Re: [PATCH 2/7] sched: Add NEED_RESCHED to the preempt_count

[Mike Galbraith]

On Wed, 2013-09-11 at 13:06 +0200, Peter Zijlstra wrote: 
> On Wed, Sep 11, 2013 at 10:25:30AM +0200, Peter Zijlstra wrote:
> > On Tue, Sep 10, 2013 at 06:59:57PM -0700, Andy Lutomirski wrote:
> 
> > > It looks like the intel_idle code can get confused if TIF_NEED_RESCHED
> > > is set but the preempt resched bit is not -- the need_resched call
> > > between monitor and mwait won't notice TIF_NEED_RESCHED.
> > > 
> > > Is this condition possible?
> > 
> > Ah indeed, I'll have to go find all idle loops out there it seems. Now
> > if only they were easy to spot :/
> > 
> > I was hoping the generic idle thing was good enough, apparently not
> > quite. Thanks for spotting that.
> 
> OK, and the reason I didn't notice is that the entire TS_POLLING thing
> is completely wrecked so we'll get the interrupt anyway.
> 
> The below might fix it.. it boots, but then it already did so that
> doesn't say much.
> 
> Mike does it fix the funny you saw?

Yup, modulo test_need_resched() not existing in master.
E5620 pipe-test
v3.7.10                  578.5 KHz
v3.7.10-nothrottle       366.7 KHz
v3.8.13                  468.3 KHz
v3.9.11                  462.0 KHz
v3.10.4                  419.4 KHz
v3.11-rc3-4-g36f571e     400.1 KHz
master-9013-gbf83e61     553.5 KHz

I still have your not quite complete hrtimer patch in there, lest menu
governor munch any improvement, as well as my throttle-nohz patch, so
seems we may be a _bit_ down vs 3.7, but reschedule_interrupt did crawl
back under its rock, and while I haven't yet booted Q6600 box, core2
Toshiba Satellite lappy is using mwait_idle_with_hints() in master.

Thanks,

-Mike

> ---
> Subject: sched, idle: Fix the idle polling state logic
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Wed Sep 11 12:43:13 CEST 2013
> 
> Mike reported that commit 7d1a9417 ("x86: Use generic idle loop")
> regressed several workloads and caused excessive reschedule
> interrupts.
> 
> The patch in question failed to notice that the x86 code had an
> inverted sense of the polling state versus the new generic code (x86:
> default polling, generic: default !polling).
> 
> Fix the two prominent x86 mwait based idle drivers and introduce a few
> new generic polling helpers (fixing the wrong smp_mb__after_clear_bit
> usage).
> 
> Also switch the idle routines to using test_need_resched() which is an
> immediate TIF_NEED_RESCHED test as opposed to need_resched which will
> end up being slightly different.
> 
> Reported-by: Mike Galbraith <bitbucket@xxxxxxxxx>
> Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/process.c     |    6 +--
>  drivers/acpi/processor_idle.c |   46 +++++-------------------
>  drivers/idle/intel_idle.c     |    2 -
>  include/linux/sched.h         |   78 ++++++++++++++++++++++++++++++++++++++----
>  kernel/cpu/idle.c             |    9 ++--
>  5 files changed, 89 insertions(+), 52 deletions(-)
> 
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -391,9 +391,9 @@ static void amd_e400_idle(void)
>  		 * The switch back from broadcast mode needs to be
>  		 * called with interrupts disabled.
>  		 */
> -		 local_irq_disable();
> -		 clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_EXIT, &cpu);
> -		 local_irq_enable();
> +		local_irq_disable();
> +		clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_EXIT, &cpu);
> +		local_irq_enable();
>  	} else
>  		default_idle();
>  }
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -119,17 +119,10 @@ static struct dmi_system_id processor_po
>   */
>  static void acpi_safe_halt(void)
>  {
> -	current_thread_info()->status &= ~TS_POLLING;
> -	/*
> -	 * TS_POLLING-cleared state must be visible before we
> -	 * test NEED_RESCHED:
> -	 */
> -	smp_mb();
> -	if (!need_resched()) {
> +	if (!test_need_resched()) {
>  		safe_halt();
>  		local_irq_disable();
>  	}
> -	current_thread_info()->status |= TS_POLLING;
>  }
>  
>  #ifdef ARCH_APICTIMER_STOPS_ON_C3
> @@ -737,6 +730,11 @@ static int acpi_idle_enter_c1(struct cpu
>  	if (unlikely(!pr))
>  		return -EINVAL;
>  
> +	if (cx->entry_method == ACPI_CSTATE_FFH) {
> +		if (current_set_polling_and_test())
> +			return -EINVAL;
> +	}
> +
>  	lapic_timer_state_broadcast(pr, cx, 1);
>  	acpi_idle_do_entry(cx);
>  
> @@ -790,18 +788,9 @@ static int acpi_idle_enter_simple(struct
>  	if (unlikely(!pr))
>  		return -EINVAL;
>  
> -	if (cx->entry_method != ACPI_CSTATE_FFH) {
> -		current_thread_info()->status &= ~TS_POLLING;
> -		/*
> -		 * TS_POLLING-cleared state must be visible before we test
> -		 * NEED_RESCHED:
> -		 */
> -		smp_mb();
> -
> -		if (unlikely(need_resched())) {
> -			current_thread_info()->status |= TS_POLLING;
> +	if (cx->entry_method == ACPI_CSTATE_FFH) {
> +		if (current_set_polling_and_test())
>  			return -EINVAL;
> -		}
>  	}
>  
>  	/*
> @@ -819,9 +808,6 @@ static int acpi_idle_enter_simple(struct
>  
>  	sched_clock_idle_wakeup_event(0);
>  
> -	if (cx->entry_method != ACPI_CSTATE_FFH)
> -		current_thread_info()->status |= TS_POLLING;
> -
>  	lapic_timer_state_broadcast(pr, cx, 0);
>  	return index;
>  }
> @@ -858,18 +844,9 @@ static int acpi_idle_enter_bm(struct cpu
>  		}
>  	}
>  
> -	if (cx->entry_method != ACPI_CSTATE_FFH) {
> -		current_thread_info()->status &= ~TS_POLLING;
> -		/*
> -		 * TS_POLLING-cleared state must be visible before we test
> -		 * NEED_RESCHED:
> -		 */
> -		smp_mb();
> -
> -		if (unlikely(need_resched())) {
> -			current_thread_info()->status |= TS_POLLING;
> +	if (cx->entry_method == ACPI_CSTATE_FFH) {
> +		if (current_set_polling_and_test())
>  			return -EINVAL;
> -		}
>  	}
>  
>  	acpi_unlazy_tlb(smp_processor_id());
> @@ -915,9 +892,6 @@ static int acpi_idle_enter_bm(struct cpu
>  
>  	sched_clock_idle_wakeup_event(0);
>  
> -	if (cx->entry_method != ACPI_CSTATE_FFH)
> -		current_thread_info()->status |= TS_POLLING;
> -
>  	lapic_timer_state_broadcast(pr, cx, 0);
>  	return index;
>  }
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
> @@ -359,7 +359,7 @@ static int intel_idle(struct cpuidle_dev
>  	if (!(lapic_timer_reliable_states & (1 << (cstate))))
>  		clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ENTER, &cpu);
>  
> -	if (!need_resched()) {
> +	if (!current_set_polling_and_test()) {
>  
>  		__monitor((void *)&current_thread_info()->flags, 0, 0);
>  		smp_mb();
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2471,34 +2471,98 @@ static inline int tsk_is_polling(struct
>  {
>  	return task_thread_info(p)->status & TS_POLLING;
>  }
> -static inline void current_set_polling(void)
> +static inline void __current_set_polling(void)
>  {
>  	current_thread_info()->status |= TS_POLLING;
>  }
>  
> -static inline void current_clr_polling(void)
> +static inline bool __must_check current_set_polling_and_test(void)
> +{
> +	__current_set_polling();
> +
> +	/*
> +	 * Polling state must be visible before we test NEED_RESCHED,
> +	 * paired by resched_task()
> +	 */
> +	smp_mb();
> +
> +	return unlikely(test_need_resched());
> +}
> +
> +static inline void __current_clr_polling(void)
>  {
>  	current_thread_info()->status &= ~TS_POLLING;
> -	smp_mb__after_clear_bit();
> +}
> +
> +static inline bool __must_check current_clr_polling_and_test(void)
> +{
> +	__current_clr_polling();
> +
> +	/*
> +	 * Polling state must be visible before we test NEED_RESCHED,
> +	 * paired by resched_task()
> +	 */
> +	smp_mb();
> +
> +	return unlikely(test_need_resched());
>  }
>  #elif defined(TIF_POLLING_NRFLAG)
>  static inline int tsk_is_polling(struct task_struct *p)
>  {
>  	return test_tsk_thread_flag(p, TIF_POLLING_NRFLAG);
>  }
> -static inline void current_set_polling(void)
> +
> +static inline void __current_set_polling(void)
>  {
>  	set_thread_flag(TIF_POLLING_NRFLAG);
>  }
>  
> -static inline void current_clr_polling(void)
> +static inline bool __must_check current_set_polling_and_test(void)
> +{
> +	__current_set_polling();
> +
> +	/*
> +	 * Polling state must be visible before we test NEED_RESCHED,
> +	 * paired by resched_task()
> +	 *
> +	 * XXX: assumes set/clear bit are identical barrier wise.
> +	 */
> +	smp_mb__after_clear_bit();
> +
> +	return unlikely(test_need_resched());
> +}
> +
> +static inline void __current_clr_polling(void)
>  {
>  	clear_thread_flag(TIF_POLLING_NRFLAG);
>  }
> +
> +static inline bool __must_check current_clr_polling_and_test(void)
> +{
> +	__current_clr_polling();
> +
> +	/*
> +	 * Polling state must be visible before we test NEED_RESCHED,
> +	 * paired by resched_task()
> +	 */
> +	smp_mb__after_clear_bit();
> +
> +	return unlikely(test_need_resched());
> +}
> +
>  #else
>  static inline int tsk_is_polling(struct task_struct *p) { return 0; }
> -static inline void current_set_polling(void) { }
> -static inline void current_clr_polling(void) { }
> +static inline void __current_set_polling(void) { }
> +static inline void __current_clr_polling(void) { }
> +
> +static inline bool __must_check current_set_polling_and_test(void)
> +{
> +	return unlikely(test_need_resched);
> +}
> +static inline bool __must_check current_clr_polling_and_test(void)
> +{
> +	return unlikely(test_need_resched);
> +}
>  #endif
>  
>  /*
> --- a/kernel/cpu/idle.c
> +++ b/kernel/cpu/idle.c
> @@ -44,7 +44,7 @@ static inline int cpu_idle_poll(void)
>  	rcu_idle_enter();
>  	trace_cpu_idle_rcuidle(0, smp_processor_id());
>  	local_irq_enable();
> -	while (!need_resched())
> +	while (!test_need_resched())
>  		cpu_relax();
>  	trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
>  	rcu_idle_exit();
> @@ -92,8 +92,7 @@ static void cpu_idle_loop(void)
>  			if (cpu_idle_force_poll || tick_check_broadcast_expired()) {
>  				cpu_idle_poll();
>  			} else {
> -				current_clr_polling();
> -				if (!need_resched()) {
> +				if (!current_clr_polling_and_test()) {
>  					stop_critical_timings();
>  					rcu_idle_enter();
>  					arch_cpu_idle();
> @@ -103,7 +102,7 @@ static void cpu_idle_loop(void)
>  				} else {
>  					local_irq_enable();
>  				}
> -				current_set_polling();
> +				__current_set_polling();
>  			}
>  			arch_cpu_idle_exit();
>  			/*
> @@ -136,7 +135,7 @@ void cpu_startup_entry(enum cpuhp_state
>  	 */
>  	boot_init_stack_canary();
>  #endif
> -	current_set_polling();
> +	__current_set_polling();
>  	arch_cpu_idle_prepare();
>  	cpu_idle_loop();
>  }
> --
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