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Subject[PATCH 07/14] x86/spinlocks: replace pv spinlocks with pv ticketlocks
Date
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

Rather than outright replacing the entire spinlock implementation in
order to paravirtualize it, keep the ticket lock implementation but add
a couple of pvops hooks on the slow patch (long spin on lock, unlocking
a contended lock).

Ticket locks have a number of nice properties, but they also have some
surprising behaviours in virtual environments. They enforce a strict
FIFO ordering on cpus trying to take a lock; however, if the hypervisor
scheduler does not schedule the cpus in the correct order, the system can
waste a huge amount of time spinning until the next cpu can take the lock.

(See Thomas Friebel's talk "Prevent Guests from Spinning Around"
http://www.xen.org/files/xensummitboston08/LHP.pdf for more details.)

To address this, we add two hooks:
- __ticket_spin_lock which is called after the cpu has been
spinning on the lock for a significant number of iterations but has
failed to take the lock (presumably because the cpu holding the lock
has been descheduled). The lock_spinning pvop is expected to block
the cpu until it has been kicked by the current lock holder.
- __ticket_spin_unlock, which on releasing a contended lock
(there are more cpus with tail tickets), it looks to see if the next
cpu is blocked and wakes it if so.

When compiled with CONFIG_PARAVIRT_SPINLOCKS disabled, a set of stub
functions causes all the extra code to go away.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
---
arch/x86/include/asm/paravirt.h | 30 +++-------------------
arch/x86/include/asm/paravirt_types.h | 8 +----
arch/x86/include/asm/spinlock.h | 44 +++++++++++++++++++++++++++------
arch/x86/kernel/paravirt-spinlocks.c | 15 +---------
arch/x86/xen/spinlock.c | 7 ++++-
5 files changed, 50 insertions(+), 54 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 18e3b8a..c864775 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -717,36 +717,14 @@ static inline void __set_fixmap(unsigned /* enum fixed_addresses */ idx,

#if defined(CONFIG_SMP) && defined(CONFIG_PARAVIRT_SPINLOCKS)

-static inline int arch_spin_is_locked(struct arch_spinlock *lock)
+static inline void __ticket_lock_spinning(struct arch_spinlock *lock, unsigned ticket)
{
- return PVOP_CALL1(int, pv_lock_ops.spin_is_locked, lock);
+ PVOP_VCALL2(pv_lock_ops.lock_spinning, lock, ticket);
}

-static inline int arch_spin_is_contended(struct arch_spinlock *lock)
+static inline void ____ticket_unlock_kick(struct arch_spinlock *lock, unsigned ticket)
{
- return PVOP_CALL1(int, pv_lock_ops.spin_is_contended, lock);
-}
-#define arch_spin_is_contended arch_spin_is_contended
-
-static __always_inline void arch_spin_lock(struct arch_spinlock *lock)
-{
- PVOP_VCALL1(pv_lock_ops.spin_lock, lock);
-}
-
-static __always_inline void arch_spin_lock_flags(struct arch_spinlock *lock,
- unsigned long flags)
-{
- PVOP_VCALL2(pv_lock_ops.spin_lock_flags, lock, flags);
-}
-
-static __always_inline int arch_spin_trylock(struct arch_spinlock *lock)
-{
- return PVOP_CALL1(int, pv_lock_ops.spin_trylock, lock);
-}
-
-static __always_inline void arch_spin_unlock(struct arch_spinlock *lock)
-{
- PVOP_VCALL1(pv_lock_ops.spin_unlock, lock);
+ PVOP_VCALL2(pv_lock_ops.unlock_kick, lock, ticket);
}

#endif
diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
index b82bac9..1078474 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -315,12 +315,8 @@ struct pv_mmu_ops {

struct arch_spinlock;
struct pv_lock_ops {
- int (*spin_is_locked)(struct arch_spinlock *lock);
- int (*spin_is_contended)(struct arch_spinlock *lock);
- void (*spin_lock)(struct arch_spinlock *lock);
- void (*spin_lock_flags)(struct arch_spinlock *lock, unsigned long flags);
- int (*spin_trylock)(struct arch_spinlock *lock);
- void (*spin_unlock)(struct arch_spinlock *lock);
+ void (*lock_spinning)(struct arch_spinlock *lock, unsigned ticket);
+ void (*unlock_kick)(struct arch_spinlock *lock, unsigned ticket);
};

/* This contains all the paravirt structures: we get a convenient
diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 3afb1a7..8e379d3 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -50,6 +50,21 @@ static __always_inline void __ticket_unlock_release(struct arch_spinlock *lock)
}
#endif

+/* How long a lock should spin before we consider blocking */
+#define SPIN_THRESHOLD (1 << 11)
+
+#ifndef CONFIG_PARAVIRT_SPINLOCKS
+
+static __always_inline void __ticket_lock_spinning(struct arch_spinlock *lock, unsigned ticket)
+{
+}
+
+static __always_inline void ____ticket_unlock_kick(struct arch_spinlock *lock, unsigned ticket)
+{
+}
+
+#endif /* CONFIG_PARAVIRT_SPINLOCKS */
+
/*
* Ticket locks are conceptually two parts, one indicating the current head of
* the queue, and the other indicating the current tail. The lock is acquired
@@ -83,6 +98,16 @@ static __always_inline struct __raw_tickets __ticket_spin_claim(struct arch_spin
return tickets;
}

+/*
+ * If a spinlock has someone waiting on it, then kick the appropriate
+ * waiting cpu.
+ */
+static __always_inline void __ticket_unlock_kick(struct arch_spinlock *lock, __ticket_t next)
+{
+ if (unlikely(lock->tickets.tail != next))
+ ____ticket_unlock_kick(lock, next);
+}
+
static __always_inline void __ticket_spin_lock(struct arch_spinlock *lock)
{
register struct __raw_tickets inc;
@@ -90,10 +115,15 @@ static __always_inline void __ticket_spin_lock(struct arch_spinlock *lock)
inc = __ticket_spin_claim(lock);

for (;;) {
- if (inc.head == inc.tail)
- goto out;
- cpu_relax();
- inc.head = ACCESS_ONCE(lock->tickets.head);
+ unsigned count = SPIN_THRESHOLD;
+
+ do {
+ if (inc.head == inc.tail)
+ goto out;
+ cpu_relax();
+ inc.head = ACCESS_ONCE(lock->tickets.head);
+ } while (--count);
+ __ticket_lock_spinning(lock, inc.tail);
}
out: barrier(); /* make sure nothing creeps before the lock is taken */
}
@@ -114,7 +144,9 @@ static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)

static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock)
{
+ __ticket_t next = lock->tickets.head + 1;
__ticket_unlock_release(lock);
+ __ticket_unlock_kick(lock, next);
barrier(); /* prevent reordering into locked region */
}

@@ -132,8 +164,6 @@ static inline int __ticket_spin_is_contended(arch_spinlock_t *lock)
return ((tmp.tail - tmp.head) & TICKET_MASK) > 1;
}

-#ifndef CONFIG_PARAVIRT_SPINLOCKS
-
static inline int arch_spin_is_locked(arch_spinlock_t *lock)
{
return __ticket_spin_is_locked(lock);
@@ -166,8 +196,6 @@ static __always_inline void arch_spin_lock_flags(arch_spinlock_t *lock,
arch_spin_lock(lock);
}

-#endif /* CONFIG_PARAVIRT_SPINLOCKS */
-
static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
{
while (arch_spin_is_locked(lock))
diff --git a/arch/x86/kernel/paravirt-spinlocks.c b/arch/x86/kernel/paravirt-spinlocks.c
index 676b8c7..c2e010e 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -7,21 +7,10 @@

#include <asm/paravirt.h>

-static inline void
-default_spin_lock_flags(arch_spinlock_t *lock, unsigned long flags)
-{
- arch_spin_lock(lock);
-}
-
struct pv_lock_ops pv_lock_ops = {
#ifdef CONFIG_SMP
- .spin_is_locked = __ticket_spin_is_locked,
- .spin_is_contended = __ticket_spin_is_contended,
-
- .spin_lock = __ticket_spin_lock,
- .spin_lock_flags = default_spin_lock_flags,
- .spin_trylock = __ticket_spin_trylock,
- .spin_unlock = __ticket_spin_unlock,
+ .lock_spinning = paravirt_nop,
+ .unlock_kick = paravirt_nop,
#endif
};
EXPORT_SYMBOL(pv_lock_ops);
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 23e061b..3d9da72 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -121,6 +121,9 @@ struct xen_spinlock {
unsigned short spinners; /* count of waiting cpus */
};

+static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
+
+#if 0
static int xen_spin_is_locked(struct arch_spinlock *lock)
{
struct xen_spinlock *xl = (struct xen_spinlock *)lock;
@@ -148,7 +151,6 @@ static int xen_spin_trylock(struct arch_spinlock *lock)
return old == 0;
}

-static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
static DEFINE_PER_CPU(struct xen_spinlock *, lock_spinners);

/*
@@ -338,6 +340,7 @@ static void xen_spin_unlock(struct arch_spinlock *lock)
if (unlikely(xl->spinners))
xen_spin_unlock_slow(xl);
}
+#endif

static irqreturn_t dummy_handler(int irq, void *dev_id)
{
@@ -373,12 +376,14 @@ void xen_uninit_lock_cpu(int cpu)

void __init xen_init_spinlocks(void)
{
+#if 0
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_lock_flags = xen_spin_lock_flags;
pv_lock_ops.spin_trylock = xen_spin_trylock;
pv_lock_ops.spin_unlock = xen_spin_unlock;
+#endif
}

#ifdef CONFIG_XEN_DEBUG_FS
--
1.7.2.3


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