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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Oct 12, 2011 at 11:57:52PM -0700, Paul E. McKenney wrote:
> 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

Which reminds me...  About the ultimate objective, getting tick-free
operation.  (Or, for the guys who want to eliminate the tick entirely,
shutting up the hrtimer stuff that they want to replace it with.)

I believe that you will then need to have two levels of not-in-dynticks
for processes, one for idle vs. not and another for when a process
switches from user-space to kernel execution.  Correct, or am I
confused?

The reason I ask is that commit e11f5981 currently only allows one
level of not-in-dynticks for processes.  It is easy to add another
level, but thought I should check beforehand.

							Thanx, Paul

> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > Cc: Mike Frysinger <vapier@gentoo.org>
> > Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
> > Cc: David Miller <davem@davemloft.net>
> > Cc: Chris Metcalf <cmetcalf@tilera.com>
> > Cc: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
> > Cc: Ralf Baechle <ralf@linux-mips.org>
> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: H. Peter Anvin <hpa@zytor.com>
> > Cc: Russell King <linux@arm.linux.org.uk>
> > Cc: Paul Mackerras <paulus@samba.org>
> > Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> > Cc: Paul Mundt <lethal@linux-sh.org>
> > ---
> >  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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