Re: [PATCH 08/11 v2] nohz: Allow rcu extended quiescent statehandling seperately from tick stop

From: Paul E. McKenney
Date: Thu Oct 13 2011 - 02:58:26 EST

On Sat, Oct 08, 2011 at 04:01:00PM +0200, Frederic Weisbecker wrote:
> It is assumed that rcu won't be used once we switch to tickless
> mode and until we restart the tick. However this is not always
> true, as in x86-64 where we dereference the idle notifiers after
> the tick is stopped.
>
> To prepare for fixing this, add two new APIs:
> tick_nohz_idle_enter_norcu() and tick_nohz_idle_exit_norcu().
>
> If no use of RCU is made in the idle loop between
> tick_nohz_enter_idle() and tick_nohz_exit_idle() calls, the arch
> must instead call the new *_norcu() version such that the arch doesn't
> need to call rcu_idle_enter() and rcu_idle_exit().
>
> Otherwise the arch must call tick_nohz_enter_idle() and
> tick_nohz_exit_idle() and also call explicitly:
>
> - rcu_idle_enter() after its last use of RCU before the CPU is put
> to sleep.
> - rcu_idle_exit() before the first use of RCU after the CPU is woken
> up.

Thank you, Frederic! I have queued this to replace the earlier
version. The set is available on branch rcu/dyntick of

https://github.com/paulmckrcu/linux

Thanx, Paul

> Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Mike Frysinger <vapier@xxxxxxxxxx>
> Cc: Guan Xuetao <gxt@xxxxxxxxxxxxxxx>
> Cc: David Miller <davem@xxxxxxxxxxxxx>
> Cc: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> Cc: Hans-Christian Egtvedt <hans-christian.egtvedt@xxxxxxxxx>
> Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
> Cc: Paul Mackerras <paulus@xxxxxxxxx>
> Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> Cc: Paul Mundt <lethal@xxxxxxxxxxxx>
> ---
> arch/arm/kernel/process.c | 4 +-
> arch/avr32/kernel/process.c | 4 +-
> arch/blackfin/kernel/process.c | 4 +-
> arch/microblaze/kernel/process.c | 4 +-
> arch/mips/kernel/process.c | 4 +-
> arch/openrisc/kernel/idle.c | 4 +-
> arch/powerpc/kernel/idle.c | 4 +-
> arch/powerpc/platforms/iseries/setup.c | 8 +++---
> arch/s390/kernel/process.c | 4 +-
> arch/sh/kernel/idle.c | 4 +-
> arch/sparc/kernel/process_64.c | 4 +-
> arch/tile/kernel/process.c | 4 +-
> arch/um/kernel/process.c | 4 +-
> arch/unicore32/kernel/process.c | 4 +-
> arch/x86/kernel/process_32.c | 4 +-
> arch/x86/kernel/process_64.c | 4 +-
> include/linux/tick.h | 45 +++++++++++++++++++++++++++++--
> kernel/time/tick-sched.c | 25 +++++++++--------
> 18 files changed, 89 insertions(+), 49 deletions(-)
>
> diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
> index f9261d0..4f83362 100644
> --- a/arch/arm/kernel/process.c
> +++ b/arch/arm/kernel/process.c
> @@ -183,7 +183,7 @@ void cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> leds_event(led_idle_start);
> while (!need_resched()) {
> #ifdef CONFIG_HOTPLUG_CPU
> @@ -210,7 +210,7 @@ void cpu_idle(void)
> }
> }
> leds_event(led_idle_end);
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/avr32/kernel/process.c b/arch/avr32/kernel/process.c
> index 6ee7952..34c8c70 100644
> --- a/arch/avr32/kernel/process.c
> +++ b/arch/avr32/kernel/process.c
> @@ -34,10 +34,10 @@ void cpu_idle(void)
> {
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> while (!need_resched())
> cpu_idle_sleep();
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/blackfin/kernel/process.c b/arch/blackfin/kernel/process.c
> index 7b141b5..57e0749 100644
> --- a/arch/blackfin/kernel/process.c
> +++ b/arch/blackfin/kernel/process.c
> @@ -88,10 +88,10 @@ void cpu_idle(void)
> #endif
> if (!idle)
> idle = default_idle;
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> while (!need_resched())
> idle();
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/microblaze/kernel/process.c b/arch/microblaze/kernel/process.c
> index 6dc123e..c6ece38 100644
> --- a/arch/microblaze/kernel/process.c
> +++ b/arch/microblaze/kernel/process.c
> @@ -103,10 +103,10 @@ void cpu_idle(void)
> if (!idle)
> idle = default_idle;
>
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> while (!need_resched())
> idle();
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
>
> preempt_enable_no_resched();
> schedule();
> diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
> index d50a005..7df2ffc 100644
> --- a/arch/mips/kernel/process.c
> +++ b/arch/mips/kernel/process.c
> @@ -56,7 +56,7 @@ void __noreturn cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> while (!need_resched() && cpu_online(cpu)) {
> #ifdef CONFIG_MIPS_MT_SMTC
> extern void smtc_idle_loop_hook(void);
> @@ -77,7 +77,7 @@ void __noreturn cpu_idle(void)
> system_state == SYSTEM_BOOTING))
> play_dead();
> #endif
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/openrisc/kernel/idle.c b/arch/openrisc/kernel/idle.c
> index fb6a9bf..2e82cd0 100644
> --- a/arch/openrisc/kernel/idle.c
> +++ b/arch/openrisc/kernel/idle.c
> @@ -51,7 +51,7 @@ void cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
>
> while (!need_resched()) {
> check_pgt_cache();
> @@ -69,7 +69,7 @@ void cpu_idle(void)
> set_thread_flag(TIF_POLLING_NRFLAG);
> }
>
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/powerpc/kernel/idle.c b/arch/powerpc/kernel/idle.c
> index 878572f..2e782a3 100644
> --- a/arch/powerpc/kernel/idle.c
> +++ b/arch/powerpc/kernel/idle.c
> @@ -56,7 +56,7 @@ void cpu_idle(void)
>
> set_thread_flag(TIF_POLLING_NRFLAG);
> while (1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> while (!need_resched() && !cpu_should_die()) {
> ppc64_runlatch_off();
>
> @@ -93,7 +93,7 @@ void cpu_idle(void)
>
> HMT_medium();
> ppc64_runlatch_on();
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> if (cpu_should_die())
> cpu_die();
> diff --git a/arch/powerpc/platforms/iseries/setup.c b/arch/powerpc/platforms/iseries/setup.c
> index e2f5fad..77ff6eb 100644
> --- a/arch/powerpc/platforms/iseries/setup.c
> +++ b/arch/powerpc/platforms/iseries/setup.c
> @@ -562,7 +562,7 @@ static void yield_shared_processor(void)
> static void iseries_shared_idle(void)
> {
> while (1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> while (!need_resched() && !hvlpevent_is_pending()) {
> local_irq_disable();
> ppc64_runlatch_off();
> @@ -576,7 +576,7 @@ static void iseries_shared_idle(void)
> }
>
> ppc64_runlatch_on();
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
>
> if (hvlpevent_is_pending())
> process_iSeries_events();
> @@ -592,7 +592,7 @@ static void iseries_dedicated_idle(void)
> set_thread_flag(TIF_POLLING_NRFLAG);
>
> while (1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> if (!need_resched()) {
> while (!need_resched()) {
> ppc64_runlatch_off();
> @@ -609,7 +609,7 @@ static void iseries_dedicated_idle(void)
> }
>
> ppc64_runlatch_on();
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/s390/kernel/process.c b/arch/s390/kernel/process.c
> index db3e930..44028ae 100644
> --- a/arch/s390/kernel/process.c
> +++ b/arch/s390/kernel/process.c
> @@ -90,10 +90,10 @@ static void default_idle(void)
> void cpu_idle(void)
> {
> for (;;) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> while (!need_resched())
> default_idle();
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/sh/kernel/idle.c b/arch/sh/kernel/idle.c
> index 6015743..ad58e75 100644
> --- a/arch/sh/kernel/idle.c
> +++ b/arch/sh/kernel/idle.c
> @@ -89,7 +89,7 @@ void cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
>
> while (!need_resched()) {
> check_pgt_cache();
> @@ -111,7 +111,7 @@ void cpu_idle(void)
> start_critical_timings();
> }
>
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/sparc/kernel/process_64.c b/arch/sparc/kernel/process_64.c
> index 1235f63..78b1bc0 100644
> --- a/arch/sparc/kernel/process_64.c
> +++ b/arch/sparc/kernel/process_64.c
> @@ -95,12 +95,12 @@ void cpu_idle(void)
> set_thread_flag(TIF_POLLING_NRFLAG);
>
> while(1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
>
> while (!need_resched() && !cpu_is_offline(cpu))
> sparc64_yield(cpu);
>
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
>
> preempt_enable_no_resched();
>
> diff --git a/arch/tile/kernel/process.c b/arch/tile/kernel/process.c
> index 920e674..53ac895 100644
> --- a/arch/tile/kernel/process.c
> +++ b/arch/tile/kernel/process.c
> @@ -85,7 +85,7 @@ void cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> while (!need_resched()) {
> if (cpu_is_offline(cpu))
> BUG(); /* no HOTPLUG_CPU */
> @@ -105,7 +105,7 @@ void cpu_idle(void)
> local_irq_enable();
> current_thread_info()->status |= TS_POLLING;
> }
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
> index 41acf59..9e7176b 100644
> --- a/arch/um/kernel/process.c
> +++ b/arch/um/kernel/process.c
> @@ -245,10 +245,10 @@ void default_idle(void)
> if (need_resched())
> schedule();
>
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> nsecs = disable_timer();
> idle_sleep(nsecs);
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> }
> }
>
> diff --git a/arch/unicore32/kernel/process.c b/arch/unicore32/kernel/process.c
> index 9999b9a..095ff5a 100644
> --- a/arch/unicore32/kernel/process.c
> +++ b/arch/unicore32/kernel/process.c
> @@ -55,7 +55,7 @@ void cpu_idle(void)
> {
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> while (!need_resched()) {
> local_irq_disable();
> stop_critical_timings();
> @@ -63,7 +63,7 @@ void cpu_idle(void)
> local_irq_enable();
> start_critical_timings();
> }
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> index ad93205..f311d096 100644
> --- a/arch/x86/kernel/process_32.c
> +++ b/arch/x86/kernel/process_32.c
> @@ -98,7 +98,7 @@ void cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> while (!need_resched()) {
>
> check_pgt_cache();
> @@ -114,7 +114,7 @@ void cpu_idle(void)
> pm_idle();
> start_critical_timings();
> }
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 9ca714e..e72daf9 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -121,7 +121,7 @@ void cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter();
> + tick_nohz_idle_enter_norcu();
> while (!need_resched()) {
>
> rmb();
> @@ -147,7 +147,7 @@ void cpu_idle(void)
> __exit_idle();
> }
>
> - tick_nohz_idle_exit();
> + tick_nohz_idle_exit_norcu();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index 0df1d50..7224396 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -7,6 +7,7 @@
> #define _LINUX_TICK_H
>
> #include <linux/clockchips.h>
> +#include <linux/irqflags.h>
>
> #ifdef CONFIG_GENERIC_CLOCKEVENTS
>
> @@ -121,18 +122,56 @@ static inline int tick_oneshot_mode_active(void) { return 0; }
> #endif /* !CONFIG_GENERIC_CLOCKEVENTS */
>
> # ifdef CONFIG_NO_HZ
> -extern void tick_nohz_idle_enter(void);
> +extern void __tick_nohz_idle_enter(void);
> +static inline void tick_nohz_idle_enter(void)
> +{
> + local_irq_disable();
> + __tick_nohz_idle_enter();
> + local_irq_enable();
> +}
> extern void tick_nohz_idle_exit(void);
> +
> +/*
> + * Call this pair of function if the arch doesn't make any use
> + * of RCU in-between. You won't need to call rcu_idle_enter() and
> + * rcu_idle_exit().
> + * Otherwise you need to call tick_nohz_idle_enter() and tick_nohz_idle_exit()
> + * and explicitly tell RCU about the window around the place the CPU enters low
> + * power mode where no RCU use is made. This is done by calling rcu_idle_enter()
> + * after the last use of RCU before the CPU is put to sleep and by calling
> + * rcu_idle_exit() before the first use of RCU after the CPU woke up.
> + */
> +static inline void tick_nohz_idle_enter_norcu(void)
> +{
> + /*
> + * Also call rcu_idle_enter() in the irq disabled section even
> + * if it disables irq itself.
> + * Just an optimization that prevents from an interrupt happening
> + * between it and __tick_nohz_idle_enter() to lose time to help completing
> + * a grace period while we could be in extended grace period already.
> + */
> + local_irq_disable();
> + __tick_nohz_idle_enter();
> + rcu_idle_enter();
> + local_irq_enable();
> +}
> +static inline void tick_nohz_idle_exit_norcu(void)
> +{
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> +}
> extern void tick_nohz_irq_exit(void);
> extern ktime_t tick_nohz_get_sleep_length(void);
> extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
> extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
> # else
> -static inline void tick_nohz_idle_enter(void)
> +static inline void tick_nohz_idle_enter(void) { }
> +static inline void tick_nohz_idle_exit(void) { }
> +static inline void tick_nohz_idle_enter_norcu(void)
> {
> rcu_idle_enter();
> }
> -static inline void tick_nohz_idle_exit(void)
> +static inline void tick_nohz_idle_exit_norcu(void)
> {
> rcu_idle_exit();
> }
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 52b7ace..360d028 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -424,18 +424,22 @@ out:
> *
> * When the next event is more than a tick into the future, stop the idle tick
> * Called when we start the idle loop.
> - * This also enters into RCU extended quiescent state so that this CPU doesn't
> - * need anymore to be part of any global grace period completion. This way
> - * the tick can be stopped safely as we don't need to report quiescent states.
> + *
> + * If no use of RCU is made in the idle loop between
> + * tick_nohz_idle_enter() and tick_nohz_idle_exit() calls, then
> + * tick_nohz_idle_enter_norcu() should be called instead and the arch
> + * doesn't need to call rcu_idle_enter() and rcu_idle_exit() explicitly.
> + *
> + * Otherwise the arch is responsible of calling:
> + *
> + * - rcu_idle_enter() after its last use of RCU before the CPU is put
> + * to sleep.
> + * - rcu_idle_exit() before the first use of RCU after the CPU is woken up.
> */
> -void tick_nohz_idle_enter(void)
> +void __tick_nohz_idle_enter(void)
> {
> struct tick_sched *ts;
>
> - WARN_ON_ONCE(irqs_disabled());
> -
> - local_irq_disable();
> -
> ts = &__get_cpu_var(tick_cpu_sched);
> /*
> * set ts->inidle unconditionally. even if the system did not
> @@ -444,9 +448,6 @@ void tick_nohz_idle_enter(void)
> */
> ts->inidle = 1;
> tick_nohz_stop_sched_tick(ts);
> - rcu_idle_enter();
> -
> - local_irq_enable();
> }
>
> /**
> @@ -522,7 +523,7 @@ void tick_nohz_idle_exit(void)
> ktime_t now;
>
> local_irq_disable();
> - rcu_idle_exit();
> +
> if (ts->idle_active || (ts->inidle && ts->tick_stopped))
> now = ktime_get();
>
> --
> 1.7.5.4
>
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