Re: [PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP systems#

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Thu, May 08, 2014 at 09:37:15PM -0400, Nicolas Pitre wrote:
> On Thu, 8 May 2014, Russell King - ARM Linux wrote:
> 
> > If you're in a preempt or SMP environment, provide a timer for udelay().
> > IF you're in an environment with IRQs which can take a long time, use
> > a timer for udelay().  If you're in an environment where the CPU clock
> > can change unexpectedly, use a timer for udelay().
> 
> Longer delays are normally not a problem.  If they are, then simply 
> disabling IRQs may solve it if absolutely required.  With much shorter 
> delays than expected this is another story.
> 
> What about the following:
> 
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 7c4fada440..10030cc5a0 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -682,6 +682,15 @@ static int cpufreq_callback(struct notifier_block *nb,
>  			cpufreq_scale(per_cpu(l_p_j_ref, cpu),
>  					per_cpu(l_p_j_ref_freq, cpu),
>  					freq->new);
> +		/*
> +		 * Another CPU might have called udelay() just before LPJ
> +		 * and a shared CPU clock is increased.  That other CPU still
> +		 * looping on the old LPJ value would return significantly
> +		 * sooner than expected.  The actual fix is to provide a
> +		 * timer based udelay() implementation instead.
> +		 */
> +		if (freq->old < freq->new)
> +			pr_warn_once("*** udelay() on SMP is racy and may be much shorter than expected ***\n");
>  	}
>  	return NOTIFY_OK;
>  }

No, because you're assuming this is just a SMP problem.  What about
preempt, where you could preempt away from a udelay loop to change
the CPU frequency, and then back again, possibly resulting in the
CPU clock rate increasing and maybe a shorter delay if the switch
from-change-clock-and-back is fast enough?  Remember that udelay()
can be used for up to 2ms delays.

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