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

Waiman Long
2016-04-13T20:59:43Z

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(),
-1 will be passed to pv_wait() instead to indicate that it doesn't
know what the current lock holder is.

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

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
---
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_paravirt.h | 19 +++++++++++++++----
kernel/locking/qspinlock_stat.h | 8 ++++----
6 files changed, 24 insertions(+), 13 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_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index 21ede57..4bec98b 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 {
@@ -275,6 +275,7 @@ static void pv_init_node(struct mcs_spinlock *node)

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

/*
@@ -290,6 +291,8 @@ 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--) {
@@ -317,7 +320,7 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
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);
+ pv_wait(&pn->state, vcpu_halted, -1);
}

/*
@@ -453,7 +456,15 @@ 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