qs_pending && rdp->passed_quiesc) {
rdp->n_rp_report_qs++;
return 1;
}