[PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP systems#
Russell King - ARM Linux
linux at arm.linux.org.uk
Fri May 9 02:18:24 PDT 2014
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),
> + /*
> + * 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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