Commit aa9b1630 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Paul E. McKenney

rcu: Precompute RCU_FAST_NO_HZ timer offsets

When a CPU is entering dyntick-idle mode, tick_nohz_stop_sched_tick()
calls rcu_needs_cpu() see if RCU needs that CPU, and, if not, computes the
next wakeup time based on the timer wheels.  Only later, when actually
entering the idle loop, rcu_prepare_for_idle() will be invoked.  In some
cases, rcu_prepare_for_idle() will post timers to wake the CPU back up.
But all for naught: The next wakeup time for the CPU has already been
computed, and posting a timer afterwards does not force that wakeup
time to be recomputed.  This means that rcu_prepare_for_idle()'s have
no effect.

This is not a problem on a busy system because something else will wake
up the CPU soon enough.  However, on lightly loaded systems, the CPU
might stay asleep for a considerable length of time.  If that CPU has
a callback that the rest of the system is waiting on, the system might
run very slowly or (in theory) even hang.

This commit avoids this problem by having rcu_needs_cpu() give
tick_nohz_stop_sched_tick() an estimate of when RCU will need the CPU
to wake back up, which tick_nohz_stop_sched_tick() takes into account
when programming the CPU's wakeup time.  An alternative approach is
for rcu_prepare_for_idle() to use hrtimers instead of normal timers,
but timers are much more efficient than are hrtimers for frequently
and repeatedly posting and cancelling a given timer, which is exactly
what RCU_FAST_NO_HZ does.
Reported-by: default avatarPascal Chapperon <pascal.chapperon@wanadoo.fr>
Reported-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: default avatarPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
Tested-by: default avatarPascal Chapperon <pascal.chapperon@wanadoo.fr>
parent 5955f7ee
......@@ -87,8 +87,9 @@ static inline void kfree_call_rcu(struct rcu_head *head,
#ifdef CONFIG_TINY_RCU
static inline int rcu_needs_cpu(int cpu)
static inline int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
{
*delta_jiffies = ULONG_MAX;
return 0;
}
......@@ -96,8 +97,9 @@ static inline int rcu_needs_cpu(int cpu)
int rcu_preempt_needs_cpu(void);
static inline int rcu_needs_cpu(int cpu)
static inline int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
{
*delta_jiffies = ULONG_MAX;
return rcu_preempt_needs_cpu();
}
......
......@@ -32,7 +32,7 @@
extern void rcu_init(void);
extern void rcu_note_context_switch(int cpu);
extern int rcu_needs_cpu(int cpu);
extern int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies);
extern void rcu_cpu_stall_reset(void);
/*
......
......@@ -1886,8 +1886,9 @@ static void __cpuinit rcu_prepare_kthreads(int cpu)
* Because we not have RCU_FAST_NO_HZ, just check whether this CPU needs
* any flavor of RCU.
*/
int rcu_needs_cpu(int cpu)
int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
{
*delta_jiffies = ULONG_MAX;
return rcu_cpu_has_callbacks(cpu);
}
......@@ -1962,28 +1963,6 @@ static void rcu_idle_count_callbacks_posted(void)
#define RCU_IDLE_GP_DELAY 6 /* Roughly one grace period. */
#define RCU_IDLE_LAZY_GP_DELAY (6 * HZ) /* Roughly six seconds. */
/*
* Allow the CPU to enter dyntick-idle mode if either: (1) There are no
* callbacks on this CPU, (2) this CPU has not yet attempted to enter
* dyntick-idle mode, or (3) this CPU is in the process of attempting to
* enter dyntick-idle mode. Otherwise, if we have recently tried and failed
* to enter dyntick-idle mode, we refuse to try to enter it. After all,
* it is better to incur scheduling-clock interrupts than to spin
* continuously for the same time duration!
*/
int rcu_needs_cpu(int cpu)
{
struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
/* Flag a new idle sojourn to the idle-entry state machine. */
rdtp->idle_first_pass = 1;
/* If no callbacks, RCU doesn't need the CPU. */
if (!rcu_cpu_has_callbacks(cpu))
return 0;
/* Otherwise, RCU needs the CPU only if it recently tried and failed. */
return rdtp->dyntick_holdoff == jiffies;
}
/*
* Does the specified flavor of RCU have non-lazy callbacks pending on
* the specified CPU? Both RCU flavor and CPU are specified by the
......@@ -2026,6 +2005,47 @@ static bool rcu_cpu_has_nonlazy_callbacks(int cpu)
rcu_preempt_cpu_has_nonlazy_callbacks(cpu);
}
/*
* Allow the CPU to enter dyntick-idle mode if either: (1) There are no
* callbacks on this CPU, (2) this CPU has not yet attempted to enter
* dyntick-idle mode, or (3) this CPU is in the process of attempting to
* enter dyntick-idle mode. Otherwise, if we have recently tried and failed
* to enter dyntick-idle mode, we refuse to try to enter it. After all,
* it is better to incur scheduling-clock interrupts than to spin
* continuously for the same time duration!
*
* The delta_jiffies argument is used to store the time when RCU is
* going to need the CPU again if it still has callbacks. The reason
* for this is that rcu_prepare_for_idle() might need to post a timer,
* but if so, it will do so after tick_nohz_stop_sched_tick() has set
* the wakeup time for this CPU. This means that RCU's timer can be
* delayed until the wakeup time, which defeats the purpose of posting
* a timer.
*/
int rcu_needs_cpu(int cpu, unsigned long *delta_jiffies)
{
struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
/* Flag a new idle sojourn to the idle-entry state machine. */
rdtp->idle_first_pass = 1;
/* If no callbacks, RCU doesn't need the CPU. */
if (!rcu_cpu_has_callbacks(cpu)) {
*delta_jiffies = ULONG_MAX;
return 0;
}
if (rdtp->dyntick_holdoff == jiffies) {
/* RCU recently tried and failed, so don't try again. */
*delta_jiffies = 1;
return 1;
}
/* Set up for the possibility that RCU will post a timer. */
if (rcu_cpu_has_nonlazy_callbacks(cpu))
*delta_jiffies = RCU_IDLE_GP_DELAY;
else
*delta_jiffies = RCU_IDLE_LAZY_GP_DELAY;
return 0;
}
/*
* Handler for smp_call_function_single(). The only point of this
* handler is to wake the CPU up, so the handler does only tracing.
......
......@@ -274,6 +274,7 @@ EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);
static void tick_nohz_stop_sched_tick(struct tick_sched *ts)
{
unsigned long seq, last_jiffies, next_jiffies, delta_jiffies;
unsigned long rcu_delta_jiffies;
ktime_t last_update, expires, now;
struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
u64 time_delta;
......@@ -322,7 +323,7 @@ static void tick_nohz_stop_sched_tick(struct tick_sched *ts)
time_delta = timekeeping_max_deferment();
} while (read_seqretry(&xtime_lock, seq));
if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) ||
if (rcu_needs_cpu(cpu, &rcu_delta_jiffies) || printk_needs_cpu(cpu) ||
arch_needs_cpu(cpu)) {
next_jiffies = last_jiffies + 1;
delta_jiffies = 1;
......@@ -330,6 +331,10 @@ static void tick_nohz_stop_sched_tick(struct tick_sched *ts)
/* Get the next timer wheel timer */
next_jiffies = get_next_timer_interrupt(last_jiffies);
delta_jiffies = next_jiffies - last_jiffies;
if (rcu_delta_jiffies < delta_jiffies) {
next_jiffies = last_jiffies + rcu_delta_jiffies;
delta_jiffies = rcu_delta_jiffies;
}
}
/*
* Do not stop the tick, if we are only one off
......
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