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Date: Mon, 21 Jul 2008 15:34:32 +0800
From: "Jack Ren" <jack.k.ren@...il.com>
To: "Thomas Gleixner" <tglx@...utronix.de>
Cc: "eric miao" <eric.y.miao@...il.com>, "Ingo Molnar" <mingo@...e.hu>,
LKML <linux-kernel@...r.kernel.org>,
"Jack Ren" <jack.ren@...vell.com>,
"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
"Dmitry Adamushko" <dmitry.adamushko@...il.com>
Subject: Re: [PATCH] sched: do not stop ticks when cpu is not idle
Hi Thomas,
I applied your patch, and rerun the test on my environment, the
issue went away. I also analyzed your patch and did not find any other
race condition so far.
cheers,
Jack Ren
On Fri, Jul 18, 2008 at 11:27 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Fri, 18 Jul 2008, eric miao wrote:
>> On Fri, Jul 18, 2008 at 9:52 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> >> Thomas, Peter, Dmitry, do you concur with the analysis? (commit below)
>> >
>> > Yes. I did not understand the issue when Jack pointed it out to me,
>> > but with Erics explanation it's really clear. Thanks for tracking that
>> > down.
>>
>> Actually, Jack did most of the analysis and came up with this quick
>> fix.
>>
>> >
>> >> It looks a bit ugly to me in the middle of schedule() - is there no wait
>> >> to solve this within kernel/time/*.c ?
>> >
>> > Hmm, yes. I think the proper fix is to enable the tick stop mechanism
>> > in the idle loop and disable it before we go to schedule. That takes
>> > an additional parameter to tick_nohz_stop_sched_tick(), but we then
>> > gain a very clear section where the nohz mimic can be active.
>> >
>> > I'll whip up a patch.
>>
>> Sounds great, thanks.
>
> Hey, thanks for tracking that down. I was banging my head against the
> wall when I understood the problem.
>
> I tried to pinpoint the occasional softlockup bug reports, but I
> probably stared too long into that code so I just saw what I expected
> to see.
>
> Can you give the patch below a try please ?
>
> Thanks,
>
> tglx
>
> ------------------->
> Subject: nohz: prevent tick stop outside of the idle loop
> From: Thomas Gleixner <tglx@...utronix.de>
>
> Jack Ran and Eric Miao tracked down the following long standing
> problem in the NOHZ code:
>
> scheduler switch to idle task
> enable interrupts
>
> Window starts here
>
> ----> interrupt happens (does not set NEED_RESCHED)
> irq_exit() stops the tick
>
> ----> interrupt happens (does set NEED_RESCHED)
>
> return from schedule()
>
> cpu_idle(): preempt_disable();
>
> Window ends here
>
> The interrupts can happen at any point inside the race window. The
> first interrupt stops the tick, the second one causes the scheduler to
> rerun and switch away from idle again and we end up with the tick
> disabled.
>
> The fact that it needs two interrupts where the first one does not set
> NEED_RESCHED and the second one does made the bug obscure and extremly
> hard to reproduce and analyse. Kudos to Jack and Eric.
>
> Solution: Limit the NOHZ functionality to the idle loop to make sure
> that we can not run into such a situation ever again.
>
> cpu_idle()
> {
> preempt_disable();
>
> while(1) {
> tick_nohz_stop_sched_tick(1); <- tell NOHZ code that we
> are in the idle loop
>
> while (!need_resched())
> halt();
>
> tick_nohz_restart_sched_tick(); <- disables NOHZ mode
> preempt_enable_no_resched();
> schedule();
> preempt_disable();
> }
> }
>
> In hindsight we should have done this forever, but ...
>
> /me grabs a large brown paperbag.
>
> Debugged-by: Jack Ren <jack.ren@...vell.com>,
> Debugged-by: eric miao <eric.y.miao@...il.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>
> ---
> diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
> index 199b368..89bfded 100644
> --- a/arch/arm/kernel/process.c
> +++ b/arch/arm/kernel/process.c
> @@ -162,7 +162,7 @@ void cpu_idle(void)
> if (!idle)
> idle = default_idle;
> leds_event(led_idle_start);
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(1);
> while (!need_resched())
> idle();
> leds_event(led_idle_end);
> diff --git a/arch/avr32/kernel/process.c b/arch/avr32/kernel/process.c
> index 6cf9df1..ff820a9 100644
> --- a/arch/avr32/kernel/process.c
> +++ b/arch/avr32/kernel/process.c
> @@ -31,7 +31,7 @@ void cpu_idle(void)
> {
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(1);
> while (!need_resched())
> cpu_idle_sleep();
> tick_nohz_restart_sched_tick();
> diff --git a/arch/blackfin/kernel/process.c b/arch/blackfin/kernel/process.c
> index 53c2cd2..77800dd 100644
> --- a/arch/blackfin/kernel/process.c
> +++ b/arch/blackfin/kernel/process.c
> @@ -105,7 +105,7 @@ void cpu_idle(void)
> #endif
> if (!idle)
> idle = default_idle;
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(1);
> while (!need_resched())
> idle();
> tick_nohz_restart_sched_tick();
> diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
> index c06f5b5..b16facd 100644
> --- a/arch/mips/kernel/process.c
> +++ b/arch/mips/kernel/process.c
> @@ -53,7 +53,7 @@ void __noreturn cpu_idle(void)
> {
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(1);
> while (!need_resched()) {
> #ifdef CONFIG_SMTC_IDLE_HOOK_DEBUG
> extern void smtc_idle_loop_hook(void);
> diff --git a/arch/powerpc/kernel/idle.c b/arch/powerpc/kernel/idle.c
> index c3cf0e8..d308a9f 100644
> --- a/arch/powerpc/kernel/idle.c
> +++ b/arch/powerpc/kernel/idle.c
> @@ -60,7 +60,7 @@ void cpu_idle(void)
>
> set_thread_flag(TIF_POLLING_NRFLAG);
> while (1) {
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(1);
> while (!need_resched() && !cpu_should_die()) {
> ppc64_runlatch_off();
>
> diff --git a/arch/powerpc/platforms/iseries/setup.c b/arch/powerpc/platforms/iseries/setup.c
> index b721207..70b688c 100644
> --- a/arch/powerpc/platforms/iseries/setup.c
> +++ b/arch/powerpc/platforms/iseries/setup.c
> @@ -561,7 +561,7 @@ static void yield_shared_processor(void)
> static void iseries_shared_idle(void)
> {
> while (1) {
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(1);
> while (!need_resched() && !hvlpevent_is_pending()) {
> local_irq_disable();
> ppc64_runlatch_off();
> @@ -591,7 +591,7 @@ static void iseries_dedicated_idle(void)
> set_thread_flag(TIF_POLLING_NRFLAG);
>
> while (1) {
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(1);
> if (!need_resched()) {
> while (!need_resched()) {
> ppc64_runlatch_off();
> diff --git a/arch/sh/kernel/process_32.c b/arch/sh/kernel/process_32.c
> index b98e37a..921892c 100644
> --- a/arch/sh/kernel/process_32.c
> +++ b/arch/sh/kernel/process_32.c
> @@ -86,7 +86,7 @@ void cpu_idle(void)
> if (!idle)
> idle = default_idle;
>
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(1);
> while (!need_resched())
> idle();
> tick_nohz_restart_sched_tick();
> diff --git a/arch/sparc64/kernel/process.c b/arch/sparc64/kernel/process.c
> index 2084f81..0798928 100644
> --- a/arch/sparc64/kernel/process.c
> +++ b/arch/sparc64/kernel/process.c
> @@ -97,7 +97,7 @@ void cpu_idle(void)
> set_thread_flag(TIF_POLLING_NRFLAG);
>
> while(1) {
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(1);
>
> while (!need_resched() && !cpu_is_offline(cpu))
> sparc64_yield(cpu);
> diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
> index 83603cf..a1c6d07 100644
> --- a/arch/um/kernel/process.c
> +++ b/arch/um/kernel/process.c
> @@ -243,7 +243,7 @@ void default_idle(void)
> if (need_resched())
> schedule();
>
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(1);
> nsecs = disable_timer();
> idle_sleep(nsecs);
> tick_nohz_restart_sched_tick();
> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> index 0c3927a..53bc653 100644
> --- a/arch/x86/kernel/process_32.c
> +++ b/arch/x86/kernel/process_32.c
> @@ -128,7 +128,7 @@ void cpu_idle(void)
>
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(1);
> while (!need_resched()) {
>
> check_pgt_cache();
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index a8e5362..9a10c18 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -120,7 +120,7 @@ void cpu_idle(void)
> current_thread_info()->status |= TS_POLLING;
> /* endless idle loop with no priority at all */
> while (1) {
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(1);
> while (!need_resched()) {
>
> rmb();
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index a881c65..d3c0269 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -49,6 +49,7 @@ struct tick_sched {
> unsigned long check_clocks;
> enum tick_nohz_mode nohz_mode;
> ktime_t idle_tick;
> + int inidle;
> int tick_stopped;
> unsigned long idle_jiffies;
> unsigned long idle_calls;
> @@ -105,14 +106,14 @@ static inline int tick_check_oneshot_change(int allow_nohz) { return 0; }
> #endif /* !CONFIG_GENERIC_CLOCKEVENTS */
>
> # ifdef CONFIG_NO_HZ
> -extern void tick_nohz_stop_sched_tick(void);
> +extern void tick_nohz_stop_sched_tick(int inidle);
> extern void tick_nohz_restart_sched_tick(void);
> extern void tick_nohz_update_jiffies(void);
> extern ktime_t tick_nohz_get_sleep_length(void);
> extern void tick_nohz_stop_idle(int cpu);
> extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
> # else
> -static inline void tick_nohz_stop_sched_tick(void) { }
> +static inline void tick_nohz_stop_sched_tick(int inidle) { }
> static inline void tick_nohz_restart_sched_tick(void) { }
> static inline void tick_nohz_update_jiffies(void) { }
> static inline ktime_t tick_nohz_get_sleep_length(void)
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 81e2fe0..f6b03d5 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -286,7 +286,7 @@ void irq_exit(void)
> #ifdef CONFIG_NO_HZ
> /* Make sure that timer wheel updates are propagated */
> if (!in_interrupt() && idle_cpu(smp_processor_id()) && !need_resched())
> - tick_nohz_stop_sched_tick();
> + tick_nohz_stop_sched_tick(0);
> rcu_irq_exit();
> #endif
> preempt_enable_no_resched();
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index beef7cc..a5c26d2 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -195,7 +195,7 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
> * Called either from the idle loop or from irq_exit() when an idle period was
> * just interrupted by an interrupt which did not cause a reschedule.
> */
> -void tick_nohz_stop_sched_tick(void)
> +void tick_nohz_stop_sched_tick(int inidle)
> {
> unsigned long seq, last_jiffies, next_jiffies, delta_jiffies, flags;
> struct tick_sched *ts;
> @@ -224,6 +224,11 @@ void tick_nohz_stop_sched_tick(void)
> if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
> goto end;
>
> + if (!inidle && !ts->inidle)
> + goto end;
> +
> + ts->inidle = 1;
> +
> if (need_resched())
> goto end;
>
> @@ -373,11 +378,14 @@ void tick_nohz_restart_sched_tick(void)
> local_irq_disable();
> tick_nohz_stop_idle(cpu);
>
> - if (!ts->tick_stopped) {
> + if (!ts->inidle || !ts->tick_stopped) {
> + ts->inidle = 0;
> local_irq_enable();
> return;
> }
>
> + ts->inidle = 0;
> +
> rcu_exit_nohz();
>
> /* Update jiffies first */
> --
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