Re: [Xen-devel] [PATCH RFC v5 4/8] pvqspinlock, x86: Allow unfair spinlock in a real PV environment

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Feb 26, 2014 at 10:14:24AM -0500, Waiman Long wrote:
> Locking is always an issue in a virtualized environment as the virtual
> CPU that is waiting on a lock may get scheduled out and hence block
> any progress in lock acquisition even when the lock has been freed.
> 
> One solution to this problem is to allow unfair lock in a
> para-virtualized environment. In this case, a new lock acquirer can
> come and steal the lock if the next-in-line CPU to get the lock is
> scheduled out. Unfair lock in a native environment is generally not a

Hmm, how do you know if the 'next-in-line CPU' is scheduled out? As
in the hypervisor knows - but you as a guest might have no idea
of it.

> good idea as there is a possibility of lock starvation for a heavily
> contended lock.

Should this then detect whether it is running under a virtualization
and only then activate itself? And when run under baremetal don't enable?

> 
> This patch add a new configuration option for the x86
> architecture to enable the use of unfair queue spinlock
> (PARAVIRT_UNFAIR_LOCKS) in a real para-virtualized guest. A jump label
> (paravirt_unfairlocks_enabled) is used to switch between a fair and
> an unfair version of the spinlock code. This jump label will only be
> enabled in a real PV guest.

As opposed to fake PV guest :-) I think you can remove the 'real'.


> 
> Enabling this configuration feature decreases the performance of an
> uncontended lock-unlock operation by about 1-2%.

Presumarily on baremetal right?

> 
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
> ---
>  arch/x86/Kconfig                     |   11 +++++
>  arch/x86/include/asm/qspinlock.h     |   74 
> ++++++++++++++++++++++++++++++++++
>  arch/x86/kernel/Makefile             |    1 +
>  arch/x86/kernel/paravirt-spinlocks.c |    7 +++
>  4 files changed, 93 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 5bf70ab..8d7c941 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -645,6 +645,17 @@ config PARAVIRT_SPINLOCKS
>  
>         If you are unsure how to answer this question, answer Y.
>  
> +config PARAVIRT_UNFAIR_LOCKS
> +     bool "Enable unfair locks in a para-virtualized guest"
> +     depends on PARAVIRT && SMP && QUEUE_SPINLOCK
> +     depends on !CONFIG_X86_OOSTORE && !CONFIG_X86_PPRO_FENCE
> +     ---help---
> +       This changes the kernel to use unfair locks in a real
> +       para-virtualized guest system. This will help performance
> +       in most cases. However, there is a possibility of lock
> +       starvation on a heavily contended lock especially in a
> +       large guest with many virtual CPUs.
> +
>  source "arch/x86/xen/Kconfig"
>  
>  config KVM_GUEST
> diff --git a/arch/x86/include/asm/qspinlock.h 
> b/arch/x86/include/asm/qspinlock.h
> index 98db42e..c278aed 100644
> --- a/arch/x86/include/asm/qspinlock.h
> +++ b/arch/x86/include/asm/qspinlock.h
> @@ -56,4 +56,78 @@ static inline void queue_spin_unlock(struct qspinlock 
> *lock)
>  
>  #include <asm-generic/qspinlock.h>
>  
> +#ifdef CONFIG_PARAVIRT_UNFAIR_LOCKS
> +/**
> + * queue_spin_lock_unfair - acquire a queue spinlock unfairly
> + * @lock: Pointer to queue spinlock structure
> + */
> +static __always_inline void queue_spin_lock_unfair(struct qspinlock *lock)
> +{
> +     union arch_qspinlock *qlock = (union arch_qspinlock *)lock;
> +
> +     if (likely(cmpxchg(&qlock->lock, 0, _QSPINLOCK_LOCKED) == 0))
> +             return;
> +     /*
> +      * Since the lock is now unfair, there is no need to activate
> +      * the 2-task quick spinning code path.
> +      */
> +     queue_spin_lock_slowpath(lock, -1);
> +}
> +
> +/**
> + * queue_spin_trylock_unfair - try to acquire the queue spinlock unfairly
> + * @lock : Pointer to queue spinlock structure
> + * Return: 1 if lock acquired, 0 if failed
> + */
> +static __always_inline int queue_spin_trylock_unfair(struct qspinlock *lock)
> +{
> +     union arch_qspinlock *qlock = (union arch_qspinlock *)lock;
> +
> +     if (!qlock->lock &&
> +        (cmpxchg(&qlock->lock, 0, _QSPINLOCK_LOCKED) == 0))
> +             return 1;
> +     return 0;
> +}
> +
> +/*
> + * Redefine arch_spin_lock and arch_spin_trylock as inline functions that 
> will
> + * jump to the unfair versions if the static key paravirt_unfairlocks_enabled
> + * is true.
> + */
> +#undef arch_spin_lock
> +#undef arch_spin_trylock
> +#undef arch_spin_lock_flags
> +
> +extern struct static_key paravirt_unfairlocks_enabled;
> +
> +/**
> + * arch_spin_lock - acquire a queue spinlock
> + * @lock: Pointer to queue spinlock structure
> + */
> +static inline void arch_spin_lock(struct qspinlock *lock)
> +{
> +     if (static_key_false(&paravirt_unfairlocks_enabled)) {
> +             queue_spin_lock_unfair(lock);
> +             return;
> +     }
> +     queue_spin_lock(lock);

What happens when you are booting and you are in the middle of using a
ticketlock (say you are waiting for it and your are in the slow-path)
 and suddenly the unfairlocks_enabled is turned on.

All the other CPUs start using the unfair version and are you still
in the ticketlock unlocker (or worst, locker and going to sleep).


> +}
> +
> +/**
> + * arch_spin_trylock - try to acquire the queue spinlock
> + * @lock : Pointer to queue spinlock structure
> + * Return: 1 if lock acquired, 0 if failed
> + */
> +static inline int arch_spin_trylock(struct qspinlock *lock)
> +{
> +     if (static_key_false(&paravirt_unfairlocks_enabled)) {
> +             return queue_spin_trylock_unfair(lock);
> +     }
> +     return queue_spin_trylock(lock);
> +}
> +
> +#define arch_spin_lock_flags(l, f)   arch_spin_lock(l)
> +
> +#endif /* CONFIG_PARAVIRT_UNFAIR_LOCKS */
> +
>  #endif /* _ASM_X86_QSPINLOCK_H */
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index cb648c8..1107a20 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -88,6 +88,7 @@ obj-$(CONFIG_DEBUG_NMI_SELFTEST) += nmi_selftest.o
>  obj-$(CONFIG_KVM_GUEST)              += kvm.o kvmclock.o
>  obj-$(CONFIG_PARAVIRT)               += paravirt.o paravirt_patch_$(BITS).o
>  obj-$(CONFIG_PARAVIRT_SPINLOCKS)+= paravirt-spinlocks.o
> +obj-$(CONFIG_PARAVIRT_UNFAIR_LOCKS)+= paravirt-spinlocks.o
>  obj-$(CONFIG_PARAVIRT_CLOCK) += pvclock.o
>  
>  obj-$(CONFIG_PCSPKR_PLATFORM)        += pcspeaker.o
> diff --git a/arch/x86/kernel/paravirt-spinlocks.c 
> b/arch/x86/kernel/paravirt-spinlocks.c
> index bbb6c73..a50032a 100644
> --- a/arch/x86/kernel/paravirt-spinlocks.c
> +++ b/arch/x86/kernel/paravirt-spinlocks.c
> @@ -8,6 +8,7 @@
>  
>  #include <asm/paravirt.h>
>  
> +#ifdef CONFIG_PARAVIRT_SPINLOCKS
>  struct pv_lock_ops pv_lock_ops = {
>  #ifdef CONFIG_SMP
>       .lock_spinning = __PV_IS_CALLEE_SAVE(paravirt_nop),
> @@ -18,3 +19,9 @@ EXPORT_SYMBOL(pv_lock_ops);
>  
>  struct static_key paravirt_ticketlocks_enabled = STATIC_KEY_INIT_FALSE;
>  EXPORT_SYMBOL(paravirt_ticketlocks_enabled);
> +#endif
> +
> +#ifdef CONFIG_PARAVIRT_UNFAIR_LOCKS
> +struct static_key paravirt_unfairlocks_enabled = STATIC_KEY_INIT_FALSE;
> +EXPORT_SYMBOL(paravirt_unfairlocks_enabled);
> +#endif
> -- 
> 1.7.1
> 

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