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Subject[PATCH RFC tip/core/rcu 2/4] rcu: Make RCU_FAST_NO_HZ use timer rather than hrtimer
Date
From: "Paul E. McKenney" <paul.mckenney@linaro.org>

The RCU_FAST_NO_HZ facility uses an hrtimer to wake up a CPU when
it is allowed to go into dyntick-idle mode, which is almost always
cancelled soon after. This is not what hrtimers are good at, so
this commit switches to the timer wheel.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree_plugin.h | 40 ++++++++++++----------------------------
1 files changed, 12 insertions(+), 28 deletions(-)

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 1e561ab..0f007b3 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1980,9 +1980,7 @@ static void rcu_prepare_for_idle(int cpu)

static DEFINE_PER_CPU(int, rcu_dyntick_drain);
static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
-static DEFINE_PER_CPU(struct hrtimer, rcu_idle_gp_timer);
-static ktime_t rcu_idle_gp_wait; /* If some non-lazy callbacks. */
-static ktime_t rcu_idle_lazy_gp_wait; /* If only lazy callbacks. */
+static DEFINE_PER_CPU(struct timer_list, rcu_idle_gp_timer);

/*
* Allow the CPU to enter dyntick-idle mode if either: (1) There are no
@@ -2051,10 +2049,9 @@ static bool rcu_cpu_has_nonlazy_callbacks(int cpu)
* real work is done upon re-entry to idle, or by the next scheduling-clock
* interrupt should idle not be re-entered.
*/
-static enum hrtimer_restart rcu_idle_gp_timer_func(struct hrtimer *hrtp)
+static void rcu_idle_gp_timer_func(unsigned long unused)
{
trace_rcu_prep_idle("Timer");
- return HRTIMER_NORESTART;
}

/*
@@ -2062,19 +2059,8 @@ static enum hrtimer_restart rcu_idle_gp_timer_func(struct hrtimer *hrtp)
*/
static void rcu_prepare_for_idle_init(int cpu)
{
- static int firsttime = 1;
- struct hrtimer *hrtp = &per_cpu(rcu_idle_gp_timer, cpu);
-
- hrtimer_init(hrtp, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
- hrtp->function = rcu_idle_gp_timer_func;
- if (firsttime) {
- unsigned int upj = jiffies_to_usecs(RCU_IDLE_GP_DELAY);
-
- rcu_idle_gp_wait = ns_to_ktime(upj * (u64)1000);
- upj = jiffies_to_usecs(RCU_IDLE_LAZY_GP_DELAY);
- rcu_idle_lazy_gp_wait = ns_to_ktime(upj * (u64)1000);
- firsttime = 0;
- }
+ setup_timer(&per_cpu(rcu_idle_gp_timer, cpu),
+ rcu_idle_gp_timer_func, 0);
}

/*
@@ -2084,7 +2070,7 @@ static void rcu_prepare_for_idle_init(int cpu)
*/
static void rcu_cleanup_after_idle(int cpu)
{
- hrtimer_cancel(&per_cpu(rcu_idle_gp_timer, cpu));
+ del_timer(&per_cpu(rcu_idle_gp_timer, cpu));
trace_rcu_prep_idle("Cleanup after idle");
}

@@ -2141,11 +2127,11 @@ static void rcu_prepare_for_idle(int cpu)
per_cpu(rcu_dyntick_drain, cpu) = 0;
per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
if (rcu_cpu_has_nonlazy_callbacks(cpu))
- hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
- rcu_idle_gp_wait, HRTIMER_MODE_REL);
+ mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
+ jiffies + RCU_IDLE_GP_DELAY);
else
- hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
- rcu_idle_lazy_gp_wait, HRTIMER_MODE_REL);
+ mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
+ jiffies + RCU_IDLE_LAZY_GP_DELAY);
return; /* Nothing more to do immediately. */
} else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
/* We have hit the limit, so time to give up. */
@@ -2193,14 +2179,12 @@ static void rcu_prepare_for_idle(int cpu)

static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
{
- struct hrtimer *hrtp = &per_cpu(rcu_idle_gp_timer, cpu);
+ struct timer_list *tltp = &per_cpu(rcu_idle_gp_timer, cpu);

- sprintf(cp, "drain=%d %c timer=%lld",
+ sprintf(cp, "drain=%d %c timer=%lu",
per_cpu(rcu_dyntick_drain, cpu),
per_cpu(rcu_dyntick_holdoff, cpu) == jiffies ? 'H' : '.',
- hrtimer_active(hrtp)
- ? ktime_to_us(hrtimer_get_remaining(hrtp))
- : -1);
+ timer_pending(tltp) ? tltp->expires - jiffies : -1);
}

#else /* #ifdef CONFIG_RCU_FAST_NO_HZ */
--
1.7.8


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