From mboxrd@z Thu Jan 1 00:00:00 1970 From: linux@arm.linux.org.uk (Russell King - ARM Linux) Date: Fri, 9 May 2014 10:18:24 +0100 Subject: [PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP systems# In-Reply-To: References: <1399504982-31181-1-git-send-email-dianders@chromium.org> <20140508192209.GH3693@n2100.arm.linux.org.uk> <20140508205223.GI3693@n2100.arm.linux.org.uk> Message-ID: <20140509091824.GL3693@n2100.arm.linux.org.uk> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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. -- FTTC broadband for 0.8mile line: now at 9.7Mbps down 460kbps up... slowly improving, and getting towards what was expected from it.