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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.
On 27/08/14 18:58, Konrad Rzeszutek Wilk wrote:
> This implements a lockless per-cpu tasklet mechanism.
>
> The existing tasklet mechanism has a single global
> spinlock that is taken every-time the global list
> is touched. And we use this lock quite a lot - when
> we call do_tasklet_work which is called via an softirq
> and from the idle loop. We take the lock on any
> operation on the tasklet_list.
>
> The problem we are facing is that there are quite a lot of
> tasklets scheduled. The most common one that is invoked is
> the one injecting the VIRQ_TIMER in the guest. Guests
> are not insane and don't set the one-shot or periodic
> clocks to be in sub 1ms intervals (causing said tasklet
> to be scheduled for such small intervalls).
>
> The problem appears when PCI passthrough devices are used
> over many sockets and we have an mix of heavy-interrupt
> guests and idle guests. The idle guests end up seeing
> 1/10 of its RUNNING timeslice eaten by the hypervisor
> (and 40% steal time).
>
> The mechanism by which we inject PCI interrupts is by
> hvm_do_IRQ_dpci which schedules the hvm_dirq_assist
> tasklet every time an interrupt is received.
> The callchain is:
>
> _asm_vmexit_handler
> -> vmx_vmexit_handler
> ->vmx_do_extint
> -> do_IRQ
> -> __do_IRQ_guest
> -> hvm_do_IRQ_dpci
> tasklet_schedule(&dpci->dirq_tasklet);
> [takes lock to put the tasklet on]
>
> [later on the schedule_tail is invoked which is 'vmx_do_resume']
>
> vmx_do_resume
> -> vmx_asm_do_vmentry
> -> call vmx_intr_assist
> -> vmx_process_softirqs
> -> do_softirq
> [executes the tasklet function, takes the
> lock again]
>
> While on other CPUs they might be sitting in a idle loop
> and invoked to deliver an VIRQ_TIMER, which also ends
> up taking the lock twice: first to schedule the
> v->arch.hvm_vcpu.assert_evtchn_irq_tasklet (accounted to
> the guests' BLOCKED_state); then to execute it - which is
> accounted for in the guest's RUNTIME_state.
>
> The end result is that on a 8 socket machine with
> PCI passthrough, where four sockets are busy with interrupts,
> and the other sockets have idle guests - we end up with
> the idle guests having around 40% steal time and 1/10
> of its timeslice (3ms out of 30 ms) being tied up
> taking the lock. The latency of the PCI interrupts delieved
> to guest is also hindered.
>
> With this patch the problem disappears completly.
> That is removing the lock for the PCI passthrough use-case
> (the 'hvm_dirq_assist' case).
>
> As such this patch introduces the code to setup
> softirq per-cpu tasklets and only modifies the PCI
> passthrough cases instead of doing it wholesale. This
> is done because:
> - We want to easily bisect it if things break.
> - We modify the code one section at a time to
> make it easier to review this core code.
>
> Now on the code itself. The Linux code (softirq.c)
> has an per-cpu implementation of tasklets on which
> this was based on. However there are differences:
> - This patch executes one tasklet at a time - similar
> to how the existing implementation does it.
> - We use a double-linked list instead of a single linked
> list. We could use a single-linked list but folks are
> more familiar with 'list_*' type macros.
> - This patch does not have the cross-CPU feeders
> implemented. That code is in the patch
> titled: tasklet: Add cross CPU feeding of per-cpu
> tasklets. This is done to support:
> "tasklet_schedule_on_cpu"
> - We add an temporary 'TASKLET_SOFTIRQ_PERCPU' which
> is can co-exist with the TASKLET_SOFTIRQ. It will be
> replaced in 'tasklet: Remove the old-softirq
> implementation."
>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> ---
> xen/arch/x86/hvm/hvm.c | 2 +-
> xen/common/tasklet.c | 129
> +++++++++++++++++++++++++++++++++++++++++--
> xen/drivers/passthrough/io.c | 2 +-
> xen/include/xen/softirq.h | 1 +
> xen/include/xen/tasklet.h | 61 ++++++++++++++++++--
> 5 files changed, 184 insertions(+), 11 deletions(-)
>
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index 94b18ba..4b4cad1 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -2270,7 +2270,7 @@ int hvm_vcpu_initialise(struct vcpu *v)
> if ( (rc = hvm_funcs.vcpu_initialise(v)) != 0 ) /* teardown:
> hvm_funcs.vcpu_destroy */
> goto fail3;
>
> - softirq_tasklet_init(
> + percpu_tasklet_init(
> &v->arch.hvm_vcpu.assert_evtchn_irq_tasklet,
> (void(*)(unsigned long))hvm_assert_evtchn_irq,
> (unsigned long)v);
> diff --git a/xen/common/tasklet.c b/xen/common/tasklet.c
> index 4e42fa7..319866f 100644
> --- a/xen/common/tasklet.c
> +++ b/xen/common/tasklet.c
> @@ -31,10 +31,30 @@ static DEFINE_PER_CPU(struct list_head,
> softirq_tasklet_list);
> /* Protects all lists and tasklet structures. */
> static DEFINE_SPINLOCK(tasklet_lock);
>
> +static DEFINE_PER_CPU(struct list_head, softirq_list);
> +
> 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
"unsigned int cpu = smp_processor_id();" outside and use "per_cpu($FOO,
cpu)" inside.
> + 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.
> + 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.
> + 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.
> + * a) 0x0: Completely empty (not scheduled nor running).
> + * b) 0x1: Scheduled but not running. Used to guard in 'tasklet_schedule'
> + * 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
> + * (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.
> +}
> +
> +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();
~Andrew
> + }
> +}
> 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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