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Date: Sat, 1 Sep 2012 18:04:23 -0700
From: Josh Triplett <josh@...htriplett.org>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
dipankar@...ibm.com, akpm@...ux-foundation.org,
mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
fweisbec@...il.com, sbw@....edu, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 01/23] rcu: Move RCU grace-period
initialization into a kthread
On Thu, Aug 30, 2012 at 11:18:16AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
>
> As the first step towards allowing grace-period initialization to be
> preemptible, this commit moves the RCU grace-period initialization
> into its own kthread. This is needed to keep large-system scheduling
> latency at reasonable levels.
>
> Reported-by: Mike Galbraith <mgalbraith@...e.de>
> Reported-by: Dimitri Sivanich <sivanich@....com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@...htriplett.org>
> kernel/rcutree.c | 191 ++++++++++++++++++++++++++++++++++++------------------
> kernel/rcutree.h | 3 +
> 2 files changed, 130 insertions(+), 64 deletions(-)
>
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index f280e54..e1c5868 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1040,6 +1040,103 @@ 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)
> +{
> + unsigned long flags;
> + struct rcu_data *rdp;
> + struct rcu_node *rnp;
> + struct rcu_state *rsp = arg;
> +
> + for (;;) {
> +
> + /* Handle grace-period start. */
> + rnp = rcu_get_root(rsp);
> + for (;;) {
> + wait_event_interruptible(rsp->gp_wq, rsp->gp_flags);
> + if (rsp->gp_flags)
> + break;
> + flush_signals(current);
> + }
> + raw_spin_lock_irqsave(&rnp->lock, flags);
> + rsp->gp_flags = 0;
> + rdp = this_cpu_ptr(rsp->rda);
> +
> + if (rcu_gp_in_progress(rsp)) {
> + /*
> + * A grace period is already in progress, so
> + * don't start another one.
> + */
> + raw_spin_unlock_irqrestore(&rnp->lock, flags);
> + continue;
> + }
> +
> + if (rsp->fqs_active) {
> + /*
> + * We need a grace period, but force_quiescent_state()
> + * is running. Tell it to start one on our behalf.
> + */
> + rsp->fqs_need_gp = 1;
> + raw_spin_unlock_irqrestore(&rnp->lock, flags);
> + continue;
> + }
> +
> + /* Advance to a new grace period and initialize state. */
> + rsp->gpnum++;
> + trace_rcu_grace_period(rsp->name, rsp->gpnum, "start");
> + WARN_ON_ONCE(rsp->fqs_state == RCU_GP_INIT);
> + 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. */
> +
> + /* Exclude any concurrent CPU-hotplug operations. */
> + raw_spin_lock(&rsp->onofflock); /* irqs already disabled. */
> +
> + /*
> + * Set the quiescent-state-needed bits in all the rcu_node
> + * structures for all currently online CPUs in breadth-first
> + * order, starting from the root rcu_node structure.
> + * This operation relies on the layout of the hierarchy
> + * within the rsp->node[] array. Note that other CPUs will
> + * access only the leaves of the hierarchy, which still
> + * 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. */
> + rcu_preempt_check_blocked_tasks(rnp);
> + rnp->qsmask = rnp->qsmaskinit;
> + rnp->gpnum = rsp->gpnum;
> + rnp->completed = rsp->completed;
> + if (rnp == rdp->mynode)
> + rcu_start_gp_per_cpu(rsp, rnp, rdp);
> + rcu_preempt_boost_start_gp(rnp);
> + 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. */
> + }
> +
> + rnp = rcu_get_root(rsp);
> + raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> + /* force_quiescent_state() now OK. */
> + rsp->fqs_state = RCU_SIGNAL_INIT;
> + raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> + raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> + }
> + return 0;
> +}
> +
> +/*
> * Start a new RCU grace period if warranted, re-initializing the hierarchy
> * in preparation for detecting the next grace period. The caller must hold
> * the root node's ->lock, which is released before return. Hard irqs must
> @@ -1056,77 +1153,20 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
> struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
> struct rcu_node *rnp = rcu_get_root(rsp);
>
> - if (!rcu_scheduler_fully_active ||
> + if (!rsp->gp_kthread ||
> !cpu_needs_another_gp(rsp, rdp)) {
> /*
> - * Either the scheduler hasn't yet spawned the first
> - * non-idle task or this CPU does not need another
> - * grace period. Either way, don't start a new grace
> - * period.
> - */
> - raw_spin_unlock_irqrestore(&rnp->lock, flags);
> - return;
> - }
> -
> - if (rsp->fqs_active) {
> - /*
> - * This CPU needs a grace period, but force_quiescent_state()
> - * is running. Tell it to start one on this CPU's behalf.
> + * Either we have not yet spawned the grace-period
> + * task or this CPU does not need another grace period.
> + * Either way, don't start a new grace period.
> */
> - rsp->fqs_need_gp = 1;
> raw_spin_unlock_irqrestore(&rnp->lock, flags);
> return;
> }
>
> - /* Advance to a new grace period and initialize state. */
> - rsp->gpnum++;
> - trace_rcu_grace_period(rsp->name, rsp->gpnum, "start");
> - WARN_ON_ONCE(rsp->fqs_state == RCU_GP_INIT);
> - rsp->fqs_state = RCU_GP_INIT; /* Hold off 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. */
> -
> - /* Exclude any concurrent CPU-hotplug operations. */
> - raw_spin_lock(&rsp->onofflock); /* irqs already disabled. */
> -
> - /*
> - * Set the quiescent-state-needed bits in all the rcu_node
> - * structures for all currently online CPUs in breadth-first
> - * order, starting from the root rcu_node structure. This
> - * operation relies on the layout of the hierarchy within the
> - * rsp->node[] array. Note that other CPUs will access only
> - * the leaves of the hierarchy, which still 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. */
> - rcu_preempt_check_blocked_tasks(rnp);
> - rnp->qsmask = rnp->qsmaskinit;
> - rnp->gpnum = rsp->gpnum;
> - rnp->completed = rsp->completed;
> - if (rnp == rdp->mynode)
> - rcu_start_gp_per_cpu(rsp, rnp, rdp);
> - rcu_preempt_boost_start_gp(rnp);
> - 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. */
> - }
> -
> - rnp = rcu_get_root(rsp);
> - raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> - rsp->fqs_state = RCU_SIGNAL_INIT; /* force_quiescent_state now OK. */
> - raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> - raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> + rsp->gp_flags = 1;
> + raw_spin_unlock_irqrestore(&rnp->lock, flags);
> + wake_up(&rsp->gp_wq);
> }
>
> /*
> @@ -2627,6 +2667,28 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
> }
>
> /*
> + * Spawn the kthread that handles this RCU flavor's grace periods.
> + */
> +static int __init rcu_spawn_gp_kthread(void)
> +{
> + unsigned long flags;
> + struct rcu_node *rnp;
> + struct rcu_state *rsp;
> + struct task_struct *t;
> +
> + for_each_rcu_flavor(rsp) {
> + t = kthread_run(rcu_gp_kthread, rsp, rsp->name);
> + BUG_ON(IS_ERR(t));
> + rnp = rcu_get_root(rsp);
> + raw_spin_lock_irqsave(&rnp->lock, flags);
> + rsp->gp_kthread = t;
> + raw_spin_unlock_irqrestore(&rnp->lock, flags);
> + }
> + return 0;
> +}
> +early_initcall(rcu_spawn_gp_kthread);
> +
> +/*
> * This function is invoked towards the end of the scheduler's initialization
> * process. Before this is called, the idle task might contain
> * RCU read-side critical sections (during which time, this idle
> @@ -2727,6 +2789,7 @@ static void __init rcu_init_one(struct rcu_state *rsp,
> }
>
> rsp->rda = rda;
> + init_waitqueue_head(&rsp->gp_wq);
> rnp = rsp->level[rcu_num_lvls - 1];
> for_each_possible_cpu(i) {
> while (i > rnp->grphi)
> diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> index 4d29169..117a150 100644
> --- a/kernel/rcutree.h
> +++ b/kernel/rcutree.h
> @@ -385,6 +385,9 @@ struct rcu_state {
> u8 boost; /* Subject to priority boost. */
> unsigned long gpnum; /* Current gp number. */
> unsigned long completed; /* # of last completed gp. */
> + struct task_struct *gp_kthread; /* Task for grace periods. */
> + wait_queue_head_t gp_wq; /* Where GP task waits. */
> + int gp_flags; /* Commands for GP task. */
>
> /* End of fields guarded by root rcu_node's lock. */
>
> --
> 1.7.8
>
--
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