From mboxrd@z Thu Jan 1 00:00:00 1970 From: nicolas.pitre@linaro.org (Nicolas Pitre) Date: Thu, 8 May 2014 21:37:15 -0400 (EDT) Subject: [PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP systems In-Reply-To: <20140508205223.GI3693@n2100.arm.linux.org.uk> 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: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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