Re: CONFIG_NO_HZ breaks blktrace timestamps

From: Guillaume Chazarain
Date: Thu Jan 10 2008 - 17:46:02 EST

David Dillow <dillowda@xxxxxxxx> wrote:

> At the moment, I'm not sure how to track this farther, or how to fix it
> properly. Any advice would be appreciated.

Just out of curiosity, could you try the appended cumulative patch and
report .clock_warps, .clock_overflows and .clock_underflows as you did.

Thanks.

commit 20fa02359d971bdb820d238184fabd42d8018e4f
Author: Guillaume Chazarain <guichaz@xxxxxxxx>
Date: Thu Jan 10 23:36:43 2008 +0100

sched: monitor clock underflows in /proc/sched_debug

We monitor clock overflows, let's also monitor clock underflows.

Signed-off-by: Guillaume Chazarain <guichaz@xxxxxxxx>

diff --git a/kernel/sched.c b/kernel/sched.c
index 37cf07a..cab9756 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -317,7 +317,7 @@ struct rq {
u64 clock, prev_clock_raw;
s64 clock_max_delta;

- unsigned int clock_warps, clock_overflows;
+ unsigned int clock_warps, clock_overflows, clock_underflows;
u64 idle_clock;
unsigned int clock_deep_idle_events;
u64 tick_timestamp;
@@ -3485,8 +3485,10 @@ void scheduler_tick(void)
/*
* Let rq->clock advance by at least TICK_NSEC:
*/
- if (unlikely(rq->clock < next_tick))
+ if (unlikely(rq->clock < next_tick)) {
rq->clock = next_tick;
+ rq->clock_underflows++;
+ }
rq->tick_timestamp = rq->clock;
update_cpu_load(rq);
if (curr != rq->idle) /* FIXME: needed? */
diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c
index 80fbbfc..9e5de09 100644
--- a/kernel/sched_debug.c
+++ b/kernel/sched_debug.c
@@ -179,6 +179,7 @@ static void print_cpu(struct seq_file *m, int cpu)
PN(prev_clock_raw);
P(clock_warps);
P(clock_overflows);
+ P(clock_underflows);
P(clock_deep_idle_events);
PN(clock_max_delta);
P(cpu_load[0]);

commit c146421cae64bb626714dc951fa39b55d2f819c1
Author: Guillaume Chazarain <guichaz@xxxxxxxx>
Date: Wed Jan 2 14:10:17 2008 +0100

commit 60c6397ce4e8c9fd7feaeaef4167ace71c3949c8

x86: scale cyc_2_nsec according to CPU frequency

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

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

Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

diff --git a/arch/x86/kernel/tsc_32.c b/arch/x86/kernel/tsc_32.c
index 9ebc0da..00bb4c1 100644
--- a/arch/x86/kernel/tsc_32.c
+++ b/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>
@@ -80,13 +81,31 @@ EXPORT_SYMBOL_GPL(check_tsc_unstable);
*
* -johnstul@xxxxxxxxxx "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 +258,9 @@ time_cpufreq_notifier(struct notifier_block *nb, unsigned long val, void *data)
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 +388,8 @@ static inline void check_geode_tsc_reliable(void) { }

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

@@ -380,7 +403,15 @@ 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 */
diff --git a/arch/x86/kernel/tsc_64.c b/arch/x86/kernel/tsc_64.c
index 9c70af4..32edd2c 100644
--- a/arch/x86/kernel/tsc_64.c
+++ b/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 +19,48 @@ 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@xxxxxxxxxx) 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@xxxxxxxxxx)
+ *
+ * -johnstul@xxxxxxxxxx "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 +133,9 @@ static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
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 +186,7 @@ static unsigned long __init tsc_read_refs(unsigned long *pm,
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 +241,9 @@ 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);
}

/*
diff --git a/include/asm-x86/timer.h b/include/asm-x86/timer.h
index 0db7e99..4f6fcb0 100644
--- a/include/asm-x86/timer.h
+++ b/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@xxxxxxxxxx)
*
* -johnstul@xxxxxxxxxx "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

commit 235fdf2d607e7139d3a7392506996a476229d12e
Author: Guillaume Chazarain <guichaz@xxxxxxxx>
Date: Thu Jan 10 23:36:44 2008 +0100

sched: fix rq->clock warps on frequency changes

Fix 2bacec8c318ca0418c0ee9ac662ee44207765dd4
(sched: touch softlockup watchdog after idling) that reintroduced warps
on frequency changes. touch_softlockup_watchdog() calls __update_rq_clock
that checks rq->clock for warps, so call it after adjusting rq->clock.

Signed-off-by: Guillaume Chazarain <guichaz@xxxxxxxx>

diff --git a/kernel/sched.c b/kernel/sched.c
index cab9756..eee610d 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -668,7 +668,6 @@ void sched_clock_idle_wakeup_event(u64 delta_ns)
struct rq *rq = cpu_rq(smp_processor_id());
u64 now = sched_clock();

- touch_softlockup_watchdog();
rq->idle_clock += delta_ns;
/*
* Override the previous timestamp and ignore all
@@ -680,6 +679,7 @@ void sched_clock_idle_wakeup_event(u64 delta_ns)
rq->prev_clock_raw = now;
rq->clock += delta_ns;
spin_unlock(&rq->lock);
+ touch_softlockup_watchdog();
}
EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);


commit 241a8c3fb4d6fd452518a8e4e8eeabe9696c82aa
Author: Guillaume Chazarain <guichaz@xxxxxxxx>
Date: Thu Jan 10 23:36:45 2008 +0100

sched: Fix rq->clock overflows detection with CONFIG_NO_HZ

When using CONFIG_NO_HZ, rq->tick_timestamp is not updated every TICK_NSEC.
We check that the number of skipped ticks matches the clock jump seen in
__update_rq_clock().


Signed-off-by: Guillaume Chazarain <guichaz@xxxxxxxx>

diff --git a/kernel/sched.c b/kernel/sched.c
index eee610d..6f1271d 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -288,6 +288,7 @@ struct rq {
unsigned long cpu_load[CPU_LOAD_IDX_MAX];
unsigned char idle_at_tick;
#ifdef CONFIG_NO_HZ
+ unsigned long last_tick_seen;
unsigned char in_nohz_recently;
#endif
/* capture load from *all* tasks on this cpu: */
@@ -379,6 +380,32 @@ static inline int cpu_of(struct rq *rq)
#endif
}

+#ifdef CONFIG_NO_HZ
+static inline bool nohz_on(int cpu)
+{
+ return tick_get_tick_sched(cpu)->nohz_mode != NOHZ_MODE_INACTIVE;
+}
+
+static inline u64 max_skipped_ticks(struct rq *rq)
+{
+ return nohz_on(cpu_of(rq)) ? jiffies - rq->last_tick_seen + 2 : 1;
+}
+
+static inline void update_last_tick_seen(struct rq *rq)
+{
+ rq->last_tick_seen = jiffies;
+}
+#else
+static inline u64 max_skipped_ticks(struct rq *rq)
+{
+ return 1;
+}
+
+static inline void update_last_tick_seen(struct rq *rq)
+{
+}
+#endif
+
/*
* Update the per-runqueue clock, as finegrained as the platform can give
* us, but without assuming monotonicity, etc.:
@@ -403,9 +430,12 @@ static void __update_rq_clock(struct rq *rq)
/*
* Catch too large forward jumps too:
*/
- if (unlikely(clock + delta > rq->tick_timestamp + TICK_NSEC)) {
- if (clock < rq->tick_timestamp + TICK_NSEC)
- clock = rq->tick_timestamp + TICK_NSEC;
+ u64 max_jump = max_skipped_ticks(rq) * TICK_NSEC;
+ u64 max_time = rq->tick_timestamp + max_jump;
+
+ if (unlikely(clock + delta > max_time)) {
+ if (clock < max_time)
+ clock = max_time;
else
clock++;
rq->clock_overflows++;
@@ -3490,6 +3520,7 @@ void scheduler_tick(void)
rq->clock_underflows++;
}
rq->tick_timestamp = rq->clock;
+ update_last_tick_seen(rq);
update_cpu_load(rq);
if (curr != rq->idle) /* FIXME: needed? */
curr->sched_class->task_tick(rq, curr);
@@ -6762,6 +6793,7 @@ void __init sched_init(void)
lockdep_set_class(&rq->lock, &rq->rq_lock_key);
rq->nr_running = 0;
rq->clock = 1;
+ update_last_tick_seen(rq);
init_cfs_rq(&rq->cfs, rq);
#ifdef CONFIG_FAIR_GROUP_SCHED
INIT_LIST_HEAD(&rq->leaf_cfs_rq_list);

commit 437f7a5480a0916e5b0877e04ca87f998ef3f3e2
Author: Guillaume Chazarain <guichaz@xxxxxxxx>
Date: Thu Jan 10 23:36:45 2008 +0100

sched: make sure jiffies is up to date before calling __update_rq_clock()

Now that __update_rq_clock() uses jiffies to detect clock overflows, make sure
jiffies are up to date before touch_softlockup_watchdog().

Removed a touch_softlockup_watchdog() call becoming redundant with the added
tick_nohz_update_jiffies().

Signed-off-by: Guillaume Chazarain <guichaz@xxxxxxxx>

diff --git a/kernel/sched.c b/kernel/sched.c
index 6f1271d..f6b3160 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -63,6 +63,7 @@
#include <linux/reciprocal_div.h>
#include <linux/unistd.h>
#include <linux/pagemap.h>
+#include <linux/tick.h>

#include <asm/tlb.h>
#include <asm/irq_regs.h>
@@ -709,7 +710,7 @@ void sched_clock_idle_wakeup_event(u64 delta_ns)
rq->prev_clock_raw = now;
rq->clock += delta_ns;
spin_unlock(&rq->lock);
- touch_softlockup_watchdog();
+ tick_nohz_update_jiffies();
}
EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index cb89fa8..2190593 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -133,14 +133,13 @@ void tick_nohz_update_jiffies(void)
if (!ts->tick_stopped)
return;

- touch_softlockup_watchdog();
-
cpu_clear(cpu, nohz_cpu_mask);
now = ktime_get();

local_irq_save(flags);
tick_do_update_jiffies64(now);
local_irq_restore(flags);
+ touch_softlockup_watchdog();
}

/**

commit 63a885d40031190e17860c899446376b0fceefa8
Author: Guillaume Chazarain <guichaz@xxxxxxxx>
Date: Thu Jan 10 23:36:45 2008 +0100

scheduler_tick() is not called every jiffies

diff --git a/kernel/sched.c b/kernel/sched.c
index f6b3160..9f73005 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -387,6 +387,11 @@ static inline bool nohz_on(int cpu)
return tick_get_tick_sched(cpu)->nohz_mode != NOHZ_MODE_INACTIVE;
}

+static inline u64 min_skipped_ticks(struct rq *rq)
+{
+ return nohz_on(cpu_of(rq)) ? jiffies - rq->last_tick_seen : 1;
+}
+
static inline u64 max_skipped_ticks(struct rq *rq)
{
return nohz_on(cpu_of(rq)) ? jiffies - rq->last_tick_seen + 2 : 1;
@@ -397,6 +402,11 @@ static inline void update_last_tick_seen(struct rq *rq)
rq->last_tick_seen = jiffies;
}
#else
+static inline u64 min_skipped_ticks(struct rq *rq)
+{
+ return 1;
+}
+
static inline u64 max_skipped_ticks(struct rq *rq)
{
return 1;
@@ -3498,25 +3508,23 @@ void account_steal_time(struct task_struct *p, cputime_t steal)
}

/*
- * This function gets called by the timer code, with HZ frequency.
+ * This function gets called by the timer code with HZ frequency or less when
+ * using CONFIG_NO_HZ.
* We call it with interrupts disabled.
- *
- * It also gets called by the fork code, when changing the parent's
- * timeslices.
*/
void scheduler_tick(void)
{
int cpu = smp_processor_id();
struct rq *rq = cpu_rq(cpu);
struct task_struct *curr = rq->curr;
- u64 next_tick = rq->tick_timestamp + TICK_NSEC;
+ u64 next_tick = rq->tick_timestamp + min_skipped_ticks(rq) * TICK_NSEC;

spin_lock(&rq->lock);
__update_rq_clock(rq);
/*
- * Let rq->clock advance by at least TICK_NSEC:
+ * Let rq->clock advance by at least the number of elapsed ticks:
*/
- if (unlikely(rq->clock < next_tick)) {
+ if (unlikely(rq->clock < next_tick - nohz_on(cpu) * TICK_NSEC)) {
rq->clock = next_tick;
rq->clock_underflows++;
}


--
Guillaume
--
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