linux-kernel - Re: [PATCH] ARM: Don't ever downscale loops_per

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Message-ID: <alpine.LFD.2.11.1405082058130.980@knanqh.ubzr>
Date:	Thu, 8 May 2014 21:37:15 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Doug Anderson <dianders@...omium.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Will Deacon <will.deacon@....com>,
	John Stultz <john.stultz@...aro.org>,
	David Riley <davidriley@...omium.org>,
	"olof@...om.net" <olof@...om.net>,
	Sonny Rao <sonnyrao@...omium.org>,
	Richard Zhao <richard.zhao@...aro.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Shawn Guo <shawn.guo@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Stephen Warren <swarren@...dia.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: Don't ever downscale loops_per_jiffy in SMP
 systems

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

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