linux-kernel - Re: [patch] sched: make cpu_clock() not use the rq clock

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Date:	Sat, 28 Jul 2007 23:03:08 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] sched: make cpu_clock() not use the rq clock

Ingo Molnar wrote:
> Subject: sched: make cpu_clock() not use the rq clock
>   

This subject doesn't make much sense to me.  All this patch does is get
the rq's current time under irq_disable rather than spinlock, right?

    J

> From: Ingo Molnar <mingo@...e.hu>
>
> it is enough to disable interrupts to get the precise rq-clock
> of the local CPU.
>
> this also solves an NMI watchdog regression: the NMI watchdog
> calls touch_softlockup_watchdog(), which might deadlock on
> rq->lock if the NMI hits an rq-locked critical section.
>
> Signed-off-by: Ingo Molnar <mingo@...e.hu>
> ---
>  kernel/sched.c |    7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> Index: linux-rt-rebase.q/kernel/sched.c
> ===================================================================
> --- linux-rt-rebase.q.orig/kernel/sched.c
> +++ linux-rt-rebase.q/kernel/sched.c
> @@ -446,13 +446,12 @@ static inline unsigned long long rq_cloc
>   */
>  unsigned long long cpu_clock(int cpu)
>  {
> -	struct rq *rq = cpu_rq(cpu);
>  	unsigned long long now;
>  	unsigned long flags;
>  
> -	spin_lock_irqsave(&rq->lock, flags);
> -	now = rq_clock(rq);
> -	spin_unlock_irqrestore(&rq->lock, flags);
> +	local_irq_save(flags);
> +	now = rq_clock(cpu_rq(cpu));
> +	local_irq_restore(flags);
>  
>  	return now;
>  }
> -
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