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Message-ID: <20111117201134.GB1865@leaf>
Date: Thu, 17 Nov 2011 12:11:34 -0800
From: Josh Triplett <josh@...htriplett.org>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
LKML <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] nohz: Remove tick_nohz_idle_enter_norcu() /
tick_nohz_idle_exit_norcu()
On Thu, Nov 17, 2011 at 06:48:14PM +0100, Frederic Weisbecker wrote:
> Those two APIs were provided to optimize the calls of
> tick_nohz_idle_enter() and rcu_idle_enter() into a single
> irq disabled section. This way no interrupt happening in-between would
> needlessly process any RCU job.
>
> Now we are talking about an optimization for which benefits
> have yet to be measured. Let's start simple and completely decouple
> idle rcu and dyntick idle logics to simplify.
>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Josh Triplett <josh@...htriplett.org>
Reviewed-by: Josh Triplett <josh@...htriplett.org>
> diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
> index 4f83362..0e42a9c 100644
> --- a/arch/arm/kernel/process.c
> +++ b/arch/arm/kernel/process.c
> @@ -183,7 +183,8 @@ void cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
> leds_event(led_idle_start);
> while (!need_resched()) {
> #ifdef CONFIG_HOTPLUG_CPU
> @@ -210,7 +211,8 @@ void cpu_idle(void)
> }
> }
> leds_event(led_idle_end);
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/avr32/kernel/process.c b/arch/avr32/kernel/process.c
> index 34c8c70..ea33957 100644
> --- a/arch/avr32/kernel/process.c
> +++ b/arch/avr32/kernel/process.c
> @@ -34,10 +34,12 @@ void cpu_idle(void)
> {
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
> while (!need_resched())
> cpu_idle_sleep();
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/blackfin/kernel/process.c b/arch/blackfin/kernel/process.c
> index 57e0749..8dd0416 100644
> --- a/arch/blackfin/kernel/process.c
> +++ b/arch/blackfin/kernel/process.c
> @@ -88,10 +88,12 @@ void cpu_idle(void)
> #endif
> if (!idle)
> idle = default_idle;
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
> while (!need_resched())
> idle();
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/microblaze/kernel/process.c b/arch/microblaze/kernel/process.c
> index c6ece38..37ed945 100644
> --- a/arch/microblaze/kernel/process.c
> +++ b/arch/microblaze/kernel/process.c
> @@ -103,10 +103,12 @@ void cpu_idle(void)
> if (!idle)
> idle = default_idle;
>
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
> while (!need_resched())
> idle();
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
>
> preempt_enable_no_resched();
> schedule();
> diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
> index 7df2ffc..7937367 100644
> --- a/arch/mips/kernel/process.c
> +++ b/arch/mips/kernel/process.c
> @@ -56,7 +56,8 @@ void __noreturn cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
> while (!need_resched() && cpu_online(cpu)) {
> #ifdef CONFIG_MIPS_MT_SMTC
> extern void smtc_idle_loop_hook(void);
> @@ -77,7 +78,8 @@ void __noreturn cpu_idle(void)
> system_state == SYSTEM_BOOTING))
> play_dead();
> #endif
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/openrisc/kernel/idle.c b/arch/openrisc/kernel/idle.c
> index 2e82cd0..e5fc7887 100644
> --- a/arch/openrisc/kernel/idle.c
> +++ b/arch/openrisc/kernel/idle.c
> @@ -51,7 +51,8 @@ void cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
>
> while (!need_resched()) {
> check_pgt_cache();
> @@ -69,7 +70,8 @@ void cpu_idle(void)
> set_thread_flag(TIF_POLLING_NRFLAG);
> }
>
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/powerpc/kernel/idle.c b/arch/powerpc/kernel/idle.c
> index 3cd73d1..9c3cd49 100644
> --- a/arch/powerpc/kernel/idle.c
> +++ b/arch/powerpc/kernel/idle.c
> @@ -62,10 +62,10 @@ void cpu_idle(void)
>
> set_thread_flag(TIF_POLLING_NRFLAG);
> while (1) {
> - if (idle_uses_rcu)
> - tick_nohz_idle_enter();
> - else
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + if (!idle_uses_rcu)
> + rcu_idle_enter();
> +
> while (!need_resched() && !cpu_should_die()) {
> ppc64_runlatch_off();
>
> @@ -102,10 +102,9 @@ void cpu_idle(void)
>
> HMT_medium();
> ppc64_runlatch_on();
> - if (idle_uses_rcu)
> - tick_nohz_idle_exit();
> - else
> - tick_nohz_idle_exit_norcu();
> + if (!idle_uses_rcu)
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> 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 77ff6eb..097f7d5 100644
> --- a/arch/powerpc/platforms/iseries/setup.c
> +++ b/arch/powerpc/platforms/iseries/setup.c
> @@ -562,7 +562,8 @@ static void yield_shared_processor(void)
> static void iseries_shared_idle(void)
> {
> while (1) {
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
> while (!need_resched() && !hvlpevent_is_pending()) {
> local_irq_disable();
> ppc64_runlatch_off();
> @@ -576,7 +577,8 @@ static void iseries_shared_idle(void)
> }
>
> ppc64_runlatch_on();
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
>
> if (hvlpevent_is_pending())
> process_iSeries_events();
> @@ -592,7 +594,8 @@ static void iseries_dedicated_idle(void)
> set_thread_flag(TIF_POLLING_NRFLAG);
>
> while (1) {
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
> if (!need_resched()) {
> while (!need_resched()) {
> ppc64_runlatch_off();
> @@ -609,7 +612,8 @@ static void iseries_dedicated_idle(void)
> }
>
> ppc64_runlatch_on();
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/s390/kernel/process.c b/arch/s390/kernel/process.c
> index 44028ae..bf2bc31 100644
> --- a/arch/s390/kernel/process.c
> +++ b/arch/s390/kernel/process.c
> @@ -90,10 +90,12 @@ static void default_idle(void)
> void cpu_idle(void)
> {
> for (;;) {
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
> while (!need_resched())
> default_idle();
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/sh/kernel/idle.c b/arch/sh/kernel/idle.c
> index ad58e75..406508d 100644
> --- a/arch/sh/kernel/idle.c
> +++ b/arch/sh/kernel/idle.c
> @@ -89,7 +89,8 @@ void cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
>
> while (!need_resched()) {
> check_pgt_cache();
> @@ -111,7 +112,8 @@ void cpu_idle(void)
> start_critical_timings();
> }
>
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/sparc/kernel/process_64.c b/arch/sparc/kernel/process_64.c
> index 78b1bc0..fde8d72 100644
> --- a/arch/sparc/kernel/process_64.c
> +++ b/arch/sparc/kernel/process_64.c
> @@ -95,12 +95,14 @@ void cpu_idle(void)
> set_thread_flag(TIF_POLLING_NRFLAG);
>
> while(1) {
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
>
> while (!need_resched() && !cpu_is_offline(cpu))
> sparc64_yield(cpu);
>
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
>
> preempt_enable_no_resched();
>
> diff --git a/arch/tile/kernel/process.c b/arch/tile/kernel/process.c
> index 53ac895..4c1ac6e 100644
> --- a/arch/tile/kernel/process.c
> +++ b/arch/tile/kernel/process.c
> @@ -85,7 +85,8 @@ void cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
> while (!need_resched()) {
> if (cpu_is_offline(cpu))
> BUG(); /* no HOTPLUG_CPU */
> @@ -105,7 +106,8 @@ void cpu_idle(void)
> local_irq_enable();
> current_thread_info()->status |= TS_POLLING;
> }
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
> index 9e7176b..b652ff1 100644
> --- a/arch/um/kernel/process.c
> +++ b/arch/um/kernel/process.c
> @@ -245,10 +245,12 @@ void default_idle(void)
> if (need_resched())
> schedule();
>
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
> nsecs = disable_timer();
> idle_sleep(nsecs);
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> }
> }
>
> diff --git a/arch/unicore32/kernel/process.c b/arch/unicore32/kernel/process.c
> index 095ff5a..52edc2b 100644
> --- a/arch/unicore32/kernel/process.c
> +++ b/arch/unicore32/kernel/process.c
> @@ -55,7 +55,8 @@ void cpu_idle(void)
> {
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
> while (!need_resched()) {
> local_irq_disable();
> stop_critical_timings();
> @@ -63,7 +64,8 @@ void cpu_idle(void)
> local_irq_enable();
> start_critical_timings();
> }
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> index f311d096..44e3384 100644
> --- a/arch/x86/kernel/process_32.c
> +++ b/arch/x86/kernel/process_32.c
> @@ -98,7 +98,8 @@ void cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_idle_enter_norcu();
> + tick_nohz_idle_enter();
> + rcu_idle_enter();
> while (!need_resched()) {
>
> check_pgt_cache();
> @@ -114,7 +115,8 @@ void cpu_idle(void)
> pm_idle();
> start_critical_timings();
> }
> - tick_nohz_idle_exit_norcu();
> + rcu_idle_exit();
> + tick_nohz_idle_exit();
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index 327434a..ab8be90 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -122,45 +122,8 @@ 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);
> -static inline void tick_nohz_idle_enter(void)
> -{
> - local_irq_disable();
> - __tick_nohz_idle_enter();
> - local_irq_enable();
> -}
> +extern void tick_nohz_idle_enter(void);
> 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);
> @@ -168,14 +131,6 @@ 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_exit(void) { }
> -static inline void tick_nohz_idle_enter_norcu(void)
> -{
> - rcu_idle_enter();
> -}
> -static inline void tick_nohz_idle_exit_norcu(void)
> -{
> - rcu_idle_exit();
> -}
>
> static inline ktime_t tick_nohz_get_sleep_length(void)
> {
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 360d028..0d887e8 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -425,21 +425,20 @@ out:
> * When the next event is more than a tick into the future, stop the idle tick
> * Called when we start the idle loop.
> *
> - * 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:
> + * 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
> @@ -448,6 +447,8 @@ void __tick_nohz_idle_enter(void)
> */
> ts->inidle = 1;
> tick_nohz_stop_sched_tick(ts);
> +
> + local_irq_enable();
> }
>
> /**
> --
> 1.7.5.4
>
--
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