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Subject[PATCH tip/core/rcu 09/23] rcu: Move quiescent-state forcing into kthread
Date
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

As the first step towards allowing quiescent-state forcing to be
preemptible, this commit moves RCU quiescent-state forcing into the
same kthread that is now used to initialize and clean up after grace
periods. This is yet another step towards keeping scheduling
latency down to a dull roar.

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>
---
kernel/rcutree.c | 198 ++++++++++++++++++-----------------------------
kernel/rcutree.h | 6 +-
kernel/rcutree_plugin.h | 8 +-
3 files changed, 82 insertions(+), 130 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index c2c036f..79c2c28 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -72,7 +72,6 @@ static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
.orphan_nxttail = &sname##_state.orphan_nxtlist, \
.orphan_donetail = &sname##_state.orphan_donelist, \
.barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
- .fqslock = __RAW_SPIN_LOCK_UNLOCKED(&sname##_state.fqslock), \
.name = #sname, \
}

@@ -226,7 +225,8 @@ int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
module_param(rcu_cpu_stall_suppress, int, 0644);
module_param(rcu_cpu_stall_timeout, int, 0644);

-static void force_quiescent_state(struct rcu_state *rsp, int relaxed);
+static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *));
+static void force_quiescent_state(struct rcu_state *rsp);
static int rcu_pending(int cpu);

/*
@@ -252,7 +252,7 @@ EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
*/
void rcu_bh_force_quiescent_state(void)
{
- force_quiescent_state(&rcu_bh_state, 0);
+ force_quiescent_state(&rcu_bh_state);
}
EXPORT_SYMBOL_GPL(rcu_bh_force_quiescent_state);

@@ -286,7 +286,7 @@ EXPORT_SYMBOL_GPL(rcutorture_record_progress);
*/
void rcu_sched_force_quiescent_state(void)
{
- force_quiescent_state(&rcu_sched_state, 0);
+ force_quiescent_state(&rcu_sched_state);
}
EXPORT_SYMBOL_GPL(rcu_sched_force_quiescent_state);

@@ -782,11 +782,11 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
else if (!trigger_all_cpu_backtrace())
dump_stack();

- /* If so configured, complain about tasks blocking the grace period. */
+ /* Complain about tasks blocking the grace period. */

rcu_print_detail_task_stall(rsp);

- force_quiescent_state(rsp, 0); /* Kick them all. */
+ force_quiescent_state(rsp); /* Kick them all. */
}

static void print_cpu_stall(struct rcu_state *rsp)
@@ -1049,7 +1049,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
struct rcu_node *rnp = rcu_get_root(rsp);

raw_spin_lock_irqsave(&rnp->lock, flags);
- rsp->gp_flags = 0;
+ rsp->gp_flags = 0; /* Clear all flags: New grace period. */

if (rcu_gp_in_progress(rsp)) {
/* Grace period already in progress, don't start another. */
@@ -1057,22 +1057,9 @@ static int rcu_gp_init(struct rcu_state *rsp)
return 0;
}

- 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);
- return 0;
- }
-
/* 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_irqrestore(&rnp->lock, flags);

@@ -1109,20 +1096,41 @@ static int rcu_gp_init(struct rcu_state *rsp)
cond_resched();
}

- rnp = rcu_get_root(rsp);
- raw_spin_lock_irqsave(&rnp->lock, flags);
- /* force_quiescent_state() now OK. */
- rsp->fqs_state = RCU_SIGNAL_INIT;
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
put_online_cpus();
-
return 1;
}

/*
+ * Do one round of quiescent-state forcing.
+ */
+int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
+{
+ unsigned long flags;
+ int fqs_state = fqs_state_in;
+ struct rcu_node *rnp = rcu_get_root(rsp);
+
+ rsp->n_force_qs++;
+ if (fqs_state == RCU_SAVE_DYNTICK) {
+ /* Collect dyntick-idle snapshots. */
+ force_qs_rnp(rsp, dyntick_save_progress_counter);
+ fqs_state = RCU_FORCE_QS;
+ } else {
+ /* Handle dyntick-idle and offline CPUs. */
+ force_qs_rnp(rsp, rcu_implicit_dynticks_qs);
+ }
+ /* Clear flag to prevent immediate re-entry. */
+ if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
+ raw_spin_lock_irqsave(&rnp->lock, flags);
+ rsp->gp_flags &= ~RCU_GP_FLAG_FQS;
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
+ }
+ return fqs_state;
+}
+
+/*
* Clean up after the old grace period.
*/
-static int rcu_gp_cleanup(struct rcu_state *rsp)
+static void rcu_gp_cleanup(struct rcu_state *rsp)
{
unsigned long flags;
unsigned long gp_duration;
@@ -1176,7 +1184,6 @@ static int rcu_gp_cleanup(struct rcu_state *rsp)
if (cpu_needs_another_gp(rsp, rdp))
rsp->gp_flags = 1;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
- return 1;
}

/*
@@ -1184,6 +1191,8 @@ static int rcu_gp_cleanup(struct rcu_state *rsp)
*/
static int rcu_gp_kthread(void *arg)
{
+ int fqs_state;
+ int ret;
struct rcu_state *rsp = arg;
struct rcu_node *rnp = rcu_get_root(rsp);

@@ -1191,25 +1200,43 @@ static int rcu_gp_kthread(void *arg)

/* Handle grace-period start. */
for (;;) {
- wait_event_interruptible(rsp->gp_wq, rsp->gp_flags);
- if (rsp->gp_flags && rcu_gp_init(rsp))
+ wait_event_interruptible(rsp->gp_wq,
+ rsp->gp_flags &
+ RCU_GP_FLAG_INIT);
+ if ((rsp->gp_flags & RCU_GP_FLAG_INIT) &&
+ rcu_gp_init(rsp))
break;
cond_resched();
flush_signals(current);
}

- /* Handle grace-period end. */
+ /* Handle quiescent-state forcing. */
+ fqs_state = RCU_SAVE_DYNTICK;
for (;;) {
- wait_event_interruptible(rsp->gp_wq,
- !ACCESS_ONCE(rnp->qsmask) &&
- !rcu_preempt_blocked_readers_cgp(rnp));
+ rsp->jiffies_force_qs = jiffies +
+ RCU_JIFFIES_TILL_FORCE_QS;
+ ret = wait_event_interruptible_timeout(rsp->gp_wq,
+ (rsp->gp_flags & RCU_GP_FLAG_FQS) ||
+ (!ACCESS_ONCE(rnp->qsmask) &&
+ !rcu_preempt_blocked_readers_cgp(rnp)),
+ RCU_JIFFIES_TILL_FORCE_QS);
+ /* If grace period done, leave loop. */
if (!ACCESS_ONCE(rnp->qsmask) &&
- !rcu_preempt_blocked_readers_cgp(rnp) &&
- rcu_gp_cleanup(rsp))
+ !rcu_preempt_blocked_readers_cgp(rnp))
break;
- cond_resched();
- flush_signals(current);
+ /* If time for quiescent-state forcing, do it. */
+ if (ret == 0 || (rsp->gp_flags & RCU_GP_FLAG_FQS)) {
+ fqs_state = rcu_gp_fqs(rsp, fqs_state);
+ cond_resched();
+ } else {
+ /* Deal with stray signal. */
+ cond_resched();
+ flush_signals(current);
+ }
}
+
+ /* Handle grace-period end. */
+ rcu_gp_cleanup(rsp);
}
return 0;
}
@@ -1793,72 +1820,20 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
* Force quiescent states on reluctant CPUs, and also detect which
* CPUs are in dyntick-idle mode.
*/
-static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
+static void force_quiescent_state(struct rcu_state *rsp)
{
unsigned long flags;
struct rcu_node *rnp = rcu_get_root(rsp);

- trace_rcu_utilization("Start fqs");
- if (!rcu_gp_in_progress(rsp)) {
- trace_rcu_utilization("End fqs");
- return; /* No grace period in progress, nothing to force. */
- }
- if (!raw_spin_trylock_irqsave(&rsp->fqslock, flags)) {
+ if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS)
+ return; /* Someone beat us to it. */
+ if (!raw_spin_trylock_irqsave(&rnp->lock, flags)) {
rsp->n_force_qs_lh++; /* Inexact, can lose counts. Tough! */
- trace_rcu_utilization("End fqs");
- return; /* Someone else is already on the job. */
- }
- if (relaxed && ULONG_CMP_GE(rsp->jiffies_force_qs, jiffies))
- goto unlock_fqs_ret; /* no emergency and done recently. */
- rsp->n_force_qs++;
- raw_spin_lock(&rnp->lock); /* irqs already disabled */
- rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
- if(!rcu_gp_in_progress(rsp)) {
- rsp->n_force_qs_ngp++;
- raw_spin_unlock(&rnp->lock); /* irqs remain disabled */
- goto unlock_fqs_ret; /* no GP in progress, time updated. */
- }
- rsp->fqs_active = 1;
- switch (rsp->fqs_state) {
- case RCU_GP_IDLE:
- case RCU_GP_INIT:
-
- break; /* grace period idle or initializing, ignore. */
-
- case RCU_SAVE_DYNTICK:
-
- raw_spin_unlock(&rnp->lock); /* irqs remain disabled */
-
- /* Record dyntick-idle state. */
- force_qs_rnp(rsp, dyntick_save_progress_counter);
- raw_spin_lock(&rnp->lock); /* irqs already disabled */
- if (rcu_gp_in_progress(rsp))
- rsp->fqs_state = RCU_FORCE_QS;
- break;
-
- case RCU_FORCE_QS:
-
- /* Check dyntick-idle state, send IPI to laggarts. */
- raw_spin_unlock(&rnp->lock); /* irqs remain disabled */
- force_qs_rnp(rsp, rcu_implicit_dynticks_qs);
-
- /* Leave state in case more forcing is required. */
-
- raw_spin_lock(&rnp->lock); /* irqs already disabled */
- break;
- }
- rsp->fqs_active = 0;
- if (rsp->fqs_need_gp) {
- raw_spin_unlock(&rsp->fqslock); /* irqs remain disabled */
- rsp->fqs_need_gp = 0;
- rcu_start_gp(rsp, flags); /* releases rnp->lock */
- trace_rcu_utilization("End fqs");
return;
}
- raw_spin_unlock(&rnp->lock); /* irqs remain disabled */
-unlock_fqs_ret:
- raw_spin_unlock_irqrestore(&rsp->fqslock, flags);
- trace_rcu_utilization("End fqs");
+ rsp->gp_flags |= RCU_GP_FLAG_FQS;
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
+ wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */
}

/*
@@ -1875,13 +1850,6 @@ __rcu_process_callbacks(struct rcu_state *rsp)
WARN_ON_ONCE(rdp->beenonline == 0);

/*
- * If an RCU GP has gone long enough, go check for dyntick
- * idle CPUs and, if needed, send resched IPIs.
- */
- if (ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs), jiffies))
- force_quiescent_state(rsp, 1);
-
- /*
* Advance callbacks in response to end of earlier grace
* period that some other CPU ended.
*/
@@ -1981,12 +1949,11 @@ static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
rdp->blimit = LONG_MAX;
if (rsp->n_force_qs == rdp->n_force_qs_snap &&
*rdp->nxttail[RCU_DONE_TAIL] != head)
- force_quiescent_state(rsp, 0);
+ force_quiescent_state(rsp);
rdp->n_force_qs_snap = rsp->n_force_qs;
rdp->qlen_last_fqs_check = rdp->qlen;
}
- } else if (ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs), jiffies))
- force_quiescent_state(rsp, 1);
+ }
}

static void
@@ -2267,17 +2234,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
/* Is the RCU core waiting for a quiescent state from this CPU? */
if (rcu_scheduler_fully_active &&
rdp->qs_pending && !rdp->passed_quiesce) {
-
- /*
- * 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->preemptible &&
- ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs) - 1,
- jiffies))
- set_need_resched();
} else if (rdp->qs_pending && rdp->passed_quiesce) {
rdp->n_rp_report_qs++;
return 1;
@@ -2307,13 +2264,6 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
return 1;
}

- /* Has an RCU GP gone long enough to send resched IPIs &c? */
- if (rcu_gp_in_progress(rsp) &&
- ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs), jiffies)) {
- rdp->n_rp_need_fqs++;
- return 1;
- }
-
/* nothing to do */
rdp->n_rp_need_nothing++;
return 0;
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 5d92b80..1f26b1f 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -413,8 +413,6 @@ struct rcu_state {
struct completion barrier_completion; /* Wake at barrier end. */
unsigned long n_barrier_done; /* ++ at start and end of */
/* _rcu_barrier(). */
- raw_spinlock_t fqslock; /* Only one task forcing */
- /* quiescent states. */
unsigned long jiffies_force_qs; /* Time at which to invoke */
/* force_quiescent_state(). */
unsigned long n_force_qs; /* Number of calls to */
@@ -433,6 +431,10 @@ struct rcu_state {
struct list_head flavors; /* List of RCU flavors. */
};

+/* Values for rcu_state structure's gp_flags field. */
+#define RCU_GP_FLAG_INIT 0x1 /* Need grace-period initialization. */
+#define RCU_GP_FLAG_FQS 0x2 /* Need grace-period quiescent-state forcing. */
+
extern struct list_head rcu_struct_flavors;
#define for_each_rcu_flavor(rsp) \
list_for_each_entry((rsp), &rcu_struct_flavors, flavors)
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index bac8cc1..befb0b2 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -119,7 +119,7 @@ EXPORT_SYMBOL_GPL(rcu_batches_completed);
*/
void rcu_force_quiescent_state(void)
{
- force_quiescent_state(&rcu_preempt_state, 0);
+ force_quiescent_state(&rcu_preempt_state);
}
EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);

@@ -2076,16 +2076,16 @@ static void rcu_prepare_for_idle(int cpu)
#ifdef CONFIG_TREE_PREEMPT_RCU
if (per_cpu(rcu_preempt_data, cpu).nxtlist) {
rcu_preempt_qs(cpu);
- force_quiescent_state(&rcu_preempt_state, 0);
+ force_quiescent_state(&rcu_preempt_state);
}
#endif /* #ifdef CONFIG_TREE_PREEMPT_RCU */
if (per_cpu(rcu_sched_data, cpu).nxtlist) {
rcu_sched_qs(cpu);
- force_quiescent_state(&rcu_sched_state, 0);
+ force_quiescent_state(&rcu_sched_state);
}
if (per_cpu(rcu_bh_data, cpu).nxtlist) {
rcu_bh_qs(cpu);
- force_quiescent_state(&rcu_bh_state, 0);
+ force_quiescent_state(&rcu_bh_state);
}

/*
--
1.7.8


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