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

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Message-ID: <20140123094804.GP30183@twins.programming.kicks-ass.net>
Date:	Thu, 23 Jan 2014 10:48:04 +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

On Wed, Jan 22, 2014 at 10:17:40PM +0100, Markus Trippelsdorf wrote:
> Yes. Thanks Peter.
> 

Ah much simpler patch that should have the same effect:

---
Subject: sched/x86/tsc: Initialize multiplier to 0
From: Peter Zijlstra <peterz@...radead.org>
Date: Wed, 22 Jan 2014 22:08:14 +0100

Since we keep the clock value linearly continuous on frequency change,
make sure the initial multiplier is 0, such that out initial value is
0. Without this we compute the initial value at whatever the TSC has
managed to reach since power-on.

Fixes: 20d1c86a57762 ("sched/clock, x86: Rewrite cyc2ns() to avoid the need to disable IRQs")
Cc: lenb@...nel.org
Cc: rjw@...ysocki.net
Cc: Eliezer Tamir <eliezer.tamir@...ux.intel.com>
Cc: rui.zhang@...el.com
Cc: jacob.jun.pan@...ux.intel.com
Cc: Mike Galbraith <bitbucket@...ine.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: hpa@...or.com
Cc: paulmck@...ux.vnet.ibm.com
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: John Stultz <john.stultz@...aro.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Sasha Levin <sasha.levin@...cle.com>
Cc: dyoung@...hat.com
Reported-by: Markus Trippelsdorf <markus@...ppelsdorf.de>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
---
 arch/x86/kernel/tsc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -180,7 +180,7 @@ static void cyc2ns_write_end(int cpu, st
 
 static void cyc2ns_data_init(struct cyc2ns_data *data)
 {
-	data->cyc2ns_mul = 1U << CYC2NS_SCALE_FACTOR;
+	data->cyc2ns_mul = 0;
 	data->cyc2ns_shift = CYC2NS_SCALE_FACTOR;
 	data->cyc2ns_offset = 0;
 	data->__count = 0;
--
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