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Subject[RFC][PATCH] sched_clock_cpu()
From
Date
Hi,

This is my current proposal to replace the rq->clock stuff (and possibly
cpu_clock()).

it _DOESN'T_ boot ;-/ and I seem to have caught a flu that makes my
whole body hurt like hell, so I'm not getting anything done.

Brain dump before I crash:

- architectures that have a 'perfect' hardware clock can set
CONFIG_HAVE_STABLE_CLOCK

- the 'jiffie' window might be superfulous when we update tick_gtod
before the __update_sched_clock() call in sched_clock_tick()

- cpu_clock() might be implemented as:

sched_clock_cpu(smp_processor_id())

if the accuracy proves good enough - how far can TSC drift in a
single jiffie when considering the filtering and idle hooks?

- what other architectures besides x86 would need this?

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
include/linux/sched.h | 29 ++++++
init/main.c | 1
kernel/Makefile | 2
kernel/sched.c | 163 ++----------------------------------
kernel/sched_clock.c | 221 ++++++++++++++++++++++++++++++++++++++++++++++++++
kernel/sched_debug.c | 7 -
kernel/sched_fair.c | 2
7 files changed, 266 insertions(+), 159 deletions(-)

Index: linux-2.6-2/kernel/sched.c
===================================================================
--- linux-2.6-2.orig/kernel/sched.c
+++ linux-2.6-2/kernel/sched.c
@@ -75,16 +75,6 @@
#include <asm/irq_regs.h>

/*
- * Scheduler clock - returns current time in nanosec units.
- * This is default implementation.
- * Architectures and sub-architectures can override this.
- */
-unsigned long long __attribute__((weak)) sched_clock(void)
-{
- return (unsigned long long)jiffies * (NSEC_PER_SEC / HZ);
-}
-
-/*
* Convert user-nice values [ -20 ... 0 ... 19 ]
* to static priority [ MAX_RT_PRIO..MAX_PRIO-1 ],
* and back.
@@ -560,13 +550,7 @@ struct rq {
unsigned long next_balance;
struct mm_struct *prev_mm;

- u64 clock, prev_clock_raw;
- s64 clock_max_delta;
-
- unsigned int clock_warps, clock_overflows, clock_underflows;
- u64 idle_clock;
- unsigned int clock_deep_idle_events;
- u64 tick_timestamp;
+ u64 clock;

atomic_t nr_iowait;

@@ -631,82 +615,6 @@ 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.:
- */
-static void __update_rq_clock(struct rq *rq)
-{
- u64 prev_raw = rq->prev_clock_raw;
- u64 now = sched_clock();
- s64 delta = now - prev_raw;
- u64 clock = rq->clock;
-
-#ifdef CONFIG_SCHED_DEBUG
- WARN_ON_ONCE(cpu_of(rq) != smp_processor_id());
-#endif
- /*
- * Protect against sched_clock() occasionally going backwards:
- */
- if (unlikely(delta < 0)) {
- clock++;
- rq->clock_warps++;
- } else {
- /*
- * Catch too large forward jumps too:
- */
- 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++;
- } else {
- if (unlikely(delta > rq->clock_max_delta))
- rq->clock_max_delta = delta;
- clock += delta;
- }
- }
-
- rq->prev_clock_raw = now;
- rq->clock = clock;
-}
-
-static void update_rq_clock(struct rq *rq)
-{
- if (likely(smp_processor_id() == cpu_of(rq)))
- __update_rq_clock(rq);
-}
-
/*
* The domain tree (rq->sd) is protected by RCU's quiescent state transition.
* See detach_destroy_domains: synchronize_sched for details.
@@ -722,6 +630,11 @@ static void update_rq_clock(struct rq *r
#define task_rq(p) cpu_rq(task_cpu(p))
#define cpu_curr(cpu) (cpu_rq(cpu)->curr)

+static inline void update_rq_clock(struct rq *rq)
+{
+ rq->clock = sched_clock_cpu(cpu_of(rq));
+}
+
/*
* Tunables that become constants when CONFIG_SCHED_DEBUG is off:
*/
@@ -935,7 +848,6 @@ static unsigned long long __cpu_clock(in
{
unsigned long long now;
unsigned long flags;
- struct rq *rq;

/*
* Only call sched_clock() if the scheduler has already been
@@ -945,9 +857,7 @@ static unsigned long long __cpu_clock(in
return 0;

local_irq_save(flags);
- rq = cpu_rq(cpu);
- update_rq_clock(rq);
- now = rq->clock;
+ now = sched_clock_cpu(cpu);
local_irq_restore(flags);

return now;
@@ -1117,43 +1027,6 @@ static struct rq *this_rq_lock(void)
return rq;
}

-/*
- * We are going deep-idle (irqs are disabled):
- */
-void sched_clock_idle_sleep_event(void)
-{
- struct rq *rq = cpu_rq(smp_processor_id());
-
- spin_lock(&rq->lock);
- __update_rq_clock(rq);
- spin_unlock(&rq->lock);
- rq->clock_deep_idle_events++;
-}
-EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
-
-/*
- * We just idled delta nanoseconds (called with irqs disabled):
- */
-void sched_clock_idle_wakeup_event(u64 delta_ns)
-{
- struct rq *rq = cpu_rq(smp_processor_id());
- u64 now = sched_clock();
-
- rq->idle_clock += delta_ns;
- /*
- * Override the previous timestamp and ignore all
- * sched_clock() deltas that occured while we idled,
- * and use the PM-provided delta_ns to advance the
- * rq clock:
- */
- spin_lock(&rq->lock);
- rq->prev_clock_raw = now;
- rq->clock += delta_ns;
- spin_unlock(&rq->lock);
- touch_softlockup_watchdog();
-}
-EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
-
static void __resched_task(struct task_struct *p, int tif_bit);

static inline void resched_task(struct task_struct *p)
@@ -1275,7 +1148,7 @@ static enum hrtimer_restart hrtick(struc
WARN_ON_ONCE(cpu_of(rq) != smp_processor_id());

spin_lock(&rq->lock);
- __update_rq_clock(rq);
+ update_rq_clock(rq);
rq->curr->sched_class->task_tick(rq, rq->curr, 1);
spin_unlock(&rq->lock);

@@ -4404,19 +4277,11 @@ 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;
+
+ sched_clock_tick();

spin_lock(&rq->lock);
- __update_rq_clock(rq);
- /*
- * Let rq->clock advance by at least TICK_NSEC:
- */
- if (unlikely(rq->clock < next_tick)) {
- rq->clock = next_tick;
- rq->clock_underflows++;
- }
- rq->tick_timestamp = rq->clock;
- update_last_tick_seen(rq);
+ update_rq_clock(rq);
update_cpu_load(rq);
curr->sched_class->task_tick(rq, curr, 0);
spin_unlock(&rq->lock);
@@ -4570,7 +4435,7 @@ need_resched_nonpreemptible:
* Do the rq-clock update outside the rq lock:
*/
local_irq_disable();
- __update_rq_clock(rq);
+ update_rq_clock(rq);
spin_lock(&rq->lock);
clear_tsk_need_resched(prev);

@@ -8149,8 +8014,6 @@ void __init sched_init(void)
spin_lock_init(&rq->lock);
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);
init_rt_rq(&rq->rt, rq);
#ifdef CONFIG_FAIR_GROUP_SCHED
@@ -8294,6 +8157,7 @@ EXPORT_SYMBOL(__might_sleep);
static void normalize_task(struct rq *rq, struct task_struct *p)
{
int on_rq;
+
update_rq_clock(rq);
on_rq = p->se.on_rq;
if (on_rq)
@@ -8325,7 +8189,6 @@ void normalize_rt_tasks(void)
p->se.sleep_start = 0;
p->se.block_start = 0;
#endif
- task_rq(p)->clock = 0;

if (!rt_task(p)) {
/*
Index: linux-2.6-2/kernel/sched_clock.c
===================================================================
--- /dev/null
+++ linux-2.6-2/kernel/sched_clock.c
@@ -0,0 +1,221 @@
+/*
+ * sched_clock for unstable cpu clocks
+ *
+ * Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
+ *
+ * Based on code by:
+ * Ingo Molnar <mingo@redhat.com>
+ * Guillaume Chazarain <guichaz@gmail.com>
+ *
+ * Create a semi stable clock from a mixture of other events, including:
+ * - gtod
+ * - jiffies
+ * - sched_clock()
+ * - explicit idle events
+ *
+ * We use gtod as base and the unstable clock deltas. The deltas are filtered,
+ * making it monotonic and keeping it within an expected window. This window
+ * is set up using jiffies.
+ *
+ * Furthermore, explicit sleep and wakeup hooks allow us to account for time
+ * that is otherwise invisible (TSC gets stopped).
+ *
+ * The clock: sched_clock_cpu() is monotonic per cpu, and should be somewhat
+ * consistent between cpus (never more than 1 jiffies difference).
+ */
+#include <linux/sched.h>
+#include <linux/percpu.h>
+#include <linux/spinlock.h>
+#include <linux/ktime.h>
+#include <linux/module.h>
+
+
+#ifndef CONFIG_HAVE_STABLE_CLOCK
+
+struct sched_clock_data {
+ spinlock_t lock;
+ unsigned long prev_jiffies;
+ u64 prev_raw;
+ u64 tick_raw;
+ u64 tick_gtod;
+ u64 clock;
+};
+
+static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
+
+static inline struct sched_clock_data *this_scd(void)
+{
+ return &__get_cpu_var(sched_clock_data);
+}
+
+static inline struct sched_clock_data *cpu_sdc(int cpu)
+{
+ return &per_cpu(sched_clock_data, cpu);
+}
+
+void sched_clock_init(void)
+{
+ u64 ktime_now = ktime_to_ns(ktime_get());
+ u64 now = 0; // sched_clock();
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ struct sched_clock_data *scd = cpu_sdc(cpu);
+
+ spin_lock_init(&scd->lock);
+ scd->prev_jiffies = jiffies;
+ scd->prev_raw = now;
+ scd->tick_raw = now;
+ scd->tick_gtod = ktime_now;
+ scd->clock = ktime_now;
+ }
+}
+
+/*
+ * update the percpu scd from the raw @now value
+ *
+ * - filter out backward motion
+ * - use jiffies to generate a min,max window to clip the raw values
+ */
+static void __update_sched_clock(struct sched_clock_data *scd, u64 now)
+{
+ unsigned long now_jiffies = jiffies;
+ long delta_jiffies = now_jiffies - scd->prev_jiffies;
+ u64 clock = scd->clock;
+ u64 min_clock, max_clock;
+ s64 delta = now - scd->prev_raw;
+
+ min_clock = scd->tick_gtod + delta_jiffies * TICK_NSEC;
+
+ if (unlikely(delta < 0)) {
+ clock++;
+ goto out;
+ }
+
+ max_clock = min_clock + TICK_NSEC;
+
+ if (unlikely(clock + delta > max_clock)) {
+ if (clock < max_clock)
+ clock = max_clock;
+ else
+ clock++;
+ } else {
+ clock += delta;
+ }
+
+ out:
+ if (unlikely(clock < min_clock))
+ clock = min_clock;
+
+ scd->prev_raw = now;
+ scd->prev_jiffies = now_jiffies;
+ scd->clock = clock;
+}
+
+static void lock_double_clock(struct sched_clock_data *data1,
+ struct sched_clock_data *data2)
+{
+ if (data1 < data2) {
+ spin_lock(&data1->lock);
+ spin_lock_nested(&data2->lock, SINGLE_DEPTH_NESTING);
+ } else {
+ spin_lock(&data2->lock);
+ spin_lock_nested(&data1->lock, SINGLE_DEPTH_NESTING);
+ }
+}
+
+u64 sched_clock_cpu(int cpu)
+{
+ struct sched_clock_data *scd = cpu_sdc(cpu);
+ u64 now = sched_clock();
+ u64 clock;
+
+ if (cpu != smp_processor_id()) {
+ /*
+ * in order to update a remote cpu's clock based on our
+ * unstable raw time rebase it against:
+ * tick_raw (offset between raw counters)
+ * tick_gotd (tick offset between cpus)
+ */
+ struct sched_clock_data *my_scd = this_scd();
+
+ lock_double_clock(scd, my_scd);
+
+ now -= my_scd->tick_raw;
+ now += scd->tick_raw;
+
+ now -= my_scd->tick_gtod;
+ now += scd->tick_gtod;
+
+ spin_unlock(&my_scd->lock);
+ } else
+ spin_lock(&scd->lock);
+
+ __update_sched_clock(scd, now);
+ clock = scd->clock;
+
+ spin_unlock(&scd->lock);
+
+ return clock;
+}
+
+void sched_clock_tick(void)
+{
+ struct sched_clock_data *scd = this_scd();
+ u64 now;
+
+ spin_lock(&scd->lock);
+ now = sched_clock();
+ __update_sched_clock(scd, now);
+ /*
+ * update tick_gtod after __update_sched_clock() because that will
+ * already observe 1 new jiffy; adding a new tick_gtod to that would
+ * increase the clock 2 jiffies.
+ */
+ scd->tick_raw = now;
+ scd->tick_gtod = ktime_to_ns(ktime_get()); // XXX get from regular tick
+ spin_unlock(&scd->lock);
+}
+
+/*
+ * We are going deep-idle (irqs are disabled):
+ */
+void sched_clock_idle_sleep_event(void)
+{
+ sched_clock_cpu(smp_processor_id());
+}
+EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
+
+/*
+ * We just idled delta nanoseconds (called with irqs disabled):
+ */
+void sched_clock_idle_wakeup_event(u64 delta_ns)
+{
+ struct sched_clock_data *scd = this_scd();
+
+ /*
+ * Override the previous timestamp and ignore all
+ * sched_clock() deltas that occured while we idled,
+ * and use the PM-provided delta_ns to advance the
+ * rq clock:
+ */
+ spin_lock(&scd->lock);
+ scd->prev_raw = sched_clock();
+ scd->clock += delta_ns;
+ spin_unlock(&scd->lock);
+
+ touch_softlockup_watchdog();
+}
+EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
+
+#endif
+
+/*
+ * Scheduler clock - returns current time in nanosec units.
+ * This is default implementation.
+ * Architectures and sub-architectures can override this.
+ */
+unsigned long long __attribute__((weak)) sched_clock(void)
+{
+ return (unsigned long long)jiffies * (NSEC_PER_SEC / HZ);
+}
Index: linux-2.6-2/include/linux/sched.h
===================================================================
--- linux-2.6-2.orig/include/linux/sched.h
+++ linux-2.6-2/include/linux/sched.h
@@ -1551,6 +1551,35 @@ static inline int set_cpus_allowed(struc

extern unsigned long long sched_clock(void);

+#ifdef CONFIG_HAVE_STABLE_CLOCK
+static inline void sched_clock_init(void)
+{
+}
+
+static inline u64 sched_clock_cpu(int cpu)
+{
+ return sched_clock();
+}
+
+static inline void sched_clock_tick(void)
+{
+}
+
+static inline void sched_clock_idle_sleep_event(void)
+{
+}
+
+static inline void sched_clock_idle_wakeup_event(u64 delta_ns)
+{
+}
+#else
+extern void sched_clock_init(void);
+extern u64 sched_clock_cpu(int cpu);
+extern void sched_clock_tick(void);
+extern void sched_clock_idle_sleep_event(void);
+extern void sched_clock_idle_wakeup_event(u64 delta_ns);
+#endif
+
/*
* For kernel-internal use: high-speed (but slightly incorrect) per-cpu
* clock constructed from sched_clock():
Index: linux-2.6-2/kernel/Makefile
===================================================================
--- linux-2.6-2.orig/kernel/Makefile
+++ linux-2.6-2/kernel/Makefile
@@ -9,7 +9,7 @@ obj-y = sched.o fork.o exec_domain.o
rcupdate.o extable.o params.o posix-timers.o \
kthread.o wait.o kfifo.o sys_ni.o posix-cpu-timers.o mutex.o \
hrtimer.o rwsem.o nsproxy.o srcu.o semaphore.o \
- notifier.o ksysfs.o pm_qos_params.o
+ notifier.o ksysfs.o pm_qos_params.o sched_clock.o

obj-$(CONFIG_SYSCTL_SYSCALL_CHECK) += sysctl_check.o
obj-$(CONFIG_STACKTRACE) += stacktrace.o
Index: linux-2.6-2/kernel/sched_debug.c
===================================================================
--- linux-2.6-2.orig/kernel/sched_debug.c
+++ linux-2.6-2/kernel/sched_debug.c
@@ -204,13 +204,6 @@ static void print_cpu(struct seq_file *m
PN(next_balance);
P(curr->pid);
PN(clock);
- PN(idle_clock);
- 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]);
P(cpu_load[1]);
P(cpu_load[2]);
Index: linux-2.6-2/kernel/sched_fair.c
===================================================================
--- linux-2.6-2.orig/kernel/sched_fair.c
+++ linux-2.6-2/kernel/sched_fair.c
@@ -957,7 +957,7 @@ static void yield_task_fair(struct rq *r
return;

if (likely(!sysctl_sched_compat_yield) && curr->policy != SCHED_BATCH) {
- __update_rq_clock(rq);
+ update_rq_clock(rq);
/*
* Update run-time statistics of the 'current'.
*/
Index: linux-2.6-2/init/main.c
===================================================================
--- linux-2.6-2.orig/init/main.c
+++ linux-2.6-2/init/main.c
@@ -602,6 +602,7 @@ asmlinkage void __init start_kernel(void
softirq_init();
timekeeping_init();
time_init();
+ sched_clock_init();
profile_init();
if (!irqs_disabled())
printk("start_kernel(): bug: interrupts were enabled early\n");



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