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rcu: Move RCU grace-period initialization into a kthread
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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.

Also change raw_spin_lock_irqsave() to raw_spin_lock_irq() as suggested
by Peter Zijlstra in review comments.

Reported-by: Mike Galbraith <mgalbraith@suse.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
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Paul E. McKenney committed Sep 23, 2012
1 parent a10d206 commit b3dbec7
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Showing 2 changed files with 129 additions and 64 deletions.
190 changes: 126 additions & 64 deletions kernel/rcutree.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1041,6 +1041,102 @@ rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
__note_new_gpnum(rsp, rnp, rdp);
}

/*
* Body of kthread that handles grace periods.
*/
static int rcu_gp_kthread(void *arg)
{
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_irq(&rnp->lock);
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_irq(&rnp->lock);
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_irq(&rnp->lock);
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_irq(&rsp->onofflock);
}
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
Expand All @@ -1058,77 +1154,20 @@ 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.
* 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.
*/
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.
*/
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);
}

/*
Expand Down Expand Up @@ -2628,6 +2667,28 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
return NOTIFY_OK;
}

/*
* 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
Expand Down Expand Up @@ -2729,6 +2790,7 @@ 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)
Expand Down
3 changes: 3 additions & 0 deletions kernel/rcutree.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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. */

Expand Down

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