[PATCH v2] locking/pvqspinlock: Add lock holder CPU argument to pv_wait()

From: Waiman Long
Date: Thu Apr 14 2016 - 14:42:37 EST

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Pan Xinhui was asking for a lock holder cpu argument in pv_wait()
to help the porting of pvqspinlock to PPC. The new argument will can
potentially help hypervisor expediate the execution of the critical
section so that the lock holder vCPU can release the lock sooner.

This patch does just that by storing the previous node vCPU number.
In pv_wait_head_or_lock(), pv_wait() will be called with that vCPU
number as it is likely to be the lock holder.

In pv_wait_node(), the newly added pv_lookup_hash() function will
be called to look up the queue head and pass in the lock holder vCPU
number stored there.

This patch introduces negligible overhead to the current pvqspinlock
code. The extra lockcpu argument isn't currently used in x86
architecture.

Suggested-by: Pan Xinhui <xinhui@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
---
v1->v2:
- In pv_wait_node(), lookup the hash table for the queue head and pass
the lock holder cpu number to pv_wait().

arch/x86/include/asm/paravirt.h | 4 +-
arch/x86/include/asm/paravirt_types.h | 2 +-
arch/x86/kernel/kvm.c | 2 +-
arch/x86/xen/spinlock.c | 2 +-
kernel/locking/qspinlock.c | 5 ++-
kernel/locking/qspinlock_paravirt.h | 58 ++++++++++++++++++++++++++++++---
kernel/locking/qspinlock_stat.h | 8 ++--
7 files changed, 65 insertions(+), 16 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 601f1b8..b89eccf 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -676,9 +676,9 @@ static __always_inline void pv_queued_spin_unlock(struct qspinlock *lock)
PVOP_VCALLEE1(pv_lock_ops.queued_spin_unlock, lock);
}

-static __always_inline void pv_wait(u8 *ptr, u8 val)
+static __always_inline void pv_wait(u8 *ptr, u8 val, int lockcpu)
{
- PVOP_VCALL2(pv_lock_ops.wait, ptr, val);
+ PVOP_VCALL3(pv_lock_ops.wait, ptr, val, lockcpu);
}

static __always_inline void pv_kick(int cpu)
diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
index e8c2326..2fc26c1 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -312,7 +312,7 @@ struct pv_lock_ops {
void (*queued_spin_lock_slowpath)(struct qspinlock *lock, u32 val);
struct paravirt_callee_save queued_spin_unlock;

- void (*wait)(u8 *ptr, u8 val);
+ void (*wait)(u8 *ptr, u8 val, int lockcpu);
void (*kick)(int cpu);
#else /* !CONFIG_QUEUED_SPINLOCKS */
struct paravirt_callee_save lock_spinning;
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index dc1207e..47ab4e1 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -590,7 +590,7 @@ static void kvm_kick_cpu(int cpu)

#include <asm/qspinlock.h>

-static void kvm_wait(u8 *ptr, u8 val)
+static void kvm_wait(u8 *ptr, u8 val, int lockcpu)
{
unsigned long flags;

diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 9e2ba5c..6f78c41 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -33,7 +33,7 @@ static void xen_qlock_kick(int cpu)
/*
* Halt the current CPU & release it back to the host
*/
-static void xen_qlock_wait(u8 *byte, u8 val)
+static void xen_qlock_wait(u8 *byte, u8 val, int lockcpu)
{
int irq = __this_cpu_read(lock_kicker_irq);

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index ce2f75e..99f31e4 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -248,7 +248,8 @@ static __always_inline void set_locked(struct qspinlock *lock)
*/

static __always_inline void __pv_init_node(struct mcs_spinlock *node) { }
-static __always_inline void __pv_wait_node(struct mcs_spinlock *node,
+static __always_inline void __pv_wait_node(struct qspinlock *lock,
+ struct mcs_spinlock *node,
struct mcs_spinlock *prev) { }
static __always_inline void __pv_kick_node(struct qspinlock *lock,
struct mcs_spinlock *node) { }
@@ -407,7 +408,7 @@ queue:
prev = decode_tail(old);
WRITE_ONCE(prev->next, node);

- pv_wait_node(node, prev);
+ pv_wait_node(lock, node, prev);
arch_mcs_spin_lock_contended(&node->locked);

/*
diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index 21ede57..895224e 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -51,6 +51,7 @@ struct pv_node {
struct mcs_spinlock __res[3];

int cpu;
+ int prev_cpu; /* Previous node cpu */
u8 state;
};

@@ -156,8 +157,7 @@ static __always_inline int trylock_clear_pending(struct qspinlock *lock)
* 256 (64-bit) or 512 (32-bit) to fully utilize a 4k page.
*
* Since we should not be holding locks from NMI context (very rare indeed) the
- * max load factor is 0.75, which is around the point where open addressing
- * breaks down.
+ * max load factor is 0.75.
*
*/
struct pv_hash_entry {
@@ -251,6 +251,31 @@ static struct pv_node *pv_unhash(struct qspinlock *lock)
}

/*
+ * Look up the given lock in the hash table
+ * Return the pv_node if found, NULL otherwise
+ */
+static struct pv_node *pv_lookup_hash(struct qspinlock *lock)
+{
+ unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
+ struct pv_hash_entry *he;
+
+ for_each_hash_entry(he, offset, hash) {
+ struct qspinlock *l = READ_ONCE(he->lock);
+
+ if (l == lock)
+ return READ_ONCE(he->node);
+ /*
+ * Presence of an empty slot signal the end of search. We
+ * may miss the entry, but that will limit the amount of
+ * time doing the search when the desired entry isn't there.
+ */
+ else if (!l)
+ break;
+ }
+ return NULL;
+}
+
+/*
* Return true if when it is time to check the previous node which is not
* in a running state.
*/
@@ -275,6 +300,7 @@ static void pv_init_node(struct mcs_spinlock *node)

pn->cpu = smp_processor_id();
pn->state = vcpu_running;
+ pn->prev_cpu = -1;
}

/*
@@ -282,7 +308,8 @@ static void pv_init_node(struct mcs_spinlock *node)
* pv_kick_node() is used to set _Q_SLOW_VAL and fill in hash table on its
* behalf.
*/
-static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
+static void pv_wait_node(struct qspinlock *lock, struct mcs_spinlock *node,
+ struct mcs_spinlock *prev)
{
struct pv_node *pn = (struct pv_node *)node;
struct pv_node *pp = (struct pv_node *)prev;
@@ -290,6 +317,8 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
int loop;
bool wait_early;

+ pn->prev_cpu = pp->cpu; /* Save previous node vCPU */
+
/* waitcnt processing will be compiled out if !QUEUED_LOCK_STAT */
for (;; waitcnt++) {
for (wait_early = false, loop = SPIN_THRESHOLD; loop; loop--) {
@@ -314,10 +343,21 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
smp_store_mb(pn->state, vcpu_halted);

if (!READ_ONCE(node->locked)) {
+ struct pv_node *hn;
+
qstat_inc(qstat_pv_wait_node, true);
qstat_inc(qstat_pv_wait_again, waitcnt);
qstat_inc(qstat_pv_wait_early, wait_early);
- pv_wait(&pn->state, vcpu_halted);
+
+ /*
+ * We try to locate the queue head pv_node by looking
+ * up the hash table. If it is not found, use the
+ * CPU in the previous node instead.
+ */
+ hn = pv_lookup_hash(lock);
+ if (!hn)
+ hn = pn;
+ pv_wait(&pn->state, vcpu_halted, hn->prev_cpu);
}

/*
@@ -453,7 +493,15 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
WRITE_ONCE(pn->state, vcpu_halted);
qstat_inc(qstat_pv_wait_head, true);
qstat_inc(qstat_pv_wait_again, waitcnt);
- pv_wait(&l->locked, _Q_SLOW_VAL);
+
+ /*
+ * Pass in the previous node vCPU nmber which is likely to be
+ * the lock holder vCPU. This additional information may help
+ * the hypervisor to give more resource to that vCPU so that
+ * it can release the lock faster. With lock stealing,
+ * however, that vCPU may not be the actual lock holder.
+ */
+ pv_wait(&l->locked, _Q_SLOW_VAL, pn->prev_cpu);

/*
* The unlocker should have freed the lock before kicking the
diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
index eb2a2c9..8728348 100644
--- a/kernel/locking/qspinlock_stat.h
+++ b/kernel/locking/qspinlock_stat.h
@@ -266,12 +266,12 @@ static inline void __pv_kick(int cpu)
/*
* Replacement function for pv_wait()
*/
-static inline void __pv_wait(u8 *ptr, u8 val)
+static inline void __pv_wait(u8 *ptr, u8 val, int lockcpu)
{
u64 *pkick_time = this_cpu_ptr(&pv_kick_time);

*pkick_time = 0;
- pv_wait(ptr, val);
+ pv_wait(ptr, val, lockcpu);
if (*pkick_time) {
this_cpu_add(qstats[qstat_pv_latency_wake],
sched_clock() - *pkick_time);
@@ -279,8 +279,8 @@ static inline void __pv_wait(u8 *ptr, u8 val)
}
}

-#define pv_kick(c) __pv_kick(c)
-#define pv_wait(p, v) __pv_wait(p, v)
+#define pv_kick(c) __pv_kick(c)
+#define pv_wait(p, v, c) __pv_wait(p, v, c)

#else /* CONFIG_QUEUED_LOCK_STAT */

--
1.7.1

Next message: Vivien Didelot: "[PATCH net 0/3] net: dsa: mv88e6xxx: fix hardware cross-chip bridging"
Previous message: Dennis Dalessandro: "Re: [PATCH 0/7] IB/hfi1: Remove write() and use ioctl() for user access"
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