From d21670acab9fcb4bc74a40b68a6941059234c55c Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 14 Apr 2010 17:39:26 -0700 Subject: rcu: reduce the number of spurious RCU_SOFTIRQ invocations 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 Signed-off-by: Paul E. McKenney --- kernel/rcutree.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) (limited to 'kernel/rcutree.c') diff --git a/kernel/rcutree.c b/kernel/rcutree.c index c60fd74..ba69969 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -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; } -- cgit v1.1