Re: [Xen-devel] [PATCH RFC V4 4/5] kvm : pv-ticketlocks support for linux guests running on KVM hypervisor

[Marcelo Tosatti <mtosatti@xxxxxxxxxx>]

On Sat, Jan 14, 2012 at 11:56:46PM +0530, Raghavendra K T wrote:
> Extends Linux guest running on KVM hypervisor to support pv-ticketlocks. 
> 
> During smp_boot_cpus  paravirtualied KVM guest detects if the hypervisor has
> required feature (KVM_FEATURE_PVLOCK_KICK) to support pv-ticketlocks. If so,
>  support for pv-ticketlocks is registered via pv_lock_ops.
> 
> Use KVM_HC_KICK_CPU hypercall to wakeup waiting/halted vcpu.
> 
> Signed-off-by: Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Suzuki Poulose <suzuki@xxxxxxxxxx>
> Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
> ---
> diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
> index 7a94987..cf5327c 100644
> --- a/arch/x86/include/asm/kvm_para.h
> +++ b/arch/x86/include/asm/kvm_para.h
> @@ -195,10 +195,20 @@ void kvm_async_pf_task_wait(u32 token);
>  void kvm_async_pf_task_wake(u32 token);
>  u32 kvm_read_and_reset_pf_reason(void);
>  extern void kvm_disable_steal_time(void);
> -#else
> -#define kvm_guest_init() do { } while (0)
> +
> +#ifdef CONFIG_PARAVIRT_SPINLOCKS
> +void __init kvm_spinlock_init(void);
> +#else /* CONFIG_PARAVIRT_SPINLOCKS */
> +static void kvm_spinlock_init(void)
> +{
> +}
> +#endif /* CONFIG_PARAVIRT_SPINLOCKS */
> +
> +#else /* CONFIG_KVM_GUEST */
> +#define kvm_guest_init() do {} while (0)
>  #define kvm_async_pf_task_wait(T) do {} while(0)
>  #define kvm_async_pf_task_wake(T) do {} while(0)
> +
>  static inline u32 kvm_read_and_reset_pf_reason(void)
>  {
>       return 0;
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index a9c2116..ec55a0b 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -33,6 +33,7 @@
>  #include <linux/sched.h>
>  #include <linux/slab.h>
>  #include <linux/kprobes.h>
> +#include <linux/debugfs.h>
>  #include <asm/timer.h>
>  #include <asm/cpu.h>
>  #include <asm/traps.h>
> @@ -545,6 +546,7 @@ static void __init kvm_smp_prepare_boot_cpu(void)
>  #endif
>       kvm_guest_cpu_init();
>       native_smp_prepare_boot_cpu();
> +     kvm_spinlock_init();
>  }
>  
>  static void __cpuinit kvm_guest_cpu_online(void *dummy)
> @@ -627,3 +629,250 @@ static __init int activate_jump_labels(void)
>       return 0;
>  }
>  arch_initcall(activate_jump_labels);
> +
> +#ifdef CONFIG_PARAVIRT_SPINLOCKS
> +
> +enum kvm_contention_stat {
> +     TAKEN_SLOW,
> +     TAKEN_SLOW_PICKUP,
> +     RELEASED_SLOW,
> +     RELEASED_SLOW_KICKED,
> +     NR_CONTENTION_STATS
> +};
> +
> +#ifdef CONFIG_KVM_DEBUG_FS
> +
> +static struct kvm_spinlock_stats
> +{
> +     u32 contention_stats[NR_CONTENTION_STATS];
> +
> +#define HISTO_BUCKETS        30
> +     u32 histo_spin_blocked[HISTO_BUCKETS+1];
> +
> +     u64 time_blocked;
> +} spinlock_stats;
> +
> +static u8 zero_stats;
> +
> +static inline void check_zero(void)
> +{
> +     u8 ret;
> +     u8 old = ACCESS_ONCE(zero_stats);
> +     if (unlikely(old)) {
> +             ret = cmpxchg(&zero_stats, old, 0);
> +             /* This ensures only one fellow resets the stat */
> +             if (ret == old)
> +                     memset(&spinlock_stats, 0, sizeof(spinlock_stats));
> +     }
> +}
> +
> +static inline void add_stats(enum kvm_contention_stat var, u32 val)
> +{
> +     check_zero();
> +     spinlock_stats.contention_stats[var] += val;
> +}
> +
> +
> +static inline u64 spin_time_start(void)
> +{
> +     return sched_clock();
> +}
> +
> +static void __spin_time_accum(u64 delta, u32 *array)
> +{
> +     unsigned index = ilog2(delta);
> +
> +     check_zero();
> +
> +     if (index < HISTO_BUCKETS)
> +             array[index]++;
> +     else
> +             array[HISTO_BUCKETS]++;
> +}
> +
> +static inline void spin_time_accum_blocked(u64 start)
> +{
> +     u32 delta = sched_clock() - start;
> +
> +     __spin_time_accum(delta, spinlock_stats.histo_spin_blocked);
> +     spinlock_stats.time_blocked += delta;
> +}
> +
> +static struct dentry *d_spin_debug;
> +static struct dentry *d_kvm_debug;
> +
> +struct dentry *kvm_init_debugfs(void)
> +{
> +     d_kvm_debug = debugfs_create_dir("kvm", NULL);
> +     if (!d_kvm_debug)
> +             printk(KERN_WARNING "Could not create 'kvm' debugfs 
> directory\n");
> +
> +     return d_kvm_debug;
> +}
> +
> +static int __init kvm_spinlock_debugfs(void)
> +{
> +     struct dentry *d_kvm = kvm_init_debugfs();
> +
> +     if (d_kvm == NULL)
> +             return -ENOMEM;
> +
> +     d_spin_debug = debugfs_create_dir("spinlocks", d_kvm);
> +
> +     debugfs_create_u8("zero_stats", 0644, d_spin_debug, &zero_stats);
> +
> +     debugfs_create_u32("taken_slow", 0444, d_spin_debug,
> +                &spinlock_stats.contention_stats[TAKEN_SLOW]);
> +     debugfs_create_u32("taken_slow_pickup", 0444, d_spin_debug,
> +                &spinlock_stats.contention_stats[TAKEN_SLOW_PICKUP]);
> +
> +     debugfs_create_u32("released_slow", 0444, d_spin_debug,
> +                &spinlock_stats.contention_stats[RELEASED_SLOW]);
> +     debugfs_create_u32("released_slow_kicked", 0444, d_spin_debug,
> +                &spinlock_stats.contention_stats[RELEASED_SLOW_KICKED]);
> +
> +     debugfs_create_u64("time_blocked", 0444, d_spin_debug,
> +                        &spinlock_stats.time_blocked);
> +
> +     debugfs_create_u32_array("histo_blocked", 0444, d_spin_debug,
> +                  spinlock_stats.histo_spin_blocked, HISTO_BUCKETS + 1);
> +
> +     return 0;
> +}
> +fs_initcall(kvm_spinlock_debugfs);
> +#else  /* !CONFIG_KVM_DEBUG_FS */
> +#define TIMEOUT                      (1 << 10)
> +static inline void add_stats(enum kvm_contention_stat var, u32 val)
> +{
> +}
> +
> +static inline u64 spin_time_start(void)
> +{
> +     return 0;
> +}
> +
> +static inline void spin_time_accum_blocked(u64 start)
> +{
> +}
> +#endif  /* CONFIG_KVM_DEBUG_FS */
> +
> +struct kvm_lock_waiting {
> +     struct arch_spinlock *lock;
> +     __ticket_t want;
> +};
> +
> +/* cpus 'waiting' on a spinlock to become available */
> +static cpumask_t waiting_cpus;
> +
> +/* Track spinlock on which a cpu is waiting */
> +static DEFINE_PER_CPU(struct kvm_lock_waiting, lock_waiting);
> +
> +static void kvm_lock_spinning(struct arch_spinlock *lock, __ticket_t want)
> +{
> +     struct kvm_lock_waiting *w = &__get_cpu_var(lock_waiting);
> +     int cpu = smp_processor_id();
> +     u64 start;
> +     unsigned long flags;
> +
> +     start = spin_time_start();
> +
> +     /*
> +      * Make sure an interrupt handler can't upset things in a
> +      * partially setup state.
> +      */
> +     local_irq_save(flags);
> +
> +     /*
> +      * The ordering protocol on this is that the "lock" pointer
> +      * may only be set non-NULL if the "want" ticket is correct.
> +      * If we're updating "want", we must first clear "lock".
> +      */
> +     w->lock = NULL;
> +     smp_wmb();
> +     w->want = want;
> +     smp_wmb();
> +     w->lock = lock;
> +
> +     add_stats(TAKEN_SLOW, 1);
> +
> +     /*
> +      * This uses set_bit, which is atomic but we should not rely on its
> +      * reordering gurantees. So barrier is needed after this call.
> +      */
> +     cpumask_set_cpu(cpu, &waiting_cpus);
> +
> +     barrier();
> +
> +     /*
> +      * Mark entry to slowpath before doing the pickup test to make
> +      * sure we don't deadlock with an unlocker.
> +      */
> +     __ticket_enter_slowpath(lock);
> +
> +     /*
> +      * check again make sure it didn't become free while
> +      * we weren't looking.
> +      */
> +     if (ACCESS_ONCE(lock->tickets.head) == want) {
> +             add_stats(TAKEN_SLOW_PICKUP, 1);
> +             goto out;
> +     }
> +
> +     /* Allow interrupts while blocked */
> +     local_irq_restore(flags);
> +
> +     /* halt until it's our turn and kicked. */
> +     halt();
> +
> +     local_irq_save(flags);
> +out:
> +     cpumask_clear_cpu(cpu, &waiting_cpus);
> +     w->lock = NULL;
> +     local_irq_restore(flags);
> +     spin_time_accum_blocked(start);
> +}
> +PV_CALLEE_SAVE_REGS_THUNK(kvm_lock_spinning);
> +
> +/* Kick a cpu by its apicid*/
> +static inline void kvm_kick_cpu(int apicid)
> +{
> +     kvm_hypercall1(KVM_HC_KICK_CPU, apicid);
> +}
> +
> +/* Kick vcpu waiting on @lock->head to reach value @ticket */
> +static void kvm_unlock_kick(struct arch_spinlock *lock, __ticket_t ticket)
> +{
> +     int cpu;
> +     int apicid;
> +
> +     add_stats(RELEASED_SLOW, 1);
> +
> +     for_each_cpu(cpu, &waiting_cpus) {
> +             const struct kvm_lock_waiting *w = &per_cpu(lock_waiting, cpu);
> +             if (ACCESS_ONCE(w->lock) == lock &&
> +                 ACCESS_ONCE(w->want) == ticket) {
> +                     add_stats(RELEASED_SLOW_KICKED, 1);
> +                     apicid = per_cpu(x86_cpu_to_apicid, cpu);
> +                     kvm_kick_cpu(apicid);
> +                     break;
> +             }
> +     }

What prevents a kick from being lost here, if say, the waiter is at
local_irq_save in kvm_lock_spinning, before the lock/want assignments?

> +
> +/*
> + * Setup pv_lock_ops to exploit KVM_FEATURE_PVLOCK_KICK if present.
> + */
> +void __init kvm_spinlock_init(void)
> +{
> +     if (!kvm_para_available())
> +             return;
> +     /* Does host kernel support KVM_FEATURE_PVLOCK_KICK? */
> +     if (!kvm_para_has_feature(KVM_FEATURE_PVLOCK_KICK))
> +             return;
> +
> +     jump_label_inc(&paravirt_ticketlocks_enabled);
> +
> +     pv_lock_ops.lock_spinning = PV_CALLEE_SAVE(kvm_lock_spinning);
> +     pv_lock_ops.unlock_kick = kvm_unlock_kick;
> +}
> +#endif       /* CONFIG_PARAVIRT_SPINLOCKS */
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c7b05fc..4d7a950 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5754,8 +5754,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  
>       local_irq_disable();
>  
> -     if (vcpu->mode == EXITING_GUEST_MODE || vcpu->requests
> -         || need_resched() || signal_pending(current)) {
> +     if (vcpu->mode == EXITING_GUEST_MODE
> +              || (vcpu->requests & ~(1UL<<KVM_REQ_PVLOCK_KICK))
> +              || need_resched() || signal_pending(current)) {
>               vcpu->mode = OUTSIDE_GUEST_MODE;
>               smp_wmb();
>               local_irq_enable();
> @@ -6711,6 +6712,7 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
>               !vcpu->arch.apf.halted)
>               || !list_empty_careful(&vcpu->async_pf.done)
>               || vcpu->arch.mp_state == KVM_MP_STATE_SIPI_RECEIVED
> +             || kvm_check_request(KVM_REQ_PVLOCK_KICK, vcpu)

The bit should only be read here (kvm_arch_vcpu_runnable), but cleared
on vcpu entry (along with the other kvm_check_request processing).

Then the first hunk becomes unnecessary.

Please do not mix host/guest patches.


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