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SubjectRe: [patch] x86: scale cyc_2_nsec according to CPU frequency

* Guillaume Chazarain <guichaz@yahoo.fr> wrote:

> Le Fri, 7 Dec 2007 14:55:25 +0100,
> Ingo Molnar <mingo@elte.hu> a ??crit :
>
> > Firstly, we dont need the 'offset' anymore because cpu_clock()
> > maintains offsets itself.
>
> Yes, but a lower quality one. __update_rq_clock tries to compensate
> large jumping clocks with a jiffy resolution, while my offset arranges
> for a very smooth frequency transition.

yes, but that would be easy to fix up via calling
sched_clock_idle_wakeup_event(0) when doing a frequency transition,
without burdening the normal sched_clock() codepath with the offset. See
the attached latest version.

Ingo

--------------->
Subject: x86: scale cyc_2_nsec according to CPU frequency
From: "Guillaume Chazarain" <guichaz@yahoo.fr>

scale the sched_clock() cyc_2_nsec scaling factor according to
CPU frequency changes.

[ mingo@elte.hu: simplified it and fixed it for SMP. ]

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/x86/kernel/tsc_32.c | 45 +++++++++++++++++++++++++++++++----
arch/x86/kernel/tsc_64.c | 59 +++++++++++++++++++++++++++++++++++++++--------
include/asm-x86/timer.h | 23 ++++++++++++++----
3 files changed, 106 insertions(+), 21 deletions(-)

Index: linux-x86.q/arch/x86/kernel/tsc_32.c
===================================================================
--- linux-x86.q.orig/arch/x86/kernel/tsc_32.c
+++ linux-x86.q/arch/x86/kernel/tsc_32.c
@@ -5,6 +5,7 @@
#include <linux/jiffies.h>
#include <linux/init.h>
#include <linux/dmi.h>
+#include <linux/percpu.h>

#include <asm/delay.h>
#include <asm/tsc.h>
@@ -78,15 +79,35 @@ EXPORT_SYMBOL_GPL(check_tsc_unstable);
* cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits.
* (mathieu.desnoyers@polymtl.ca)
*
+ * ns += offset to avoid sched_clock jumps with cpufreq
+ *
* -johnstul@us.ibm.com "math is hard, lets go shopping!"
*/
-unsigned long cyc2ns_scale __read_mostly;

-#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */
+DEFINE_PER_CPU(unsigned long, cyc2ns);

-static inline void set_cyc2ns_scale(unsigned long cpu_khz)
+static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
{
- cyc2ns_scale = (1000000 << CYC2NS_SCALE_FACTOR)/cpu_khz;
+ unsigned long flags, prev_scale, *scale;
+ unsigned long long tsc_now, ns_now;
+
+ local_irq_save(flags);
+ sched_clock_idle_sleep_event();
+
+ scale = &per_cpu(cyc2ns, cpu);
+
+ rdtscll(tsc_now);
+ ns_now = __cycles_2_ns(tsc_now);
+
+ prev_scale = *scale;
+ if (cpu_khz)
+ *scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;
+
+ /*
+ * Start smoothly with the new frequency:
+ */
+ sched_clock_idle_wakeup_event(0);
+ local_irq_restore(flags);
}

/*
@@ -239,7 +260,9 @@ time_cpufreq_notifier(struct notifier_bl
ref_freq, freq->new);
if (!(freq->flags & CPUFREQ_CONST_LOOPS)) {
tsc_khz = cpu_khz;
- set_cyc2ns_scale(cpu_khz);
+ preempt_disable();
+ set_cyc2ns_scale(cpu_khz, smp_processor_id());
+ preempt_enable();
/*
* TSC based sched_clock turns
* to junk w/ cpufreq
@@ -367,6 +390,8 @@ static inline void check_geode_tsc_relia

void __init tsc_init(void)
{
+ int cpu;
+
if (!cpu_has_tsc || tsc_disable)
goto out_no_tsc;

@@ -380,7 +405,15 @@ void __init tsc_init(void)
(unsigned long)cpu_khz / 1000,
(unsigned long)cpu_khz % 1000);

- set_cyc2ns_scale(cpu_khz);
+ /*
+ * Secondary CPUs do not run through tsc_init(), so set up
+ * all the scale factors for all CPUs, assuming the same
+ * speed as the bootup CPU. (cpufreq notifiers will fix this
+ * up if their speed diverges)
+ */
+ for_each_possible_cpu(cpu)
+ set_cyc2ns_scale(cpu_khz, cpu);
+
use_tsc_delay();

/* Check and install the TSC clocksource */
Index: linux-x86.q/arch/x86/kernel/tsc_64.c
===================================================================
--- linux-x86.q.orig/arch/x86/kernel/tsc_64.c
+++ linux-x86.q/arch/x86/kernel/tsc_64.c
@@ -10,6 +10,7 @@

#include <asm/hpet.h>
#include <asm/timex.h>
+#include <asm/timer.h>

static int notsc __initdata = 0;

@@ -18,16 +19,50 @@ EXPORT_SYMBOL(cpu_khz);
unsigned int tsc_khz;
EXPORT_SYMBOL(tsc_khz);

-static unsigned int cyc2ns_scale __read_mostly;
+/* Accelerators for sched_clock()
+ * convert from cycles(64bits) => nanoseconds (64bits)
+ * basic equation:
+ * ns = cycles / (freq / ns_per_sec)
+ * ns = cycles * (ns_per_sec / freq)
+ * ns = cycles * (10^9 / (cpu_khz * 10^3))
+ * ns = cycles * (10^6 / cpu_khz)
+ *
+ * Then we use scaling math (suggested by george@mvista.com) to get:
+ * ns = cycles * (10^6 * SC / cpu_khz) / SC
+ * ns = cycles * cyc2ns_scale / SC
+ *
+ * And since SC is a constant power of two, we can convert the div
+ * into a shift.
+ *
+ * We can use khz divisor instead of mhz to keep a better precision, since
+ * cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits.
+ * (mathieu.desnoyers@polymtl.ca)
+ *
+ * ns += offset to avoid sched_clock jumps with cpufreq
+ *
+ * -johnstul@us.ibm.com "math is hard, lets go shopping!"
+ */
+DEFINE_PER_CPU(unsigned long, cyc2ns);

-static inline void set_cyc2ns_scale(unsigned long khz)
+static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
{
- cyc2ns_scale = (NSEC_PER_MSEC << NS_SCALE) / khz;
-}
+ unsigned long flags, prev_scale, *scale;
+ unsigned long long tsc_now, ns_now;

-static unsigned long long cycles_2_ns(unsigned long long cyc)
-{
- return (cyc * cyc2ns_scale) >> NS_SCALE;
+ local_irq_save(flags);
+ sched_clock_idle_sleep_event();
+
+ scale = &per_cpu(cyc2ns, cpu);
+
+ rdtscll(tsc_now);
+ ns_now = __cycles_2_ns(tsc_now);
+
+ prev_scale = *scale;
+ if (cpu_khz)
+ *scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;
+
+ sched_clock_idle_wakeup_event(0);
+ local_irq_restore(flags);
}

unsigned long long sched_clock(void)
@@ -100,7 +135,9 @@ static int time_cpufreq_notifier(struct
mark_tsc_unstable("cpufreq changes");
}

- set_cyc2ns_scale(tsc_khz_ref);
+ preempt_disable();
+ set_cyc2ns_scale(tsc_khz_ref, smp_processor_id());
+ preempt_enable();

return 0;
}
@@ -151,7 +188,7 @@ static unsigned long __init tsc_read_ref
void __init tsc_calibrate(void)
{
unsigned long flags, tsc1, tsc2, tr1, tr2, pm1, pm2, hpet1, hpet2;
- int hpet = is_hpet_enabled();
+ int hpet = is_hpet_enabled(), cpu;

local_irq_save(flags);

@@ -206,7 +243,9 @@ void __init tsc_calibrate(void)
}

tsc_khz = tsc2 / tsc1;
- set_cyc2ns_scale(tsc_khz);
+
+ for_each_possible_cpu(cpu)
+ set_cyc2ns_scale(tsc_khz, cpu);
}

/*
Index: linux-x86.q/include/asm-x86/timer.h
===================================================================
--- linux-x86.q.orig/include/asm-x86/timer.h
+++ linux-x86.q/include/asm-x86/timer.h
@@ -2,6 +2,7 @@
#define _ASMi386_TIMER_H
#include <linux/init.h>
#include <linux/pm.h>
+#include <linux/percpu.h>

#define TICK_SIZE (tick_nsec / 1000)

@@ -16,7 +17,7 @@ extern int recalibrate_cpu_khz(void);
#define calculate_cpu_khz() native_calculate_cpu_khz()
#endif

-/* Accellerators for sched_clock()
+/* Accelerators for sched_clock()
* convert from cycles(64bits) => nanoseconds (64bits)
* basic equation:
* ns = cycles / (freq / ns_per_sec)
@@ -31,20 +32,32 @@ extern int recalibrate_cpu_khz(void);
* And since SC is a constant power of two, we can convert the div
* into a shift.
*
- * We can use khz divisor instead of mhz to keep a better percision, since
+ * We can use khz divisor instead of mhz to keep a better precision, since
* cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits.
* (mathieu.desnoyers@polymtl.ca)
*
* -johnstul@us.ibm.com "math is hard, lets go shopping!"
*/
-extern unsigned long cyc2ns_scale __read_mostly;
+
+DECLARE_PER_CPU(unsigned long, cyc2ns);

#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */

-static inline unsigned long long cycles_2_ns(unsigned long long cyc)
+static inline unsigned long long __cycles_2_ns(unsigned long long cyc)
{
- return (cyc * cyc2ns_scale) >> CYC2NS_SCALE_FACTOR;
+ return cyc * per_cpu(cyc2ns, smp_processor_id()) >> CYC2NS_SCALE_FACTOR;
}

+static inline unsigned long long cycles_2_ns(unsigned long long cyc)
+{
+ unsigned long long ns;
+ unsigned long flags;
+
+ local_irq_save(flags);
+ ns = __cycles_2_ns(cyc);
+ local_irq_restore(flags);
+
+ return ns;
+}

#endif

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