[PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP systems
dianders at chromium.org
Thu May 8 21:43:04 PDT 2014
On Thu, May 8, 2014 at 6:37 PM, Nicolas Pitre <nicolas.pitre at linaro.org> 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),
> + /*
> + * 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");
I would be OK with that. At least someone would have a clue what to do.
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