[GIT PULL, v2] scheduler fixes

Ingo Molnar
2010-12-19T22:30:37Z

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Sun, Dec 19, 2010 at 7:27 AM, Ingo Molnar <mingo@elte.hu> wrote:
> > @@ -3943,6 +4128,7 @@ need_resched_nonpreemptible:
> >                rq->nr_switches++;
> >                rq->curr = next;
> >                ++*switch_count;
> > +               WARN_ON_ONCE(test_tsk_need_resched(next));
> >
> >                context_switch(rq, prev, next); /* unlocks the rq */
> >                /*
> > diff --git a/kernel/timer.c b/kern
>
> Please don't add like this in an -rc. It looks like it's some
> debugging aid, I don't think it should have gone in now.

Yeah - sorry about that. The warning is bogus - please pull the updated tree below.
(Can also be pulled on top of the previous one.)

Thanks,

Ingo

git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git sched-fixes-for-linus

------------------>
Heiko Carstens (1):
nohz: Fix get_next_timer_interrupt() vs cpu hotplug

Ingo Molnar (1):
sched: Remove debugging check

Mike Galbraith (1):
Sched: fix skip_clock_update optimization

Peter Zijlstra (3):
sched: Cure more NO_HZ load average woes
sched: Fix the irqtime code to deal with u64 wraps
sched: Fix the irqtime code for 32bit

include/linux/sched.h | 2 +-
kernel/fork.c | 1 +
kernel/sched.c | 287 ++++++++++++++++++++++++++++++++++++++++---------
kernel/timer.c | 8 ++-
4 files changed, 245 insertions(+), 53 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2c79e92..2238745 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -143,7 +143,7 @@ extern unsigned long nr_iowait_cpu(int cpu);
extern unsigned long this_cpu_load(void);

-extern void calc_global_load(void);
+extern void calc_global_load(unsigned long ticks);

extern unsigned long get_parent_ip(unsigned long addr);

diff --git a/kernel/fork.c b/kernel/fork.c
index 3b159c5..5447dc7 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -273,6 +273,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)

setup_thread_stack(tsk, orig);
clear_user_return_notifier(tsk);
+ clear_tsk_need_resched(tsk);
stackend = end_of_stack(tsk);
*stackend = STACK_END_MAGIC; /* for overflow detection */

diff --git a/kernel/sched.c b/kernel/sched.c
index dc91a4d..297d1a0 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -636,22 +636,18 @@ static inline struct task_group *task_group(struct task_struct *p)

#endif /* CONFIG_CGROUP_SCHED */

-static u64 irq_time_cpu(int cpu);
-static void sched_irq_time_avg_update(struct rq *rq, u64 irq_time);
+static void update_rq_clock_task(struct rq *rq, s64 delta);

-inline void update_rq_clock(struct rq *rq)
+static void update_rq_clock(struct rq *rq)
{
- if (!rq->skip_clock_update) {
- int cpu = cpu_of(rq);
- u64 irq_time;
+ s64 delta;

- rq->clock = sched_clock_cpu(cpu);
- irq_time = irq_time_cpu(cpu);
- if (rq->clock - irq_time > rq->clock_task)
- rq->clock_task = rq->clock - irq_time;
+ if (rq->skip_clock_update)
+ return;

- sched_irq_time_avg_update(rq, irq_time);
- }
+ delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
+ rq->clock += delta;
+ update_rq_clock_task(rq, delta);
}

/*
@@ -1924,10 +1920,9 @@ static void deactivate_task(struct rq *rq, struct task_struct *p, int flags)
* They are read and saved off onto struct rq in update_rq_clock().
* This may result in other CPU reading this CPU's irq time and can
* race with irq/account_system_vtime on this CPU. We would either get old
- * or new value (or semi updated value on 32 bit) with a side effect of
- * accounting a slice of irq time to wrong task when irq is in progress
- * while we read rq->clock. That is a worthy compromise in place of having
- * locks on each irq in account_system_time.
+ * or new value with a side effect of accounting a slice of irq time to wrong
+ * task when irq is in progress while we read rq->clock. That is a worthy
+ * compromise in place of having locks on each irq in account_system_time.
*/
static DEFINE_PER_CPU(u64, cpu_hardirq_time);
static DEFINE_PER_CPU(u64, cpu_softirq_time);
@@ -1945,19 +1940,58 @@ void disable_sched_clock_irqtime(void)
sched_clock_irqtime = 0;
}

-static u64 irq_time_cpu(int cpu)
+#ifndef CONFIG_64BIT
+static DEFINE_PER_CPU(seqcount_t, irq_time_seq);
+
+static inline void irq_time_write_begin(void)
{
- if (!sched_clock_irqtime)
- return 0;
+ __this_cpu_inc(irq_time_seq.sequence);
+ smp_wmb();
+}
+
+static inline void irq_time_write_end(void)
+{
+ smp_wmb();
+ __this_cpu_inc(irq_time_seq.sequence);
+}
+
+static inline u64 irq_time_read(int cpu)
+{
+ u64 irq_time;
+ unsigned seq;

+ do {
+ seq = read_seqcount_begin(&per_cpu(irq_time_seq, cpu));
+ irq_time = per_cpu(cpu_softirq_time, cpu) +
+ per_cpu(cpu_hardirq_time, cpu);
+ } while (read_seqcount_retry(&per_cpu(irq_time_seq, cpu), seq));
+
+ return irq_time;
+}
+#else /* CONFIG_64BIT */
+static inline void irq_time_write_begin(void)
+{
+}
+
+static inline void irq_time_write_end(void)
+{
+}
+
+static inline u64 irq_time_read(int cpu)
+{
return per_cpu(cpu_softirq_time, cpu) + per_cpu(cpu_hardirq_time, cpu);
}
+#endif /* CONFIG_64BIT */

+/*
+ * Called before incrementing preempt_count on {soft,}irq_enter
+ * and before decrementing preempt_count on {soft,}irq_exit.
+ */
void account_system_vtime(struct task_struct *curr)
{
unsigned long flags;
+ s64 delta;
int cpu;
- u64 now, delta;

if (!sched_clock_irqtime)
return;
@@ -1965,9 +1999,10 @@ void account_system_vtime(struct task_struct *curr)
local_irq_save(flags);

cpu = smp_processor_id();
- now = sched_clock_cpu(cpu);
- delta = now - per_cpu(irq_start_time, cpu);
- per_cpu(irq_start_time, cpu) = now;
+ delta = sched_clock_cpu(cpu) - __this_cpu_read(irq_start_time);
+ __this_cpu_add(irq_start_time, delta);
+
+ irq_time_write_begin();
/*
* We do not account for softirq time from ksoftirqd here.
* We want to continue accounting softirq time to ksoftirqd thread
@@ -1975,33 +2010,55 @@ void account_system_vtime(struct task_struct *curr)
* that do not consume any time, but still wants to run.
*/
if (hardirq_count())
- per_cpu(cpu_hardirq_time, cpu) += delta;
+ __this_cpu_add(cpu_hardirq_time, delta);
else if (in_serving_softirq() && !(curr->flags & PF_KSOFTIRQD))
- per_cpu(cpu_softirq_time, cpu) += delta;
+ __this_cpu_add(cpu_softirq_time, delta);

+ irq_time_write_end();
local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(account_system_vtime);

-static void sched_irq_time_avg_update(struct rq *rq, u64 curr_irq_time)
+static void update_rq_clock_task(struct rq *rq, s64 delta)
{
- if (sched_clock_irqtime && sched_feat(NONIRQ_POWER)) {
- u64 delta_irq = curr_irq_time - rq->prev_irq_time;
- rq->prev_irq_time = curr_irq_time;
- sched_rt_avg_update(rq, delta_irq);
- }
+ s64 irq_delta;
+
+ irq_delta = irq_time_read(cpu_of(rq)) - rq->prev_irq_time;
+
+ /*
+ * Since irq_time is only updated on {soft,}irq_exit, we might run into
+ * this case when a previous update_rq_clock() happened inside a
+ * {soft,}irq region.
+ *
+ * When this happens, we stop ->clock_task and only update the
+ * prev_irq_time stamp to account for the part that fit, so that a next
+ * update will consume the rest. This ensures ->clock_task is
+ * monotonic.
+ *
+ * It does however cause some slight miss-attribution of {soft,}irq
+ * time, a more accurate solution would be to update the irq_time using
+ * the current rq->clock timestamp, except that would require using
+ * atomic ops.
+ */
+ if (irq_delta > delta)
+ irq_delta = delta;
+
+ rq->prev_irq_time += irq_delta;
+ delta -= irq_delta;
+ rq->clock_task += delta;
+
+ if (irq_delta && sched_feat(NONIRQ_POWER))
+ sched_rt_avg_update(rq, irq_delta);
}

-#else
+#else /* CONFIG_IRQ_TIME_ACCOUNTING */

-static u64 irq_time_cpu(int cpu)
+static void update_rq_clock_task(struct rq *rq, s64 delta)
{
- return 0;
+ rq->clock_task += delta;
}

-static void sched_irq_time_avg_update(struct rq *rq, u64 curr_irq_time) { }
-
-#endif
+#endif /* CONFIG_IRQ_TIME_ACCOUNTING */

#include "sched_idletask.c"
#include "sched_fair.c"
@@ -2129,7 +2186,7 @@ static void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
* A queue event has occurred, and we're going to schedule. In
* this case, we can save a useless back to back clock update.
*/
- if (test_tsk_need_resched(rq->curr))
+ if (rq->curr->se.on_rq && test_tsk_need_resched(rq->curr))
rq->skip_clock_update = 1;
}

@@ -3119,6 +3176,15 @@ static long calc_load_fold_active(struct rq *this_rq)
return delta;
}

+static unsigned long
+calc_load(unsigned long load, unsigned long exp, unsigned long active)
+{
+ load *= exp;
+ load += active * (FIXED_1 - exp);
+ load += 1UL << (FSHIFT - 1);
+ return load >> FSHIFT;
+}
+
#ifdef CONFIG_NO_HZ
/*
* For NO_HZ we delay the active fold to the next LOAD_FREQ update.
@@ -3148,6 +3214,128 @@ static long calc_load_fold_idle(void)

return delta;
}
+
+/**
+ * fixed_power_int - compute: x^n, in O(log n) time
+ *
+ * @x: base of the power
+ * @frac_bits: fractional bits of @x
+ * @n: power to raise @x to.
+ *
+ * By exploiting the relation between the definition of the natural power
+ * function: x^n := x*x*...*x (x multiplied by itself for n times), and
+ * the binary encoding of numbers used by computers: n := \Sum n_i * 2^i,
+ * (where: n_i \elem {0, 1}, the binary vector representing n),
+ * we find: x^n := x^(\Sum n_i * 2^i) := \Prod x^(n_i * 2^i), which is
+ * of course trivially computable in O(log_2 n), the length of our binary
+ * vector.
+ */
+static unsigned long
+fixed_power_int(unsigned long x, unsigned int frac_bits, unsigned int n)
+{
+ unsigned long result = 1UL << frac_bits;
+
+ if (n) for (;;) {
+ if (n & 1) {
+ result *= x;
+ result += 1UL << (frac_bits - 1);
+ result >>= frac_bits;
+ }
+ n >>= 1;
+ if (!n)
+ break;
+ x *= x;
+ x += 1UL << (frac_bits - 1);
+ x >>= frac_bits;
+ }
+
+ return result;
+}
+
+/*
+ * a1 = a0 * e + a * (1 - e)
+ *
+ * a2 = a1 * e + a * (1 - e)
+ * = (a0 * e + a * (1 - e)) * e + a * (1 - e)
+ * = a0 * e^2 + a * (1 - e) * (1 + e)
+ *
+ * a3 = a2 * e + a * (1 - e)
+ * = (a0 * e^2 + a * (1 - e) * (1 + e)) * e + a * (1 - e)
+ * = a0 * e^3 + a * (1 - e) * (1 + e + e^2)
+ *
+ * ...
+ *
+ * an = a0 * e^n + a * (1 - e) * (1 + e + ... + e^n-1) [1]
+ * = a0 * e^n + a * (1 - e) * (1 - e^n)/(1 - e)
+ * = a0 * e^n + a * (1 - e^n)
+ *
+ * [1] application of the geometric series:
+ *
+ * n 1 - x^(n+1)
+ * S_n := \Sum x^i = -------------
+ * i=0 1 - x
+ */
+static unsigned long
+calc_load_n(unsigned long load, unsigned long exp,
+ unsigned long active, unsigned int n)
+{
+
+ return calc_load(load, fixed_power_int(exp, FSHIFT, n), active);
+}
+
+/*
+ * NO_HZ can leave us missing all per-cpu ticks calling
+ * calc_load_account_active(), but since an idle CPU folds its delta into
+ * calc_load_tasks_idle per calc_load_account_idle(), all we need to do is fold
+ * in the pending idle delta if our idle period crossed a load cycle boundary.
+ *
+ * Once we've updated the global active value, we need to apply the exponential
+ * weights adjusted to the number of cycles missed.
+ */
+static void calc_global_nohz(unsigned long ticks)
+{
+ long delta, active, n;
+
+ if (time_before(jiffies, calc_load_update))
+ return;
+
+ /*
+ * If we crossed a calc_load_update boundary, make sure to fold
+ * any pending idle changes, the respective CPUs might have
+ * missed the tick driven calc_load_account_active() update
+ * due to NO_HZ.
+ */
+ delta = calc_load_fold_idle();
+ if (delta)
+ atomic_long_add(delta, &calc_load_tasks);
+
+ /*
+ * If we were idle for multiple load cycles, apply them.
+ */
+ if (ticks >= LOAD_FREQ) {
+ n = ticks / LOAD_FREQ;
+
+ active = atomic_long_read(&calc_load_tasks);
+ active = active > 0 ? active * FIXED_1 : 0;
+
+ avenrun[0] = calc_load_n(avenrun[0], EXP_1, active, n);
+ avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n);
+ avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n);
+
+ calc_load_update += n * LOAD_FREQ;
+ }
+
+ /*
+ * Its possible the remainder of the above division also crosses
+ * a LOAD_FREQ period, the regular check in calc_global_load()
+ * which comes after this will take care of that.
+ *
+ * Consider us being 11 ticks before a cycle completion, and us
+ * sleeping for 4*LOAD_FREQ + 22 ticks, then the above code will
+ * age us 4 cycles, and the test in calc_global_load() will
+ * pick up the final one.
+ */
+}
#else
static void calc_load_account_idle(struct rq *this_rq)
{
@@ -3157,6 +3345,10 @@ static inline long calc_load_fold_idle(void)
{
return 0;
}
+
+static void calc_global_nohz(unsigned long ticks)
+{
+}
#endif

/**
@@ -3174,24 +3366,17 @@ void get_avenrun(unsigned long *loads, unsigned long offset, int shift)
loads[2] = (avenrun[2] + offset) << shift;
}

-static unsigned long
-calc_load(unsigned long load, unsigned long exp, unsigned long active)
-{
- load *= exp;
- load += active * (FIXED_1 - exp);
- return load >> FSHIFT;
-}
-
/*
* calc_load - update the avenrun load estimates 10 ticks after the
* CPUs have updated calc_load_tasks.
*/
-void calc_global_load(void)
+void calc_global_load(unsigned long ticks)
{
- unsigned long upd = calc_load_update + 10;
long active;

- if (time_before(jiffies, upd))
+ calc_global_nohz(ticks);
+
+ if (time_before(jiffies, calc_load_update + 10))
return;

active = atomic_long_read(&calc_load_tasks);
@@ -3845,7 +4030,6 @@ static void put_prev_task(struct rq *rq, struct task_struct *prev)
{
if (prev->se.on_rq)
update_rq_clock(rq);
- rq->skip_clock_update = 0;
prev->sched_class->put_prev_task(rq, prev);
}

@@ -3903,7 +4087,6 @@ need_resched_nonpreemptible:
hrtick_clear(rq);

raw_spin_lock_irq(&rq->lock);
- clear_tsk_need_resched(prev);

switch_count = &prev->nivcsw;
if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
@@ -3935,6 +4118,8 @@ need_resched_nonpreemptible:

put_prev_task(rq, prev);
next = pick_next_task(rq);
+ clear_tsk_need_resched(prev);
+ rq->skip_clock_update = 0;

if (likely(prev != next)) {
sched_info_switch(prev, next);
diff --git a/kernel/timer.c b/kernel/timer.c
index 68a9ae7..353b922 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1252,6 +1252,12 @@ unsigned long get_next_timer_interrupt(unsigned long now)
struct tvec_base *base = __get_cpu_var(tvec_bases);
unsigned long expires;

+ /*
+ * Pretend that there is no timer pending if the cpu is offline.
+ * Possible pending timers will be migrated later to an active cpu.
+ */
+ if (cpu_is_offline(smp_processor_id()))
+ return now + NEXT_TIMER_MAX_DELTA;
spin_lock(&base->lock);
if (time_before_eq(base->next_timer, base->timer_jiffies))
base->next_timer = __next_timer_interrupt(base);
@@ -1319,7 +1325,7 @@ void do_timer(unsigned long ticks)
{
jiffies_64 += ticks;
update_wall_time();
- calc_global_load();
+ calc_global_load(ticks);
}

#ifdef __ARCH_WANT_SYS_ALARM