linux-kernel - Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM

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Message-ID: <BANLkTincaA75JR3V2Fd6MDCw6yMhUmDdWA@mail.gmail.com>
Date:	Fri, 27 May 2011 15:01:38 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Ingo Molnar <mingo@...e.hu>,
	Frank Rowand <frank.rowand@...sony.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()"
 locks up on ARM

On Fri, May 27, 2011 at 1:23 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> We'd end up with something like the below, which isn't too different
> from what I've now got queued.
>
> It has the extra cpu == smp_processor_id() check, but I'm not sure this
> whole case is worth the trouble. I could go stick some counters in to
> verify how often all this happens I guess.
>
> ---
>  arch/x86/include/asm/system.h |    2 ++
>  kernel/sched.c                |   14 +++++++++++---
>  kernel/sched_debug.c          |    7 +++++++
>  3 files changed, 20 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
> index c2ff2a1..2c597e8 100644
> --- a/arch/x86/include/asm/system.h
> +++ b/arch/x86/include/asm/system.h
> @@ -10,6 +10,8 @@
>  #include <linux/kernel.h>
>  #include <linux/irqflags.h>
>
> +#define __ARCH_WANT_INTERRUPTS_ON_CTXSW
> +
>  /* entries in ARCH_DLINFO: */
>  #if defined(CONFIG_IA32_EMULATION) || !defined(CONFIG_X86_64)
>  # define AT_VECTOR_SIZE_ARCH 2
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 2d12893..e4f7a9f 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2636,9 +2636,17 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>                 * to spin on ->on_cpu if p is current, since that would
>                 * deadlock.
>                 */
> -               if (p == current) {
> -                       ttwu_queue(p, cpu);
> -                       goto stat;
> +               if (cpu == smp_processor_id()) {
> +                       struct rq *rq;
> +
> +                       rq = __task_rq_lock(p);
> +                       if (p->on_cpu) {

As Oleg has said, I also think we don't need this check.

> +                               ttwu_activate(rq, p, ENQUEUE_WAKEUP);
> +                               ttwu_do_wakeup(rq, p, wake_flags);

And the difference with ttwu_queue() is ttwu_queue() calls
ttwu_activate() with another flag ENQUEUE_WAKING, so if
we call ->task_waking() before  ttwu_queue(), I guess it will work
too.
But I like this version, because we call ->task_waking() and
ttwu_activate() on the local cpu, that means the calculations on
vruntime in that two functions are accumulated into noop.

Thanks,
Yong

> +                               __task_rq_unlock(rq);
> +                               goto stat;
> +                       }
> +                       __task_rq_unlock(rq);
>                }
>  #endif
>                cpu_relax();
> diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c
> index a6710a1..f0ff1de 100644
> --- a/kernel/sched_debug.c
> +++ b/kernel/sched_debug.c
> @@ -332,6 +332,13 @@ static int sched_debug_show(struct seq_file *m, void *v)
>                (int)strcspn(init_utsname()->version, " "),
>                init_utsname()->version);
>
> +#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
> +       SEQ_printf(m, "__ARCH_WANT_INTERRUPTS_ON_CTXSW\n");
> +#endif
> +#ifdef __ARCH_WANT_UNLOCKED_CTXSW
> +       SEQ_printf(m, "__ARCH_WANT_UNLOCKED_CTXSW\n");
> +#endif
> +
>  #define P(x) \
>        SEQ_printf(m, "%-40s: %Ld\n", #x, (long long)(x))
>  #define PN(x) \
>
>
>



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