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Date:	Thu, 23 Feb 2012 10:30:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Aaron Durbin <adurbin@...gle.com>,
	Paul Turner <pjt@...gle.com>,
	Yong Zhang <yong.zhang0@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Extend mwait idle to optimize away CAL and RES
 interrupts to an idle CPU -v1

On Wed, 2012-02-22 at 16:36 -0800, Venkatesh Pallipadi wrote:

> Changes since previous versions:

>   Moved the changes into arch specific code as per PeterZ suggestion

You failed:

>  include/linux/sched.h               |    2 +
>  kernel/sched/core.c                 |   13 ++++++



> diff --git a/arch/x86/include/asm/ipiless_poke.h b/arch/x86/include/asm/ipiless_poke.h
> new file mode 100644
> index 0000000..58670c7
> --- /dev/null
> +++ b/arch/x86/include/asm/ipiless_poke.h
> @@ -0,0 +1,82 @@
> +#ifndef _ASM_X86_IPILESS_POKE_H
> +#define _ASM_X86_IPILESS_POKE_H
> +
> +#include <linux/sched.h>
> +#include <asm/thread_info.h>
> +
> +#ifdef CONFIG_SMP
> +
> +DECLARE_PER_CPU(atomic_t *, idle_task_ti_flags);
> +
> +/*
> + * Use 2 bits in idle_task's thead info flags:
> + * TIF_IPILESS_IDLE marks enter to and exit from idle states with ipiless
> + * wakeup capability.
> + * TIF_IPI_PENDING set by IPI source CPU if it finds that the IPI target CPU
> + * is in TIF_IPILESS_IDLE state (and TIF_IPI_PENDING is not already set).
> + * Setting of TIF_IPI_PENDING bit brings the target CPU out of idle state.
> + */
> +
> +static inline void ipiless_idle_enter(void)
> +{
> +	set_thread_flag(TIF_IPILESS_IDLE);
> +}
> +
> +static inline void ipiless_idle_exit(void)
> +{
> +	clear_thread_flag(TIF_IPILESS_IDLE);
> +}
> +
> +static inline int is_ipi_pending(void)
> +{
> +	return unlikely(test_thread_flag(TIF_IPI_PENDING));
> +}
> +
> +static inline int need_wakeup(void)
> +{
> +	return need_resched() || is_ipi_pending();
> +}
> +
> +static inline void ipiless_pending_work(void)
> +{
> +	if (is_ipi_pending()) {
> +		clear_thread_flag(TIF_IPI_PENDING);
> +		local_bh_disable();
> +		local_irq_disable();

That local_bh_disable() is completely superfluous, disabling IRQs
already kills bh.

> +		generic_smp_call_function_single_interrupt();
> +		__scheduler_ipi();

Why not scheduler_ipi()?

Also, you could keep a pending vector bitmask in a per-cpu variable to
avoid having to call all handlers. not sure that's worth it for just
two, but something to keep in mind for if/when this starts expanding.

> +		local_irq_enable();
> +		local_bh_enable();
> +	}
> +}

Why doesn't ipiless_idle_exit() call this? That would keep it isolated
to just those idle routines that actually use mwait instead of bothering
the generic idle paths with this.

> +static inline int ipiless_magic_poke(int cpu)
> +{
> +	int val;
> +	atomic_t *idle_flag = per_cpu(idle_task_ti_flags, cpu);

What's this atomic_t nonsense about? thread_info::flags is __u32,
casting it to atomic_t is complete rubbish.

> +
> +	val = atomic_read(idle_flag);

The __u32 version would look like: val = ACCESS_ONCE(*idle_flag);

> +	if (unlikely(val & _TIF_IPI_PENDING))
> +		return 1;
> +
> +	if (!(val & _TIF_IPILESS_IDLE))
> +		return 0;
> +
> +	if (val == atomic_cmpxchg(idle_flag, val, val | _TIF_IPI_PENDING))

The __u32 version would look like:

  if (val == cmpxchg(idle_flag, val, val | _TIF_IPI_PENDING))

Bonus win for being shorter!

> +		return 1;
> +
> +	return 0;
> +}
> +
> +#else
> +static inline void ipiless_pending_work(void) { }
> +static inline void ipiless_idle_enter(void) { }
> +static inline void ipiless_idle_exit(void) { }
> +
> +static inline int need_wakeup(void)
> +{
> +	return need_resched();
> +}
> +#endif
> +
> +#endif /* _ASM_X86_IPILESS_POKE_H */


> diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> index a8ff227..e66a4c8 100644
> --- a/arch/x86/kernel/smp.c
> +++ b/arch/x86/kernel/smp.c
> @@ -27,6 +27,7 @@
>  #include <asm/mtrr.h>
>  #include <asm/tlbflush.h>
>  #include <asm/mmu_context.h>
> +#include <asm/ipiless_poke.h>
>  #include <asm/proto.h>
>  #include <asm/apic.h>
>  #include <asm/nmi.h>
> @@ -109,6 +110,14 @@
>   *	about nothing of note with C stepping upwards.
>   */
>  
> +DEFINE_PER_CPU(atomic_t *, idle_task_ti_flags);
> +
> +void __cpuinit thread_idle_state_setup(struct task_struct *idle, int cpu)
> +{
> +	per_cpu(idle_task_ti_flags, cpu) =
> +				(atomic_t *)(&(task_thread_info(idle)->flags));
> +}

As Ingo already pointed out, its the architecture that actually sets up
the idle threads, so putting callbacks into the generic code to find
them is kinda backwards.

Again, *yuck* at that atomic_t business.


> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 5255c9d..1558316 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1451,6 +1451,14 @@ static void sched_ttwu_pending(void)
>  	raw_spin_unlock(&rq->lock);
>  }
>  
> +void __scheduler_ipi(void)
> +{
> +	if (llist_empty(&this_rq()->wake_list))
> +		return;
> +
> +	sched_ttwu_pending();
> +}

FAIL!! It should be 100% identical to a normal IPI, that calls
scheduler_ipi() so this should too.

>  void scheduler_ipi(void)
>  {
>  	if (llist_empty(&this_rq()->wake_list) && !got_nohz_idle_kick())
> @@ -4827,6 +4835,10 @@ void __cpuinit init_idle_bootup_task(struct task_struct *idle)
>  	idle->sched_class = &idle_sched_class;
>  }
>  
> +void __cpuinit __weak thread_idle_state_setup(struct task_struct *idle, int cpu)
> +{
> +}
> +
>  /**
>   * init_idle - set up an idle thread for a given CPU
>   * @idle: task in question
> @@ -4869,6 +4881,7 @@ void __cpuinit init_idle(struct task_struct *idle, int cpu)
>  
>  	/* Set the preempt count _outside_ the spinlocks! */
>  	task_thread_info(idle)->preempt_count = 0;
> +	thread_idle_state_setup(idle, cpu);

I suggest you put this in smpboot.c someplace ;-)
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