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

[Nicolas Pitre]

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;
 }


Nicolas