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Subject[PATCH RFC 8/9] RCU: Make RCU priority boosting consume less power
Work in progress, not for inclusion.

This patch modified the RCU priority booster to explicitly sleep when
there are no RCU readers in need of priority boosting. This should be
a power-consumption improvement over the one-second polling cycle in
the underlying RCU priority-boosting patch.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

include/linux/rcupreempt.h | 15 ++++++
kernel/rcupreempt.c | 102 ++++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 115 insertions(+), 2 deletions(-)

diff -urpNa -X dontdiff linux-2.6.22-G-boosttorture/include/linux/rcupreempt.h linux-2.6.22-H-boostsleep/include/linux/rcupreempt.h
--- linux-2.6.22-G-boosttorture/include/linux/rcupreempt.h 2007-08-24 11:24:59.000000000 -0700
+++ linux-2.6.22-H-boostsleep/include/linux/rcupreempt.h 2007-08-24 18:12:41.000000000 -0700
@@ -60,6 +60,21 @@ enum rcu_boost_state {

#define N_RCU_BOOST_STATE (RCU_BOOST_INVALID + 1)

+/*
+ * RCU-booster state with respect to sleeping. The RCU booster
+ * sleeps when no task has recently been seen sleeping in an RCU
+ * read-side critical section, and is awakened when a new sleeper
+ * appears.
+ */
+enum rcu_booster_state {
+ RCU_BOOSTER_ACTIVE = 0, /* RCU booster actively scanning. */
+ RCU_BOOSTER_DROWSY = 1, /* RCU booster is considering sleeping. */
+ RCU_BOOSTER_SLEEPING = 2, /* RCU booster is asleep. */
+ RCU_BOOSTER_INVALID = 3, /* For bogus state sightings. */
+};
+
+#define N_RCU_BOOSTER_STATE (RCU_BOOSTER_INVALID + 1)
+
#endif /* #ifdef CONFIG_PREEMPT_RCU_BOOST */

#define call_rcu_bh(head, rcu) call_rcu(head, rcu)
diff -urpNa -X dontdiff linux-2.6.22-G-boosttorture/kernel/rcupreempt.c linux-2.6.22-H-boostsleep/kernel/rcupreempt.c
--- linux-2.6.22-G-boosttorture/kernel/rcupreempt.c 2007-08-27 15:42:57.000000000 -0700
+++ linux-2.6.22-H-boostsleep/kernel/rcupreempt.c 2007-08-27 15:42:37.000000000 -0700
@@ -108,6 +108,7 @@ struct rcu_boost_dat {
unsigned long rbs_unboosted;
#ifdef CONFIG_PREEMPT_RCU_BOOST_STATS
unsigned long rbs_stats[N_RCU_BOOST_DAT_EVENTS][N_RCU_BOOST_STATE];
+ unsigned long rbs_qw_stats[N_RCU_BOOSTER_STATE];
#endif /* #ifdef CONFIG_PREEMPT_RCU_BOOST_STATS */
};
#define RCU_BOOST_ELEMENTS 4
@@ -115,6 +116,10 @@ struct rcu_boost_dat {
static int rcu_boost_idx = -1; /* invalid value for early RCU use. */
static DEFINE_PER_CPU(struct rcu_boost_dat, rcu_boost_dat[RCU_BOOST_ELEMENTS]);
static struct task_struct *rcu_boost_task;
+static DEFINE_SPINLOCK(rcu_boost_quiesce_lock);
+static enum rcu_booster_state rcu_booster_quiesce_state = RCU_BOOSTER_ACTIVE;
+static unsigned long rbs_qs_stats[2][N_RCU_BOOSTER_STATE];
+wait_queue_head_t rcu_booster_quiesce_wq;

#ifdef CONFIG_PREEMPT_RCU_BOOST_STATS

@@ -171,6 +176,15 @@ static char *rcu_boost_state_error[] = {
"? ?", /* unlock */
};

+/* Labels for RCU booster state printout. */
+
+static char *rcu_booster_state_label[] = {
+ "Active",
+ "Drowsy",
+ "Sleeping",
+ "???",
+};
+
/*
* Print out RCU booster task statistics at the specified interval.
*/
@@ -221,6 +235,14 @@ static void rcu_boost_dat_stat_print(voi
cpu)[i].rbs_stats[event][state];
}
}
+ for (state = 0; state < N_RCU_BOOSTER_STATE; state++) {
+ sum.rbs_qw_stats[state] = 0;
+ for_each_possible_cpu(cpu)
+ for (i = 0; i < RCU_BOOST_ELEMENTS; i++)
+ sum.rbs_qw_stats[state] +=
+ per_cpu(rcu_boost_dat,
+ cpu)[i].rbs_qw_stats[state];
+ }

/* Print them out! */

@@ -240,6 +262,24 @@ static void rcu_boost_dat_stat_print(voi
rcu_boost_state_event[event], buf);
}

+ printk(KERN_INFO "RCU booster state: %s\n",
+ rcu_booster_quiesce_state >= 0 &&
+ rcu_booster_quiesce_state < N_RCU_BOOSTER_STATE
+ ? rcu_booster_state_label[rcu_booster_quiesce_state]
+ : "???");
+ i = 0;
+ for (state = 0; state < N_RCU_BOOSTER_STATE; state++)
+ i += sprintf(&buf[i], " %ld", rbs_qs_stats[0][state]);
+ printk(KERN_INFO "No tasks found: %s\n", buf);
+ i = 0;
+ for (state = 0; state < N_RCU_BOOSTER_STATE; state++)
+ i += sprintf(&buf[i], " %ld", rbs_qs_stats[1][state]);
+ printk(KERN_INFO "Tasks found: %s\n", buf);
+ i = 0;
+ for (state = 0; state < N_RCU_BOOSTER_STATE; state++)
+ i += sprintf(&buf[i], " %ld", sum.rbs_qw_stats[state]);
+ printk(KERN_INFO "Awaken opportunities: %s\n", buf);
+
/* Go away and don't come back for awhile. */

lastprint = xtime.tv_sec;
@@ -293,6 +333,8 @@ static void init_rcu_boost_early(void)
for (j = 0; j < N_RCU_BOOST_DAT_EVENTS; j++)
for (k = 0; k < N_RCU_BOOST_STATE; k++)
rbdp[i].rbs_stats[j][k] = 0;
+ for (j = 0; j < N_RCU_BOOSTER_STATE; j++)
+ rbdp[i].rbs_qw_stats[j] = 0;
}
#endif /* #ifdef CONFIG_PREEMPT_RCU_BOOST_STATS */
}
@@ -378,10 +420,11 @@ static void rcu_unboost_prio(struct task
/*
* Boost all of the RCU-reader tasks on the specified list.
*/
-static void rcu_boost_one_reader_list(struct rcu_boost_dat *rbdp)
+static int rcu_boost_one_reader_list(struct rcu_boost_dat *rbdp)
{
LIST_HEAD(list);
unsigned long flags;
+ int retval = 0;
struct task_struct *taskp;

/*
@@ -397,6 +440,7 @@ static void rcu_boost_one_reader_list(st
list_splice_init(&rbdp->rbs_toboost, &list);
list_splice_init(&rbdp->rbs_boosted, &list);
while (!list_empty(&list)) {
+ retval = 1;

/*
* Pause for a bit before boosting each task.
@@ -438,6 +482,36 @@ static void rcu_boost_one_reader_list(st
list_add_tail(&taskp->rcub_entry, &rbdp->rbs_boosted);
}
spin_unlock_irqrestore(&rbdp->rbs_lock, flags);
+ return retval;
+}
+
+/*
+ * Examine state to see if it is time to sleep.
+ */
+static void rcu_booster_try_sleep(int yo)
+{
+ spin_lock(&rcu_boost_quiesce_lock);
+ if (rcu_booster_quiesce_state < 0 ||
+ rcu_booster_quiesce_state >= N_RCU_BOOSTER_STATE)
+ rcu_booster_quiesce_state = RCU_BOOST_INVALID;
+ rbs_qs_stats[yo != 0][rcu_booster_quiesce_state]++;
+ if (yo != 0) {
+ rcu_booster_quiesce_state = RCU_BOOSTER_ACTIVE;
+ } else {
+ if (rcu_booster_quiesce_state == RCU_BOOSTER_ACTIVE) {
+ rcu_booster_quiesce_state = RCU_BOOSTER_DROWSY;
+ } else if (rcu_booster_quiesce_state == RCU_BOOSTER_DROWSY) {
+ rcu_booster_quiesce_state = RCU_BOOSTER_SLEEPING;
+ spin_unlock(&rcu_boost_quiesce_lock);
+ __wait_event(rcu_booster_quiesce_wq,
+ rcu_booster_quiesce_state ==
+ RCU_BOOSTER_ACTIVE);
+ spin_lock(&rcu_boost_quiesce_lock);
+ } else {
+ rcu_booster_quiesce_state = RCU_BOOSTER_ACTIVE;
+ }
+ }
+ spin_unlock(&rcu_boost_quiesce_lock);
}

/*
@@ -448,15 +522,21 @@ static int rcu_booster(void *arg)
{
int cpu;
struct sched_param sp = { .sched_priority = PREEMPT_RCU_BOOSTER_PRIO, };
+ int yo = 0;

sched_setscheduler(current, SCHED_RR, &sp);
current->flags |= PF_NOFREEZE;
+ init_waitqueue_head(&rcu_booster_quiesce_wq);

do {

/* Advance the lists of tasks. */

rcu_boost_idx = (rcu_boost_idx + 1) % RCU_BOOST_ELEMENTS;
+ if (rcu_boost_idx == 0) {
+ rcu_booster_try_sleep(yo);
+ yo = 0;
+ }
for_each_possible_cpu(cpu) {

/*
@@ -469,7 +549,7 @@ static int rcu_booster(void *arg)
* nothing.
*/

- rcu_boost_one_reader_list(rcu_rbd_boosting(cpu));
+ yo += rcu_boost_one_reader_list(rcu_rbd_boosting(cpu));

/*
* Large SMP systems may need to sleep sometimes
@@ -511,6 +591,23 @@ void init_rcu_boost_late(void)
}

/*
+ * Awaken the RCU priority booster if neecessary.
+ */
+static void rcu_preempt_wake(struct rcu_boost_dat *rbdp)
+{
+ spin_lock(&rcu_boost_quiesce_lock);
+ if (rcu_booster_quiesce_state >= N_RCU_BOOSTER_STATE)
+ rcu_booster_quiesce_state = RCU_BOOSTER_INVALID;
+ rbdp->rbs_qw_stats[rcu_booster_quiesce_state]++;
+ if (rcu_booster_quiesce_state == RCU_BOOSTER_SLEEPING) {
+ rcu_booster_quiesce_state = RCU_BOOSTER_ACTIVE;
+ wake_up(&rcu_booster_quiesce_wq);
+ } else if (rcu_booster_quiesce_state != RCU_BOOSTER_ACTIVE)
+ rcu_booster_quiesce_state = RCU_BOOSTER_ACTIVE;
+ spin_unlock(&rcu_boost_quiesce_lock);
+}
+
+/*
* Update task's RCU-boost state to reflect blocking in RCU read-side
* critical section, so that the RCU-boost task can find it in case it
* later needs its priority boosted.
@@ -532,6 +629,7 @@ void __rcu_preempt_boost(void)
}
spin_lock(&rbdp->rbs_lock);
rbdp->rbs_blocked++;
+ rcu_preempt_wake(rbdp);

/*
* Update state. We hold the lock and aren't yet on the list,
-
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