[Xen-devel] [PATCH RFC V4 01/11] x86/spinlock: replace pv spinlocks with pv ticketlocks

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

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@xxxxxxxxxx>
---
 arch/x86/include/asm/paravirt.h       |   30 ++-----------------
 arch/x86/include/asm/paravirt_types.h |   10 ++----
 arch/x86/include/asm/spinlock.h       |   50 ++++++++++++++++++++++++++------
 arch/x86/include/asm/spinlock_types.h |    4 --
 arch/x86/kernel/paravirt-spinlocks.c  |   15 +--------
 arch/x86/xen/spinlock.c               |    7 ++++-
 6 files changed, 56 insertions(+), 60 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index a7d2db9..76cae7a 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -750,36 +750,14 @@ 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 __always_inline void __ticket_lock_spinning(struct arch_spinlock *lock, 
__ticket_t 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 __always_inline void ____ticket_unlock_kick(struct arch_spinlock *lock, 
__ticket_t 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 8e8b9a4..005e24d 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -327,13 +327,11 @@ struct pv_mmu_ops {
 };
 
 struct arch_spinlock;
+#include <asm/spinlock_types.h>
+
 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, __ticket_t ticket);
+       void (*unlock_kick)(struct arch_spinlock *lock, __ticket_t 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 a82c2bf..5efd2f9 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -37,6 +37,32 @@
 # define UNLOCK_LOCK_PREFIX
 #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, 
__ticket_t ticket)
+{
+}
+
+static __always_inline void ____ticket_unlock_kick(struct arch_spinlock *lock, 
__ticket_t ticket)
+{
+}
+
+#endif /* CONFIG_PARAVIRT_SPINLOCKS */
+
+
+/* 
+ * 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);
+}
+
 /*
  * 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
@@ -50,19 +76,24 @@
  * in the high part, because a wide xadd increment of the low part would carry
  * up and contaminate the high part.
  */
-static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
+static __always_inline void __ticket_spin_lock(struct arch_spinlock *lock)
 {
        register struct __raw_tickets inc = { .tail = 1 };
 
        inc = xadd(&lock->tickets, inc);
 
        for (;;) {
-               if (inc.head == inc.tail)
-                       break;
-               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);
        }
-       barrier();              /* make sure nothing creeps before the lock is 
taken */
+out:   barrier();              /* make sure nothing creeps before the lock is 
taken */
 }
 
 static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock)
@@ -81,7 +112,10 @@ 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;
+
        __add(&lock->tickets.head, 1, UNLOCK_LOCK_PREFIX);
+       __ticket_unlock_kick(lock, next);
 }
 
 static inline int __ticket_spin_is_locked(arch_spinlock_t *lock)
@@ -98,8 +132,6 @@ 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);
@@ -132,8 +164,6 @@ 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/include/asm/spinlock_types.h 
b/arch/x86/include/asm/spinlock_types.h
index 8ebd5df..dbe223d 100644
--- a/arch/x86/include/asm/spinlock_types.h
+++ b/arch/x86/include/asm/spinlock_types.h
@@ -1,10 +1,6 @@
 #ifndef _ASM_X86_SPINLOCK_TYPES_H
 #define _ASM_X86_SPINLOCK_TYPES_H
 
-#ifndef __LINUX_SPINLOCK_TYPES_H
-# error "please don't include this file directly"
-#endif
-
 #include <linux/types.h>
 
 #if (CONFIG_NR_CPUS < 256)
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 cc9b1e1..23af06a 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
-- 
1.7.6.4


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