linux-kernel - Re: [PATCH 13/15] sched: Use a static_key for sched_clock

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Date:	Wed, 22 Jan 2014 22:08:14 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Arjan van de Ven <arjan@...ux.intel.com>, lenb@...nel.org,
	rjw@...ysocki.net, Eliezer Tamir <eliezer.tamir@...ux.intel.com>,
	rui.zhang@...el.com, jacob.jun.pan@...ux.intel.com,
	Mike Galbraith <bitbucket@...ine.de>,
	Ingo Molnar <mingo@...nel.org>, hpa@...or.com,
	paulmck@...ux.vnet.ibm.com, Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Andy Lutomirski <luto@...capital.net>,
	linux-kernel@...r.kernel.org, dyoung@...hat.com
Subject: Re: [PATCH 13/15] sched: Use a static_key for sched_clock_stable

> 
> I think its the right region to look through. My current suspect is the
> linear continuity fit with the initial 'random' multiplier.
> 
> That initial 'random' multiplier can get us quite high, and we'll fit
> the function to match that but continue at a sane rate.
> 
> I'll try and prod a little more later this evening as time permits.

Does this cure things?

---
 arch/x86/kernel/tsc.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index a3acbac2ee72..bb04148c5fe0 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -237,7 +237,7 @@ static inline unsigned long long cycles_2_ns(unsigned long long cyc)
 /* XXX surely we already have this someplace in the kernel?! */
 #define DIV_ROUND(n, d) (((n) + ((d) / 2)) / (d))
 
-static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
+static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu, bool origin)
 {
 	unsigned long long tsc_now, ns_now;
 	struct cyc2ns_data *data;
@@ -252,7 +252,10 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
 	data = cyc2ns_write_begin(cpu);
 
 	rdtscll(tsc_now);
-	ns_now = cycles_2_ns(tsc_now);
+	if (origin)
+		ns_now = 0;
+	else
+		ns_now = cycles_2_ns(tsc_now);
 
 	/*
 	 * Compute a new multiplier as per the above comment and ensure our
@@ -926,7 +929,7 @@ static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
 			mark_tsc_unstable("cpufreq changes");
 	}
 
-	set_cyc2ns_scale(tsc_khz, freq->cpu);
+	set_cyc2ns_scale(tsc_khz, freq->cpu, false);
 
 	return 0;
 }
@@ -1199,7 +1202,7 @@ void __init tsc_init(void)
 	 */
 	for_each_possible_cpu(cpu) {
 		cyc2ns_init(cpu);
-		set_cyc2ns_scale(cpu_khz, cpu);
+		set_cyc2ns_scale(cpu_khz, cpu, true);
 	}
 
 	if (tsc_disabled > 0)
--
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