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Date: Thu, 22 Sep 2011 12:00:40 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Mike Galbraith <efault@....de>
Cc: linux-rt-users <linux-rt-users@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
LKML <linux-kernel@...r.kernel.org>,
Oleg Nesterov <oleg@...hat.com>,
Miklos Szeredi <miklos@...redi.hu>, mingo <mingo@...hat.com>
Subject: Re: rt14: strace -> migrate_disable_atomic imbalance
On Thu, 2011-09-22 at 10:38 +0200, Peter Zijlstra wrote:
> On Wed, 2011-09-21 at 20:50 +0200, Peter Zijlstra wrote:
> > +static void wait_task_inactive_sched_out(struct preempt_notifier *n,
> > + struct task_struct *next)
> > +{
> > + struct task_struct *p;
> > + struct wait_task_inactive_blocked *blocked =
> > + container_of(n, struct wait_task_inactive_blocked, notifier);
> > +
> > + if (current->on_rq) /* we're not inactive yet */
> > + return;
> > +
> > + hlist_del(&n->link);
> > +
> > + p = ACCESS_ONCE(blocked->waiter);
> > + blocked->waiter = NULL;
> > + wake_up_process(p);
> > +}
>
> Trying a wakeup from there isn't going to actually ever work of-course..
> Duh!
OK, this one seems to be better.. But its quite vile, not sure I
actually like it anymore.
---
arch/ia64/kvm/Kconfig | 1
arch/powerpc/kvm/Kconfig | 1
arch/s390/kvm/Kconfig | 1
arch/tile/kvm/Kconfig | 1
arch/x86/kvm/Kconfig | 1
include/linux/kvm_host.h | 2
include/linux/preempt.h | 4 -
include/linux/sched.h | 2
init/Kconfig | 3
kernel/sched.c | 188 +++++++++++++++++++++--------------------------
10 files changed, 85 insertions(+), 119 deletions(-)
Index: linux-2.6/arch/ia64/kvm/Kconfig
===================================================================
--- linux-2.6.orig/arch/ia64/kvm/Kconfig
+++ linux-2.6/arch/ia64/kvm/Kconfig
@@ -22,7 +22,6 @@ config KVM
depends on HAVE_KVM && MODULES && EXPERIMENTAL
# for device assignment:
depends on PCI
- select PREEMPT_NOTIFIERS
select ANON_INODES
select HAVE_KVM_IRQCHIP
select KVM_APIC_ARCHITECTURE
Index: linux-2.6/arch/powerpc/kvm/Kconfig
===================================================================
--- linux-2.6.orig/arch/powerpc/kvm/Kconfig
+++ linux-2.6/arch/powerpc/kvm/Kconfig
@@ -18,7 +18,6 @@ if VIRTUALIZATION
config KVM
bool
- select PREEMPT_NOTIFIERS
select ANON_INODES
config KVM_BOOK3S_HANDLER
Index: linux-2.6/arch/s390/kvm/Kconfig
===================================================================
--- linux-2.6.orig/arch/s390/kvm/Kconfig
+++ linux-2.6/arch/s390/kvm/Kconfig
@@ -19,7 +19,6 @@ config KVM
def_tristate y
prompt "Kernel-based Virtual Machine (KVM) support"
depends on HAVE_KVM && EXPERIMENTAL
- select PREEMPT_NOTIFIERS
select ANON_INODES
---help---
Support hosting paravirtualized guest machines using the SIE
Index: linux-2.6/arch/tile/kvm/Kconfig
===================================================================
--- linux-2.6.orig/arch/tile/kvm/Kconfig
+++ linux-2.6/arch/tile/kvm/Kconfig
@@ -19,7 +19,6 @@ if VIRTUALIZATION
config KVM
tristate "Kernel-based Virtual Machine (KVM) support"
depends on HAVE_KVM && MODULES && EXPERIMENTAL
- select PREEMPT_NOTIFIERS
select ANON_INODES
---help---
Support hosting paravirtualized guest machines.
Index: linux-2.6/arch/x86/kvm/Kconfig
===================================================================
--- linux-2.6.orig/arch/x86/kvm/Kconfig
+++ linux-2.6/arch/x86/kvm/Kconfig
@@ -24,7 +24,6 @@ config KVM
depends on PCI
# for TASKSTATS/TASK_DELAY_ACCT:
depends on NET
- select PREEMPT_NOTIFIERS
select MMU_NOTIFIER
select ANON_INODES
select HAVE_KVM_IRQCHIP
Index: linux-2.6/include/linux/kvm_host.h
===================================================================
--- linux-2.6.orig/include/linux/kvm_host.h
+++ linux-2.6/include/linux/kvm_host.h
@@ -111,9 +111,7 @@ enum {
struct kvm_vcpu {
struct kvm *kvm;
-#ifdef CONFIG_PREEMPT_NOTIFIERS
struct preempt_notifier preempt_notifier;
-#endif
int cpu;
int vcpu_id;
int srcu_idx;
Index: linux-2.6/include/linux/preempt.h
===================================================================
--- linux-2.6.orig/include/linux/preempt.h
+++ linux-2.6/include/linux/preempt.h
@@ -101,8 +101,6 @@ do { \
#endif /* CONFIG_PREEMPT_COUNT */
-#ifdef CONFIG_PREEMPT_NOTIFIERS
-
struct preempt_notifier;
/**
@@ -147,6 +145,4 @@ static inline void preempt_notifier_init
notifier->ops = ops;
}
-#endif
-
#endif /* __LINUX_PREEMPT_H */
Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -1236,10 +1236,8 @@ struct task_struct {
struct sched_entity se;
struct sched_rt_entity rt;
-#ifdef CONFIG_PREEMPT_NOTIFIERS
/* list of struct preempt_notifier: */
struct hlist_head preempt_notifiers;
-#endif
/*
* fpu_counter contains the number of consecutive context switches
Index: linux-2.6/init/Kconfig
===================================================================
--- linux-2.6.orig/init/Kconfig
+++ linux-2.6/init/Kconfig
@@ -1403,9 +1403,6 @@ config STOP_MACHINE
source "block/Kconfig"
-config PREEMPT_NOTIFIERS
- bool
-
config PADATA
depends on SMP
bool
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -2387,6 +2387,57 @@ struct migration_arg {
static int migration_cpu_stop(void *data);
+struct wait_task_inactive_blocked {
+ struct preempt_notifier notifier;
+ struct task_struct *waiter;
+};
+
+static void
+preempt_ops_sched_out_nop(struct preempt_notifier *n, struct task_struct *next)
+{
+}
+
+static void wait_task_inactive_sched_in(struct preempt_notifier *n, int cpu)
+{
+ struct task_struct *p;
+ struct wait_task_inactive_blocked *blocked =
+ container_of(n, struct wait_task_inactive_blocked, notifier);
+
+ hlist_del(&n->link);
+
+ p = ACCESS_ONCE(blocked->waiter);
+ blocked->waiter = NULL;
+ wake_up_process(p);
+}
+
+static struct preempt_ops wait_task_inactive_ops_post = {
+ .sched_in = wait_task_inactive_sched_in,
+ .sched_out = preempt_ops_sched_out_nop,
+};
+
+static void preempt_ops_sched_in_nop(struct preempt_notifier *n, int cpu)
+{
+}
+
+static void
+wait_task_inactive_sched_out(struct preempt_notifier *n, struct task_struct *next)
+{
+ struct wait_task_inactive_blocked *blocked =
+ container_of(n, struct wait_task_inactive_blocked, notifier);
+
+ if (current->on_rq) /* we're not inactive yet */
+ return;
+
+ hlist_del(&n->link);
+ blocked->notifier.ops = &wait_task_inactive_ops_post;
+ hlist_add_head(&n->link, &next->preempt_notifiers);
+}
+
+static struct preempt_ops wait_task_inactive_ops_pre = {
+ .sched_in = preempt_ops_sched_in_nop,
+ .sched_out = wait_task_inactive_sched_out,
+};
+
/*
* wait_task_inactive - wait for a thread to unschedule.
*
@@ -2405,93 +2456,45 @@ static int migration_cpu_stop(void *data
*/
unsigned long wait_task_inactive(struct task_struct *p, long match_state)
{
+ unsigned long ncsw = 0;
unsigned long flags;
- int running, on_rq;
- unsigned long ncsw;
struct rq *rq;
- for (;;) {
- /*
- * We do the initial early heuristics without holding
- * any task-queue locks at all. We'll only try to get
- * the runqueue lock when things look like they will
- * work out!
- */
- rq = task_rq(p);
-
- /*
- * If the task is actively running on another CPU
- * still, just relax and busy-wait without holding
- * any locks.
- *
- * NOTE! Since we don't hold any locks, it's not
- * even sure that "rq" stays as the right runqueue!
- * But we don't care, since "task_running()" will
- * return false if the runqueue has changed and p
- * is actually now running somewhere else!
- */
- while (task_running(rq, p)) {
- if (match_state && unlikely(p->state != match_state))
- return 0;
- cpu_relax();
- }
-
- /*
- * Ok, time to look more closely! We need the rq
- * lock now, to be *sure*. If we're wrong, we'll
- * just go back and repeat.
- */
- rq = task_rq_lock(p, &flags);
- trace_sched_wait_task(p);
- running = task_running(rq, p);
- on_rq = p->on_rq;
- ncsw = 0;
- if (!match_state || p->state == match_state)
- ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
- task_rq_unlock(rq, p, &flags);
-
- /*
- * If it changed from the expected state, bail out now.
- */
- if (unlikely(!ncsw))
- break;
+ struct wait_task_inactive_blocked blocked = {
+ .notifier = {
+ .ops = &wait_task_inactive_ops_pre,
+ },
+ .waiter = current,
+ };
- /*
- * Was it really running after all now that we
- * checked with the proper locks actually held?
- *
- * Oops. Go back and try again..
- */
- if (unlikely(running)) {
- cpu_relax();
- continue;
- }
+ /* if we don't match the expected state, bail */
+ if (match_state && unlikely(p->state != match_state))
+ return 0;
- /*
- * It's not enough that it's not actively running,
- * it must be off the runqueue _entirely_, and not
- * preempted!
- *
- * So if it was still runnable (but just not actively
- * running right now), it's preempted, and we should
- * yield - it could be a while.
- */
- if (unlikely(on_rq)) {
- ktime_t to = ktime_set(0, NSEC_PER_SEC/HZ);
+ rq = task_rq_lock(p, &flags);
+ if (!p->on_rq) /* we're already blocked */
+ goto done;
- set_current_state(TASK_UNINTERRUPTIBLE);
- schedule_hrtimeout(&to, HRTIMER_MODE_REL);
- continue;
- }
+ hlist_add_head(&blocked.notifier.link, &p->preempt_notifiers);
+ task_rq_unlock(rq, p, &flags);
- /*
- * Ahh, all good. It wasn't running, and it wasn't
- * runnable, which means that it will never become
- * running in the future either. We're all done!
- */
- break;
+ for (;;) {
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ if (!blocked.waiter)
+ break;
+ schedule();
}
+ __set_current_state(TASK_RUNNING);
+ /*
+ * Serializes against the completion of the previously observed context
+ * switch.
+ */
+ rq = task_rq_lock(p, &flags);
+done:
+ if (!match_state || p->state == match_state)
+ ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
+ task_rq_unlock(rq, p, &flags);
return ncsw;
}
@@ -2967,10 +2970,7 @@ static void __sched_fork(struct task_str
#endif
INIT_LIST_HEAD(&p->rt.run_list);
-
-#ifdef CONFIG_PREEMPT_NOTIFIERS
INIT_HLIST_HEAD(&p->preempt_notifiers);
-#endif
}
/*
@@ -3084,8 +3084,6 @@ void wake_up_new_task(struct task_struct
task_rq_unlock(rq, p, &flags);
}
-#ifdef CONFIG_PREEMPT_NOTIFIERS
-
/**
* preempt_notifier_register - tell me when current is being preempted & rescheduled
* @notifier: notifier struct to register
@@ -3111,9 +3109,9 @@ EXPORT_SYMBOL_GPL(preempt_notifier_unreg
static void fire_sched_in_preempt_notifiers(struct task_struct *curr)
{
struct preempt_notifier *notifier;
- struct hlist_node *node;
+ struct hlist_node *node, *n;
- hlist_for_each_entry(notifier, node, &curr->preempt_notifiers, link)
+ hlist_for_each_entry_safe(notifier, node, n, &curr->preempt_notifiers, link)
notifier->ops->sched_in(notifier, raw_smp_processor_id());
}
@@ -3122,26 +3120,12 @@ fire_sched_out_preempt_notifiers(struct
struct task_struct *next)
{
struct preempt_notifier *notifier;
- struct hlist_node *node;
+ struct hlist_node *node, *n;
- hlist_for_each_entry(notifier, node, &curr->preempt_notifiers, link)
+ hlist_for_each_entry_safe(notifier, node, n, &curr->preempt_notifiers, link)
notifier->ops->sched_out(notifier, next);
}
-#else /* !CONFIG_PREEMPT_NOTIFIERS */
-
-static void fire_sched_in_preempt_notifiers(struct task_struct *curr)
-{
-}
-
-static void
-fire_sched_out_preempt_notifiers(struct task_struct *curr,
- struct task_struct *next)
-{
-}
-
-#endif /* CONFIG_PREEMPT_NOTIFIERS */
-
/**
* prepare_task_switch - prepare to switch tasks
* @rq: the runqueue preparing to switch
@@ -8312,9 +8296,7 @@ void __init sched_init(void)
set_load_weight(&init_task);
-#ifdef CONFIG_PREEMPT_NOTIFIERS
INIT_HLIST_HEAD(&init_task.preempt_notifiers);
-#endif
#ifdef CONFIG_SMP
open_softirq(SCHED_SOFTIRQ, run_rebalance_domains);
--
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