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SubjectRe: [PATCH RFC] reduce runqueue lock contention
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On Mon, 2010-06-21 at 12:02 +0200, Peter Zijlstra wrote:
> To return the favour, here's a scary patch that renders your machine a
> doorstop :-) Sadly it just hangs, no splat, no sysrq, no nmi, no tripple
> fault.
>
> It looses the ttwu task_running() check, as I must admit I'm not quite
> sure what it does.. Ingo?
>
> It makes the whole TASK_WAKING thing very interesting again :-)
>
> It also re-introduces a bunch of races because we now need to run
> ->select_task_rq() without holding the rq->lock. We cannot defer running
> that because it really wants to know the cpu the wakeup originated from
> (affine wakeups and the like).


Progress, this one gave spectacular fireworks ;-)

---
arch/x86/kernel/smp.c | 1 +
include/linux/sched.h | 7 +-
kernel/sched.c | 247 +++++++++++++++++++++++++----------------------
kernel/sched_fair.c | 10 +-
kernel/sched_idletask.c | 2 +-
kernel/sched_rt.c | 4 +-
6 files changed, 143 insertions(+), 128 deletions(-)
diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index d801210..1e487d6 100644
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -202,6 +202,7 @@ void smp_reschedule_interrupt(struct pt_regs *regs)
/*
* KVM uses this interrupt to force a cpu out of guest mode
*/
+ sched_ttwu_pending();
}

void smp_call_function_interrupt(struct pt_regs *regs)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index a61c08c..9ae9fdb 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1003,6 +1003,7 @@ partition_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
}
#endif /* !CONFIG_SMP */

+void sched_ttwu_pending(void);

struct io_context; /* See blkdev.h */

@@ -1046,12 +1047,11 @@ struct sched_class {
void (*put_prev_task) (struct rq *rq, struct task_struct *p);

#ifdef CONFIG_SMP
- int (*select_task_rq)(struct rq *rq, struct task_struct *p,
- int sd_flag, int flags);
+ int (*select_task_rq)(struct task_struct *p, int sd_flag, int flags);

void (*pre_schedule) (struct rq *this_rq, struct task_struct *task);
void (*post_schedule) (struct rq *this_rq);
- void (*task_waking) (struct rq *this_rq, struct task_struct *task);
+ void (*task_waking) (struct task_struct *task);
void (*task_woken) (struct rq *this_rq, struct task_struct *task);

void (*set_cpus_allowed)(struct task_struct *p,
@@ -1174,6 +1174,7 @@ struct task_struct {
int lock_depth; /* BKL lock depth */

#ifdef CONFIG_SMP
+ struct task_struct *wake_entry;
#ifdef __ARCH_WANT_UNLOCKED_CTXSW
int oncpu;
#endif
diff --git a/kernel/sched.c b/kernel/sched.c
index b697606..5fe442c 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -518,6 +518,8 @@ struct rq {
u64 age_stamp;
u64 idle_stamp;
u64 avg_idle;
+
+ struct task_struct *wake_list;
#endif

/* calc_load related fields */
@@ -944,7 +946,7 @@ static inline struct rq *__task_rq_lock(struct task_struct *p)
for (;;) {
rq = task_rq(p);
raw_spin_lock(&rq->lock);
- if (likely(rq == task_rq(p)))
+ if (likely(rq == task_rq(p)) && !task_is_waking(p))
return rq;
raw_spin_unlock(&rq->lock);
}
@@ -964,7 +966,7 @@ static struct rq *task_rq_lock(struct task_struct *p, unsigned long *flags)
local_irq_save(*flags);
rq = task_rq(p);
raw_spin_lock(&rq->lock);
- if (likely(rq == task_rq(p)))
+ if (likely(rq == task_rq(p)) && !task_is_waking(p))
return rq;
raw_spin_unlock_irqrestore(&rq->lock, *flags);
}
@@ -2257,9 +2259,9 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
* The caller (fork, wakeup) owns TASK_WAKING, ->cpus_allowed is stable.
*/
static inline
-int select_task_rq(struct rq *rq, struct task_struct *p, int sd_flags, int wake_flags)
+int select_task_rq(struct task_struct *p, int sd_flags, int wake_flags)
{
- int cpu = p->sched_class->select_task_rq(rq, p, sd_flags, wake_flags);
+ int cpu = p->sched_class->select_task_rq(p, sd_flags, wake_flags);

/*
* In order not to call set_task_cpu() on a blocking task we need
@@ -2285,9 +2287,8 @@ static void update_avg(u64 *avg, u64 sample)
}
#endif

-static inline void ttwu_activate(struct task_struct *p, struct rq *rq,
- bool is_sync, bool is_migrate, bool is_local,
- unsigned long en_flags)
+static inline void
+ttwu_stat(struct task_struct *p, bool is_sync, bool is_migrate, bool is_local)
{
schedstat_inc(p, se.statistics.nr_wakeups);
if (is_sync)
@@ -2298,8 +2299,6 @@ static inline void ttwu_activate(struct task_struct *p, struct rq *rq,
schedstat_inc(p, se.statistics.nr_wakeups_local);
else
schedstat_inc(p, se.statistics.nr_wakeups_remote);
-
- activate_task(rq, p, en_flags);
}

static inline void ttwu_post_activation(struct task_struct *p, struct rq *rq,
@@ -2330,111 +2329,6 @@ static inline void ttwu_post_activation(struct task_struct *p, struct rq *rq,
}

/**
- * try_to_wake_up - wake up a thread
- * @p: the thread to be awakened
- * @state: the mask of task states that can be woken
- * @wake_flags: wake modifier flags (WF_*)
- *
- * Put it on the run-queue if it's not already there. The "current"
- * thread is always on the run-queue (except when the actual
- * re-schedule is in progress), and as such you're allowed to do
- * the simpler "current->state = TASK_RUNNING" to mark yourself
- * runnable without the overhead of this.
- *
- * Returns %true if @p was woken up, %false if it was already running
- * or @state didn't match @p's state.
- */
-static int try_to_wake_up(struct task_struct *p, unsigned int state,
- int wake_flags)
-{
- int cpu, orig_cpu, this_cpu, success = 0;
- unsigned long flags;
- unsigned long en_flags = ENQUEUE_WAKEUP;
- struct rq *rq;
-
- this_cpu = get_cpu();
-
- smp_wmb();
- rq = task_rq_lock(p, &flags);
- if (!(p->state & state))
- goto out;
-
- if (p->se.on_rq)
- goto out_running;
-
- cpu = task_cpu(p);
- orig_cpu = cpu;
-
-#ifdef CONFIG_SMP
- if (unlikely(task_running(rq, p)))
- goto out_activate;
-
- /*
- * In order to handle concurrent wakeups and release the rq->lock
- * we put the task in TASK_WAKING state.
- *
- * First fix up the nr_uninterruptible count:
- */
- if (task_contributes_to_load(p)) {
- if (likely(cpu_online(orig_cpu)))
- rq->nr_uninterruptible--;
- else
- this_rq()->nr_uninterruptible--;
- }
- p->state = TASK_WAKING;
-
- if (p->sched_class->task_waking) {
- p->sched_class->task_waking(rq, p);
- en_flags |= ENQUEUE_WAKING;
- }
-
- cpu = select_task_rq(rq, p, SD_BALANCE_WAKE, wake_flags);
- if (cpu != orig_cpu)
- set_task_cpu(p, cpu);
- __task_rq_unlock(rq);
-
- rq = cpu_rq(cpu);
- raw_spin_lock(&rq->lock);
-
- /*
- * We migrated the task without holding either rq->lock, however
- * since the task is not on the task list itself, nobody else
- * will try and migrate the task, hence the rq should match the
- * cpu we just moved it to.
- */
- WARN_ON(task_cpu(p) != cpu);
- WARN_ON(p->state != TASK_WAKING);
-
-#ifdef CONFIG_SCHEDSTATS
- schedstat_inc(rq, ttwu_count);
- if (cpu == this_cpu)
- schedstat_inc(rq, ttwu_local);
- else {
- struct sched_domain *sd;
- for_each_domain(this_cpu, sd) {
- if (cpumask_test_cpu(cpu, sched_domain_span(sd))) {
- schedstat_inc(sd, ttwu_wake_remote);
- break;
- }
- }
- }
-#endif /* CONFIG_SCHEDSTATS */
-
-out_activate:
-#endif /* CONFIG_SMP */
- ttwu_activate(p, rq, wake_flags & WF_SYNC, orig_cpu != cpu,
- cpu == this_cpu, en_flags);
- success = 1;
-out_running:
- ttwu_post_activation(p, rq, wake_flags, success);
-out:
- task_rq_unlock(rq, &flags);
- put_cpu();
-
- return success;
-}
-
-/**
* try_to_wake_up_local - try to wake up a local task with rq lock held
* @p: the thread to be awakened
*
@@ -2459,12 +2353,131 @@ static void try_to_wake_up_local(struct task_struct *p)
schedstat_inc(rq, ttwu_count);
schedstat_inc(rq, ttwu_local);
}
- ttwu_activate(p, rq, false, false, true, ENQUEUE_WAKEUP);
+ ttwu_stat(p, false, false, true);
+ activate_task(rq, p, ENQUEUE_WAKEUP);
success = true;
}
ttwu_post_activation(p, rq, 0, success);
}

+static int ttwu_running(struct task_struct *p, int wake_flags)
+{
+ struct rq *rq;
+
+ for (;;) {
+ rq = task_rq(p);
+ raw_spin_lock(&rq->lock);
+ if (likely(rq == task_rq(p)))
+ break;
+ raw_spin_unlock(&rq->lock);
+ }
+
+ if (!p->se.on_rq) {
+ raw_spin_unlock(&rq->lock);
+ return 0;
+ }
+
+ ttwu_post_activation(p, rq, 0, true);
+ return 1;
+}
+
+static void ttwu_do_activate(struct rq *rq, struct task_struct *p, bool local)
+{
+ set_task_cpu(p, cpu_of(rq));
+ activate_task(rq, p, ENQUEUE_WAKEUP
+#ifdef CONFIG_SMP
+ | ENQUEUE_WAKING
+#endif
+ );
+ ttwu_post_activation(p, rq, 0, true);
+}
+
+void sched_ttwu_pending(void)
+{
+#ifdef CONFIG_SMP
+ struct rq *rq = this_rq();
+ struct task_struct *list = xchg(&rq->wake_list, NULL);
+
+ if (!list)
+ return;
+
+ raw_spin_lock(&rq->lock);
+
+ while (list) {
+ struct task_struct *p = list;
+ list = list->wake_entry;
+ ttwu_do_activate(rq, p, false);
+ }
+
+ raw_spin_unlock(&rq->lock);
+#endif
+}
+
+static void ttwu_queue(struct task_struct *p, int cpu)
+{
+ struct task_struct *next = NULL;
+ struct rq *rq = cpu_rq(cpu);
+
+ if (cpu == smp_processor_id()) {
+ raw_spin_lock(&rq->lock);
+ ttwu_do_activate(rq, p, true);
+ raw_spin_unlock(&rq->lock);
+ return;
+ }
+
+#ifdef CONFIG_SMP
+ for (;;) {
+ struct task_struct *old = next;
+
+ p->wake_entry = next;
+ next = cmpxchg(&rq->wake_list, old, p);
+ if (next == old)
+ break;
+ }
+
+ if (!next)
+ smp_send_reschedule(cpu);
+#endif
+}
+
+static int
+try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
+{
+ unsigned long flags;
+ int success = 0;
+ int cpu = task_cpu(p);
+
+ local_irq_save(flags);
+ for (;;) {
+ unsigned int task_state = p->state;
+
+ if (!(task_state & state))
+ goto out;
+
+ if (cmpxchg(&p->state, task_state, TASK_WAKING) == task_state)
+ break;
+ }
+
+ success = 1;
+
+ if (unlikely(p->se.on_rq) && ttwu_running(p, wake_flags))
+ goto out;
+
+#ifdef CONFIG_SMP
+ if (p->sched_class->task_waking)
+ p->sched_class->task_waking(p);
+
+ cpu = select_task_rq(p, SD_BALANCE_WAKE, wake_flags);
+#endif
+ ttwu_stat(p, wake_flags & WF_SYNC, /* sync */
+ cpu != task_cpu(p), /* migrate */
+ cpu == smp_processor_id()); /* local */
+ ttwu_queue(p, cpu);
+out:
+ local_irq_restore(flags);
+ return success;
+}
+
/**
* wake_up_process - Wake up a specific process
* @p: The process to be woken up.
@@ -2604,7 +2617,7 @@ void wake_up_new_task(struct task_struct *p, unsigned long clone_flags)
* We set TASK_WAKING so that select_task_rq() can drop rq->lock
* without people poking at ->cpus_allowed.
*/
- cpu = select_task_rq(rq, p, SD_BALANCE_FORK, 0);
+ cpu = select_task_rq(p, SD_BALANCE_FORK, 0);
set_task_cpu(p, cpu);

p->state = TASK_RUNNING;
@@ -3200,7 +3213,7 @@ void sched_exec(void)
int dest_cpu;

rq = task_rq_lock(p, &flags);
- dest_cpu = p->sched_class->select_task_rq(rq, p, SD_BALANCE_EXEC, 0);
+ dest_cpu = p->sched_class->select_task_rq(p, SD_BALANCE_EXEC, 0);
if (dest_cpu == smp_processor_id())
goto unlock;

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 5e8f98c..2bd145f 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1128,11 +1128,12 @@ static void yield_task_fair(struct rq *rq)

#ifdef CONFIG_SMP

-static void task_waking_fair(struct rq *rq, struct task_struct *p)
+static void task_waking_fair(struct task_struct *p)
{
struct sched_entity *se = &p->se;
struct cfs_rq *cfs_rq = cfs_rq_of(se);

+ // XXX racy again,.. we need to read cfs_rq->min_vruntime atomically
se->vruntime -= cfs_rq->min_vruntime;
}

@@ -1443,7 +1444,7 @@ static int select_idle_sibling(struct task_struct *p, int target)
* preempt must be disabled.
*/
static int
-select_task_rq_fair(struct rq *rq, struct task_struct *p, int sd_flag, int wake_flags)
+select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
{
struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;
int cpu = smp_processor_id();
@@ -1516,11 +1517,8 @@ select_task_rq_fair(struct rq *rq, struct task_struct *p, int sd_flag, int wake_
if (affine_sd && (!tmp || affine_sd->span_weight > sd->span_weight))
tmp = affine_sd;

- if (tmp) {
- raw_spin_unlock(&rq->lock);
+ if (tmp)
update_shares(tmp);
- raw_spin_lock(&rq->lock);
- }
}
#endif

diff --git a/kernel/sched_idletask.c b/kernel/sched_idletask.c
index 9fa0f40..efaed8c 100644
--- a/kernel/sched_idletask.c
+++ b/kernel/sched_idletask.c
@@ -7,7 +7,7 @@

#ifdef CONFIG_SMP
static int
-select_task_rq_idle(struct rq *rq, struct task_struct *p, int sd_flag, int flags)
+select_task_rq_idle(struct task_struct *p, int sd_flag, int flags)
{
return task_cpu(p); /* IDLE tasks as never migrated */
}
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index d10c80e..1011cdc 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -949,8 +949,10 @@ static void yield_task_rt(struct rq *rq)
static int find_lowest_rq(struct task_struct *task);

static int
-select_task_rq_rt(struct rq *rq, struct task_struct *p, int sd_flag, int flags)
+select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
{
+ struct rq *rq = task_rq(p); // XXX racy
+
if (sd_flag != SD_BALANCE_WAKE)
return smp_processor_id();



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