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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [RFC PATCH v1 1/5] tasklet: Introduce per-cpu tasklet for softirq.
> > static void tasklet_enqueue(struct tasklet *t)
> > {
> > unsigned int cpu = t->scheduled_on;
> >
> > + if ( t->is_percpu )
> > + {
> > + unsigned long flags;
> > + struct list_head *list;
> > +
> > + INIT_LIST_HEAD(&t->list);
> > + BUG_ON( !t->is_softirq );
> > + BUG_ON( cpu != smp_processor_id() ); /* Not implemented yet. */
> > +
> > + local_irq_save(flags);
> > +
> > + list = &__get_cpu_var(softirq_list);
> > + list_add_tail(&t->list, list);
> > + raise_softirq(TASKLET_SOFTIRQ_PERCPU);
> > +
> > + local_irq_restore(flags);
>
> The raise_softirq() call can be done with interrupts re-enabled, which
> reduces the critical window.
>
> __get_cpu_var() does some inspection of the stack pointer behind your
> back. It would be far more efficient in the critical window to take
Exactly 4 lines of assembler code :-)
> "unsigned int cpu = smp_processor_id();" outside and use "per_cpu($FOO,
> cpu)" inside.
<nods> That would indeed be better.
>
> > + return;
> > + }
> > if ( t->is_softirq )
> > {
> > struct list_head *list = &per_cpu(softirq_tasklet_list, cpu);
> > @@ -56,16 +76,25 @@ void tasklet_schedule_on_cpu(struct tasklet *t,
> > unsigned int cpu)
> > {
> > unsigned long flags;
> >
> > - spin_lock_irqsave(&tasklet_lock, flags);
> > + if ( !tasklets_initialised || t->is_dead )
> > + return;
> >
> > - if ( tasklets_initialised && !t->is_dead )
> > + if ( t->is_percpu )
> > {
> > - t->scheduled_on = cpu;
> > - if ( !t->is_running )
> > + if ( !test_and_set_bit(TASKLET_STATE_SCHED, &t->state) )
> > {
> > - list_del(&t->list);
> > + t->scheduled_on = cpu;
> > tasklet_enqueue(t);
> > }
> > + return;
> > + }
> > + spin_lock_irqsave(&tasklet_lock, flags);
> > +
> > + t->scheduled_on = cpu;
> > + if ( !t->is_running )
> > + {
> > + list_del(&t->list);
> > + tasklet_enqueue(t);
> > }
> >
> > spin_unlock_irqrestore(&tasklet_lock, flags);
> > @@ -104,6 +133,66 @@ static void do_tasklet_work(unsigned int cpu, struct
> > list_head *list)
> > }
> > }
> >
> > +void do_tasklet_work_percpu(void)
> > +{
> > + struct tasklet *t = NULL;
> > + struct list_head *head;
> > + bool_t poke = 0;
> > +
> > + local_irq_disable();
> > + head = &__get_cpu_var(softirq_list);
> > +
> > + if ( !list_empty(head) )
> > + {
> > + t = list_entry(head->next, struct tasklet, list);
> > +
> > + if ( head->next == head->prev ) /* One singular item. Re-init
> > head. */
> > + INIT_LIST_HEAD(&__get_cpu_var(softirq_list));
>
> It would be most efficient to hoist "struct list_head *this_softirq_list
> = &this_cpu(softirq_list);" outside the critical region.
<nods>
>
> > + else
> > + {
> > + /* Multiple items, update head to skip 't'. */
> > + struct list_head *list;
> > +
> > + /* One item past 't'. */
> > + list = head->next->next;
> > +
> > + BUG_ON(list == NULL);
> > +
> > + /* And update head to skip 't'. Note that t->list.prev still
> > + * points to head, but we don't care as we only process one
> > tasklet
> > + * and once done the tasklet list is re-init one way or
> > another.
> > + */
> > + head->next = list;
> > + poke = 1;
> > + }
> > + }
> > + local_irq_enable();
> > +
> > + if ( !t )
> > + return; /* Never saw it happend, but we might have a spurious
> > case? */
> > +
> > + if ( tasklet_trylock(t) )
> > + {
> > + if ( !test_and_clear_bit(TASKLET_STATE_SCHED, &t->state) )
> > + BUG();
> > + sync_local_execstate();
> > + t->func(t->data);
> > + tasklet_unlock(t);
> > + if ( poke )
> > + raise_softirq(TASKLET_SOFTIRQ_PERCPU);
> > + /* We could reinit the t->list but tasklet_enqueue does it for us.
> > */
> > + return;
> > + }
> > +
> > + local_irq_disable();
> > +
> > + INIT_LIST_HEAD(&t->list);
> > + list_add_tail(&t->list, &__get_cpu_var(softirq_list));
> > + smp_wmb();
>
> Is this needed? all of this infrastructure is local to the cpu.
Artifact of debugging. Thought I do wonder what to do about
ARM. As I understand it, the world of ARM is a wild place
where there is a need for these barriers to exist. But maybe
I have just heard to many stories.
>
> > + raise_softirq(TASKLET_SOFTIRQ_PERCPU);
> > + local_irq_enable();
> > +}
> > +
> > /* VCPU context work */
> > void do_tasklet(void)
> > {
> > @@ -147,10 +236,29 @@ static void tasklet_softirq_action(void)
> > spin_unlock_irq(&tasklet_lock);
> > }
> >
> > +/* Per CPU softirq context work. */
> > +static void tasklet_softirq_percpu_action(void)
> > +{
> > + do_tasklet_work_percpu();
> > +}
> > +
> > void tasklet_kill(struct tasklet *t)
> > {
> > unsigned long flags;
> >
> > + if ( t->is_percpu )
> > + {
> > + while ( test_and_set_bit(TASKLET_STATE_SCHED, &t->state) )
> > + {
> > + do {
> > + process_pending_softirqs();
> > + } while ( test_bit(TASKLET_STATE_SCHED, &t->state) );
> > + }
> > + tasklet_unlock_wait(t);
> > + clear_bit(TASKLET_STATE_SCHED, &t->state);
> > + t->is_dead = 1;
> > + return;
> > + }
> > spin_lock_irqsave(&tasklet_lock, flags);
> >
> > if ( !list_empty(&t->list) )
> > @@ -208,6 +316,14 @@ void softirq_tasklet_init(
> > t->is_softirq = 1;
> > }
> >
> > +void percpu_tasklet_init(
> > + struct tasklet *t, void (*func)(unsigned long), unsigned long data)
> > +{
> > + tasklet_init(t, func, data);
> > + t->is_softirq = 1;
> > + t->is_percpu = 1;
> > +}
> > +
> > static int cpu_callback(
> > struct notifier_block *nfb, unsigned long action, void *hcpu)
> > {
> > @@ -218,11 +334,13 @@ static int cpu_callback(
> > case CPU_UP_PREPARE:
> > INIT_LIST_HEAD(&per_cpu(tasklet_list, cpu));
> > INIT_LIST_HEAD(&per_cpu(softirq_tasklet_list, cpu));
> > + INIT_LIST_HEAD(&per_cpu(softirq_list, cpu));
> > break;
> > case CPU_UP_CANCELED:
> > case CPU_DEAD:
> > migrate_tasklets_from_cpu(cpu, &per_cpu(tasklet_list, cpu));
> > migrate_tasklets_from_cpu(cpu, &per_cpu(softirq_tasklet_list,
> > cpu));
> > + migrate_tasklets_from_cpu(cpu, &per_cpu(softirq_list, cpu));
> > break;
> > default:
> > break;
> > @@ -242,6 +360,7 @@ void __init tasklet_subsys_init(void)
> > cpu_callback(&cpu_nfb, CPU_UP_PREPARE, hcpu);
> > register_cpu_notifier(&cpu_nfb);
> > open_softirq(TASKLET_SOFTIRQ, tasklet_softirq_action);
> > + open_softirq(TASKLET_SOFTIRQ_PERCPU, tasklet_softirq_percpu_action);
> > tasklets_initialised = 1;
> > }
> >
> > diff --git a/xen/drivers/passthrough/io.c b/xen/drivers/passthrough/io.c
> > index ef75b94..740cee5 100644
> > --- a/xen/drivers/passthrough/io.c
> > +++ b/xen/drivers/passthrough/io.c
> > @@ -114,7 +114,7 @@ int pt_irq_create_bind(
> > spin_unlock(&d->event_lock);
> > return -ENOMEM;
> > }
> > - softirq_tasklet_init(
> > + percpu_tasklet_init(
> > &hvm_irq_dpci->dirq_tasklet,
> > hvm_dirq_assist, (unsigned long)d);
> > for ( i = 0; i < NR_HVM_IRQS; i++ )
> > diff --git a/xen/include/xen/softirq.h b/xen/include/xen/softirq.h
> > index 0c0d481..8b15c8c 100644
> > --- a/xen/include/xen/softirq.h
> > +++ b/xen/include/xen/softirq.h
> > @@ -7,6 +7,7 @@ enum {
> > SCHEDULE_SOFTIRQ,
> > NEW_TLBFLUSH_CLOCK_PERIOD_SOFTIRQ,
> > RCU_SOFTIRQ,
> > + TASKLET_SOFTIRQ_PERCPU,
> > TASKLET_SOFTIRQ,
> > NR_COMMON_SOFTIRQS
> > };
> > diff --git a/xen/include/xen/tasklet.h b/xen/include/xen/tasklet.h
> > index 8c3de7e..9497c47 100644
> > --- a/xen/include/xen/tasklet.h
> > +++ b/xen/include/xen/tasklet.h
> > @@ -17,21 +17,24 @@
> > struct tasklet
> > {
> > struct list_head list;
> > + unsigned long state;
> > int scheduled_on;
> > bool_t is_softirq;
> > bool_t is_running;
> > bool_t is_dead;
> > + bool_t is_percpu;
> > void (*func)(unsigned long);
> > unsigned long data;
> > };
> >
> > -#define _DECLARE_TASKLET(name, func, data, softirq) \
> > +#define _DECLARE_TASKLET(name, func, data, softirq, percpu) \
> > struct tasklet name = { \
> > - LIST_HEAD_INIT(name.list), -1, softirq, 0, 0, func, data }
> > + LIST_HEAD_INIT(name.list), 0, -1, softirq, 0, 0, percpu, \
> > + func, data }
> > #define DECLARE_TASKLET(name, func, data) \
> > - _DECLARE_TASKLET(name, func, data, 0)
> > + _DECLARE_TASKLET(name, func, data, 0, 0)
> > #define DECLARE_SOFTIRQ_TASKLET(name, func, data) \
> > - _DECLARE_TASKLET(name, func, data, 1)
> > + _DECLARE_TASKLET(name, func, data, 1, 0)
> >
> > /* Indicates status of tasklet work on each CPU. */
> > DECLARE_PER_CPU(unsigned long, tasklet_work_to_do);
> > @@ -40,6 +43,54 @@ DECLARE_PER_CPU(unsigned long, tasklet_work_to_do);
> > #define TASKLET_enqueued (1ul << _TASKLET_enqueued)
> > #define TASKLET_scheduled (1ul << _TASKLET_scheduled)
> >
> > +/* These fancy bit manipulations (bit 0 and bit 1) along with using a lock
> > + * operation allow us to have four stages in tasklet life-time.
s/./:/
> > + * a) 0x0: Completely empty (not scheduled nor running).
> > + * b) 0x1: Scheduled but not running. Used to guard in 'tasklet_schedule'
s/to guard/as a guard/
> > + * such that we will only schedule one. If it is scheduled and had
> > never
> > + * run (hence never clearing STATE_SCHED bit), tasklet_kill will spin
> > + * forever on said tasklet. However 'tasklet_schedule' raises the
> > + * softirq associated with the per-cpu - so it will run, albeit there
> > might
> > + * be a race (tasklet_kill spinning until the softirq handler runs).
> > + * c) 0x2: it is running (only on one CPU) and can be scheduled on any
> > + * CPU. The bit 0 - scheduled is cleared at this stage allowing
> > + * 'tasklet_schedule' to succesfully schedule.
> > + * d) 0x3: scheduled and running - only possible if the running tasklet
> > calls
> > + * tasklet_schedule (on same CPU) or the tasklet is scheduled from
> > another
> > + * CPU while the tasklet is running on another CPU.
> > + *
> > + * The two bits play a vital role in assuring that the tasklet is scheduled
> > + * once and runs only once. The steps are:
> > + *
> > + * 1) tasklet_schedule: STATE_SCHED bit set (0x1), added on the per cpu
> > list.
> > + * 2) tasklet_softirq_percpu_action picks one tasklet from the list.
> > Schedules
> > + * itself later if there are more tasklets on it. Tries to set STATE_RUN
> > bit
> > + * (0x3) - if it fails adds tasklet back to the per-cpu list. If it
> > succeeds
> > + * clears the STATE_SCHED bit (0x2). Once tasklet completed, unsets
> > STATE_RUN
s/completed/completes/
> > + * (0x0 or 0x1 if tasklet called tasklet_schedule).
> > + */
> > +enum {
> > + TASKLET_STATE_SCHED, /* Bit 0 */
> > + TASKLET_STATE_RUN
> > +};
> > +
> > +static inline int tasklet_trylock(struct tasklet *t)
> > +{
> > + return !test_and_set_bit(TASKLET_STATE_RUN, &(t)->state);
>
> No need for brackets around (t) for these static inlines.
<nods>
>
> > +}
> > +
> > +static inline void tasklet_unlock(struct tasklet *t)
> > +{
> > + barrier();
>
> clear_bit() has a memory clobber. This barrier() is entirely redundant.
>
> > + clear_bit(TASKLET_STATE_RUN, &(t)->state);
> > +}
> > +static inline void tasklet_unlock_wait(struct tasklet *t)
> > +{
> > + while (test_bit(TASKLET_STATE_RUN, &(t)->state))
> > + {
> > + barrier();
>
> cpu_relax();
Ah yes!
>
> ~Andrew
Thank you for your quick review! (less than hour after posting?!
Is that the record?
>
> > + }
> > +}
> > void tasklet_schedule_on_cpu(struct tasklet *t, unsigned int cpu);
> > void tasklet_schedule(struct tasklet *t);
> > void do_tasklet(void);
> > @@ -48,6 +99,8 @@ void tasklet_init(
> > struct tasklet *t, void (*func)(unsigned long), unsigned long data);
> > void softirq_tasklet_init(
> > struct tasklet *t, void (*func)(unsigned long), unsigned long data);
> > +void percpu_tasklet_init(
> > + struct tasklet *t, void (*func)(unsigned long), unsigned long data);
> > void tasklet_subsys_init(void);
> >
> > #endif /* __XEN_TASKLET_H__ */
>
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