Re: [PATCH RFC 4/4] xen: implement Xen-specific spinlocks

[Johannes Weiner]

Hi,

Jeremy Fitzhardinge <jeremy@xxxxxxxx> writes:

> The standard ticket spinlocks are very expensive in a virtual
> environment, because their performance depends on Xen's scheduler
> giving vcpus time in the order that they're supposed to take the
> spinlock.
>
> This implements a Xen-specific spinlock, which should be much more
> efficient.
>
> The fast-path is essentially the old Linux-x86 locks, using a single
> lock byte.  The locker decrements the byte; if the result is 0, then
> they have the lock.  If the lock is negative, then locker must spin
> until the lock is positive again.
>
> When there's contention, the locker spin for 2^16[*] iterations waiting
> to get the lock.  If it fails to get the lock in that time, it adds
> itself to the contention count in the lock and blocks on a per-cpu
> event channel.
>
> When unlocking the spinlock, the locker looks to see if there's anyone
> blocked waiting for the lock by checking for a non-zero waiter count.
> If there's a waiter, it traverses the per-cpu "lock_spinners"
> variable, which contains which lock each CPU is waiting on.  It picks
> one CPU waiting on the lock and sends it an event to wake it up.
>
> This allows efficient fast-path spinlock operation, while allowing
> spinning vcpus to give up their processor time while waiting for a
> contended lock.
>
> [*] 2^16 iterations is threshold at which 98% locks have been taken
> according to Thomas Friebel's Xen Summit talk "Preventing Guests from
> Spinning Around".  Therefore, we'd expect the lock and unlock slow
> paths will only be entered 2% of the time.
>
> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
> Cc: Thomas Friebel <thomas.friebel@xxxxxxx>
> ---
>  arch/x86/xen/smp.c           |  172 +++++++++++++++++++++++++++++++++++++++++-
>  drivers/xen/events.c         |   27 ++++++
>  include/asm-x86/xen/events.h |    1 
>  include/xen/events.h         |    7 +
>  4 files changed, 206 insertions(+), 1 deletion(-)
>
> ===================================================================
> --- a/arch/x86/xen/smp.c
> +++ b/arch/x86/xen/smp.c
> @@ -15,6 +15,7 @@
>   * This does not handle HOTPLUG_CPU yet.
>   */
>  #include <linux/sched.h>
> +#include <linux/kernel_stat.h>
>  #include <linux/err.h>
>  #include <linux/smp.h>
>  
> @@ -34,6 +35,8 @@
>  
>  #include "xen-ops.h"
>  #include "mmu.h"
> +
> +static void __cpuinit xen_init_lock_cpu(int cpu);
>  
>  cpumask_t xen_cpu_initialized_map;
>  
> @@ -179,6 +182,8 @@
>  {
>  	unsigned cpu;
>  
> +	xen_init_lock_cpu(0);
> +
>  	smp_store_cpu_info(0);
>  	cpu_data(0).x86_max_cores = 1;
>  	set_cpu_sibling_map(0);
> @@ -301,6 +306,7 @@
>  	clear_tsk_thread_flag(idle, TIF_FORK);
>  #endif
>  	xen_setup_timer(cpu);
> +	xen_init_lock_cpu(cpu);
>  
>  	per_cpu(cpu_state, cpu) = CPU_UP_PREPARE;
>  
> @@ -413,6 +419,170 @@
>  	return IRQ_HANDLED;
>  }
>  
> +struct xen_spinlock {
> +	unsigned char lock;		/* 0 -> free; 1 -> locked */
> +	unsigned short spinners;	/* count of waiting cpus */
> +};
> +
> +static int xen_spin_is_locked(struct raw_spinlock *lock)
> +{
> +	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
> +
> +	return xl->lock != 0;
> +}
> +
> +static int xen_spin_is_contended(struct raw_spinlock *lock)
> +{
> +	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
> +
> +	/* Not strictly true; this is only the count of contended
> +	   lock-takers entering the slow path. */
> +	return xl->spinners != 0;
> +}
> +
> +static int xen_spin_trylock(struct raw_spinlock *lock)
> +{
> +	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
> +	u8 old = 1;
> +
> +	asm("xchgb %b0,%1"
> +	    : "+q" (old), "+m" (xl->lock) : : "memory");
> +
> +	return old == 0;
> +}
> +
> +static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
> +static DEFINE_PER_CPU(struct xen_spinlock *, lock_spinners);

The plural is a bit misleading, as this is a single pointer per CPU.

> +static inline void spinning_lock(struct xen_spinlock *xl)
> +{
> +	__get_cpu_var(lock_spinners) = xl;
> +	wmb();			/* set lock of interest before count */
> +	asm(LOCK_PREFIX " incw %0"
> +	    : "+m" (xl->spinners) : : "memory");
> +}
> +
> +static inline void unspinning_lock(struct xen_spinlock *xl)
> +{
> +	asm(LOCK_PREFIX " decw %0"
> +	    : "+m" (xl->spinners) : : "memory");
> +	wmb();			/* decrement count before clearing lock */
> +	__get_cpu_var(lock_spinners) = NULL;
> +}
> +
> +static noinline int xen_spin_lock_slow(struct raw_spinlock *lock)
> +{
> +	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
> +	int irq = __get_cpu_var(lock_kicker_irq);
> +	int ret;
> +
> +	/* If kicker interrupts not initialized yet, just spin */
> +	if (irq == -1)
> +		return 0;
> +
> +	/* announce we're spinning */
> +	spinning_lock(xl);
> +
> +	/* clear pending */
> +	xen_clear_irq_pending(irq);
> +
> +	/* check again make sure it didn't become free while
> +	   we weren't looking  */
> +	ret = xen_spin_trylock(lock);
> +	if (ret)
> +		goto out;
> +
> +	/* block until irq becomes pending */
> +	xen_poll_irq(irq);
> +	kstat_this_cpu.irqs[irq]++;
> +
> +out:
> +	unspinning_lock(xl);
> +	return ret;
> +}
> +
> +static void xen_spin_lock(struct raw_spinlock *lock)
> +{
> +	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
> +	int timeout;
> +	u8 oldval;
> +
> +	do {
> +		timeout = 1 << 10;
> +
> +		asm("1: xchgb %1,%0\n"
> +		    "   testb %1,%1\n"
> +		    "   jz 3f\n"
> +		    "2: rep;nop\n"
> +		    "   cmpb $0,%0\n"
> +		    "   je 1b\n"
> +		    "   dec %2\n"
> +		    "   jnz 2b\n"
> +		    "3:\n"
> +		    : "+m" (xl->lock), "=q" (oldval), "+r" (timeout)
> +		    : "1" (1)
> +		    : "memory");
> +
> +	} while (unlikely(oldval != 0 && !xen_spin_lock_slow(lock)));
> +}
> +
> +static noinline void xen_spin_unlock_slow(struct xen_spinlock *xl)
> +{
> +	int cpu;
> +
> +	for_each_online_cpu(cpu) {

Would it be feasible to have a bitmap for the spinning CPUs in order to
do a for_each_spinning_cpu() here instead?  Or is setting a bit in
spinning_lock() and unsetting it in unspinning_lock() more overhead than
going over all CPUs here?

> +		/* XXX should mix up next cpu selection */
> +		if (per_cpu(lock_spinners, cpu) == xl) {
> +			xen_send_IPI_one(cpu, XEN_SPIN_UNLOCK_VECTOR);
> +			break;
> +		}
> +	}
> +}
> +
> +static void xen_spin_unlock(struct raw_spinlock *lock)
> +{
> +	struct xen_spinlock *xl = (struct xen_spinlock *)lock;
> +
> +	smp_wmb();		/* make sure no writes get moved after unlock */
> +	xl->lock = 0;		/* release lock */
> +
> +	/* make sure unlock happens before kick */
> +	barrier();
> +
> +	if (unlikely(xl->spinners))
> +		xen_spin_unlock_slow(xl);
> +}
> +
> +static __cpuinit void xen_init_lock_cpu(int cpu)
> +{
> +	int irq;
> +	const char *name;
> +
> +	name = kasprintf(GFP_KERNEL, "spinlock%d", cpu);
> +	irq = bind_ipi_to_irqhandler(XEN_SPIN_UNLOCK_VECTOR,
> +				     cpu,
> +				     xen_reschedule_interrupt,
> +				     IRQF_DISABLED|IRQF_PERCPU|IRQF_NOBALANCING,
> +				     name,
> +				     NULL);
> +
> +	if (irq >= 0) {
> +		disable_irq(irq); /* make sure it's never delivered */
> +		per_cpu(lock_kicker_irq, cpu) = irq;
> +	}
> +
> +	printk("cpu %d spinlock event irq %d\n", cpu, irq);
> +}
> +
> +static void __init xen_init_spinlocks(void)
> +{
> +	pv_lock_ops.spin_is_locked = xen_spin_is_locked;
> +	pv_lock_ops.spin_is_contended = xen_spin_is_contended;
> +	pv_lock_ops.spin_lock = xen_spin_lock;
> +	pv_lock_ops.spin_trylock = xen_spin_trylock;
> +	pv_lock_ops.spin_unlock = xen_spin_unlock;
> +}
> +
>  static const struct smp_ops xen_smp_ops __initdata = {
>  	.smp_prepare_boot_cpu = xen_smp_prepare_boot_cpu,
>  	.smp_prepare_cpus = xen_smp_prepare_cpus,
> @@ -430,5 +600,5 @@
>  {
>  	smp_ops = xen_smp_ops;
>  	xen_fill_possible_map();
> -	paravirt_use_bytelocks();
> +	xen_init_spinlocks();
>  }
> ===================================================================
> --- a/drivers/xen/events.c
> +++ b/drivers/xen/events.c
> @@ -741,6 +741,33 @@
>  	}
>  }
>  
> +/* Clear an irq's pending state, in preparation for polling on it */
> +void xen_clear_irq_pending(int irq)
> +{
> +	int evtchn = evtchn_from_irq(irq);
> +
> +	if (VALID_EVTCHN(evtchn))
> +		clear_evtchn(evtchn);
> +}
> +
> +/* Poll waiting for an irq to become pending.  In the usual case, the
> +   irq will be disabled so it won't deliver an interrupt. */
> +void xen_poll_irq(int irq)
> +{
> +	evtchn_port_t evtchn = evtchn_from_irq(irq);
> +
> +	if (VALID_EVTCHN(evtchn)) {
> +		struct sched_poll poll;
> +
> +		poll.nr_ports = 1;
> +		poll.timeout = 0;
> +		poll.ports = &evtchn;
> +
> +		if (HYPERVISOR_sched_op(SCHEDOP_poll, &poll) != 0)
> +			BUG();
> +	}
> +}
> +
>  void xen_irq_resume(void)
>  {
>  	unsigned int cpu, irq, evtchn;
> ===================================================================
> --- a/include/asm-x86/xen/events.h
> +++ b/include/asm-x86/xen/events.h
> @@ -5,6 +5,7 @@
>  	XEN_RESCHEDULE_VECTOR,
>  	XEN_CALL_FUNCTION_VECTOR,
>  	XEN_CALL_FUNCTION_SINGLE_VECTOR,
> +	XEN_SPIN_UNLOCK_VECTOR,
>  
>  	XEN_NR_IPIS,
>  };
> ===================================================================
> --- a/include/xen/events.h
> +++ b/include/xen/events.h
> @@ -45,4 +45,11 @@
>  extern void xen_irq_resume(void);
>  extern void xen_evtchn_do_upcall(struct pt_regs *regs);
>  
> +/* Clear an irq's pending state, in preparation for polling on it */
> +void xen_clear_irq_pending(int irq);
> +
> +/* Poll waiting for an irq to become pending.  In the usual case, the
> +   irq will be disabled so it won't deliver an interrupt. */
> +void xen_poll_irq(int irq);
> +
>  #endif	/* _XEN_EVENTS_H */

	Hannes
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