From: "Paul E. McKenney" <[email protected]> RCU grace-period initialization is currently carried out with interrupts disabled, which can result in 200-microsecond latency spikes on systems on which RCU has been configured for 4096 CPUs. This patch therefore makes the RCU grace-period initialization be preemptible, which should eliminate those latency spikes. Similar spikes from grace-period cleanup and the forcing of quiescent states will be dealt with similarly by later patches.
Reported-by: Mike Galbraith <[email protected]> Reported-by: Dimitri Sivanich <[email protected]> Signed-off-by: Paul E. McKenney <[email protected]> Reviewed-by: Josh Triplett <[email protected]> --- kernel/rcutree.c | 26 +++++++++++--------------- 1 files changed, 11 insertions(+), 15 deletions(-) diff --git a/kernel/rcutree.c b/kernel/rcutree.c index 0df9aaa..59c528f 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -1028,7 +1028,7 @@ rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat /* * Body of kthread that handles grace periods. */ -static int rcu_gp_kthread(void *arg) +static int __noreturn rcu_gp_kthread(void *arg) { struct rcu_data *rdp; struct rcu_node *rnp; @@ -1054,6 +1054,7 @@ static int rcu_gp_kthread(void *arg) * don't start another one. */ raw_spin_unlock_irq(&rnp->lock); + cond_resched(); continue; } @@ -1064,6 +1065,7 @@ static int rcu_gp_kthread(void *arg) */ rsp->fqs_need_gp = 1; raw_spin_unlock_irq(&rnp->lock); + cond_resched(); continue; } @@ -1074,10 +1076,10 @@ static int rcu_gp_kthread(void *arg) rsp->fqs_state = RCU_GP_INIT; /* Stop force_quiescent_state. */ rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS; record_gp_stall_check_time(rsp); - raw_spin_unlock(&rnp->lock); /* leave irqs disabled. */ + raw_spin_unlock_irq(&rnp->lock); /* Exclude any concurrent CPU-hotplug operations. */ - raw_spin_lock(&rsp->onofflock); /* irqs already disabled. */ + get_online_cpus(); /* * Set the quiescent-state-needed bits in all the rcu_node @@ -1089,15 +1091,9 @@ static int rcu_gp_kthread(void *arg) * indicate that no grace period is in progress, at least * until the corresponding leaf node has been initialized. * In addition, we have excluded CPU-hotplug operations. - * - * Note that the grace period cannot complete until - * we finish the initialization process, as there will - * be at least one qsmask bit set in the root node until - * that time, namely the one corresponding to this CPU, - * due to the fact that we have irqs disabled. */ rcu_for_each_node_breadth_first(rsp, rnp) { - raw_spin_lock(&rnp->lock); /* irqs already disabled. */ + raw_spin_lock_irq(&rnp->lock); rcu_preempt_check_blocked_tasks(rnp); rnp->qsmask = rnp->qsmaskinit; rnp->gpnum = rsp->gpnum; @@ -1108,17 +1104,17 @@ static int rcu_gp_kthread(void *arg) trace_rcu_grace_period_init(rsp->name, rnp->gpnum, rnp->level, rnp->grplo, rnp->grphi, rnp->qsmask); - raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */ + raw_spin_unlock_irq(&rnp->lock); + cond_resched(); } rnp = rcu_get_root(rsp); - raw_spin_lock(&rnp->lock); /* irqs already disabled. */ + raw_spin_lock_irq(&rnp->lock); /* force_quiescent_state() now OK. */ rsp->fqs_state = RCU_SIGNAL_INIT; - raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */ - raw_spin_unlock_irq(&rsp->onofflock); + raw_spin_unlock_irq(&rnp->lock); + put_online_cpus(); } - return 0; } /* -- 1.7.8 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
