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SubjectRe: [PATCH] Extend mwait idle to optimize away CAL and RES interrupts to an idle CPU -v1
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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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