Give the queue head the ability to stop stealers. After a number of
spins without sucessfully acquiring the lock, the queue head employs
this, which will assure it is the next owner.
---
arch/powerpc/include/asm/qspinlock_types.h | 10 +++-
arch/powerpc/lib/qspinlock.c | 56 +++++++++++++++++++++-
2 files changed, 63 insertions(+), 3 deletions(-)
diff --git a/arch/powerpc/include/asm/qspinlock_types.h
b/arch/powerpc/include/asm/qspinlock_types.h
index 210adf05b235..8b20f5e22bba 100644
--- a/arch/powerpc/include/asm/qspinlock_types.h
+++ b/arch/powerpc/include/asm/qspinlock_types.h
@@ -29,7 +29,8 @@ typedef struct qspinlock {
* Bitfields in the lock word:
*
* 0: locked bit
- * 16-31: tail cpu (+1)
+ * 16: must queue bit
+ * 17-31: tail cpu (+1)
*/
#define _Q_SET_MASK(type) (((1U << _Q_ ## type ## _BITS) - 1)\
<< _Q_ ## type ## _OFFSET)
@@ -38,7 +39,12 @@ typedef struct qspinlock {
#define _Q_LOCKED_MASK _Q_SET_MASK(LOCKED)
#define _Q_LOCKED_VAL (1U << _Q_LOCKED_OFFSET)
-#define _Q_TAIL_CPU_OFFSET 16
+#define _Q_MUST_Q_OFFSET 16
+#define _Q_MUST_Q_BITS 1
+#define _Q_MUST_Q_MASK _Q_SET_MASK(MUST_Q)
+#define _Q_MUST_Q_VAL (1U << _Q_MUST_Q_OFFSET)
+
+#define _Q_TAIL_CPU_OFFSET 17
#define _Q_TAIL_CPU_BITS (32 - _Q_TAIL_CPU_OFFSET)
#define _Q_TAIL_CPU_MASK _Q_SET_MASK(TAIL_CPU)
diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
index 1625cce714b2..a906cc8f15fa 100644
--- a/arch/powerpc/lib/qspinlock.c
+++ b/arch/powerpc/lib/qspinlock.c
@@ -22,6 +22,7 @@ struct qnodes {
/* Tuning parameters */
static int STEAL_SPINS __read_mostly = (1<<5);
static bool MAYBE_STEALERS __read_mostly = true;
+static int HEAD_SPINS __read_mostly = (1<<8);
static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
@@ -30,6 +31,11 @@ static __always_inline int get_steal_spins(void)
return STEAL_SPINS;
}
+static __always_inline int get_head_spins(void)
+{
+ return HEAD_SPINS;
+}
+
static inline u32 encode_tail_cpu(void)
{
return (smp_processor_id() + 1) << _Q_TAIL_CPU_OFFSET;
@@ -142,6 +148,23 @@ static __always_inline u32 publish_tail_cpu(struct
qspinlock *lock, u32 tail)
return prev;
}
+static __always_inline u32 lock_set_mustq(struct qspinlock *lock)
+{
+ u32 new = _Q_MUST_Q_VAL;
+ u32 prev;
+
+ asm volatile(
+"1: lwarx %0,0,%1 # lock_set_mustq \n"
+" or %0,%0,%2 \n"
+" stwcx. %0,0,%1 \n"
+" bne- 1b \n"
+ : "=&r" (prev)
+ : "r" (&lock->val), "r" (new)
+ : "cr0", "memory");
+
+ return prev;
+}
+
static struct qnode *get_tail_qnode(struct qspinlock *lock, u32 val)
{
int cpu = get_tail_cpu(val);
@@ -165,6 +188,9 @@ static inline bool try_to_steal_lock(struct qspinlock *lock)
for (;;) {
u32 val = READ_ONCE(lock->val);
+ if (val & _Q_MUST_Q_VAL)
+ break;
+
if (unlikely(!(val & _Q_LOCKED_VAL))) {
if (trylock_with_tail_cpu(lock, val))
return true;
@@ -246,11 +272,22 @@ static inline void queued_spin_lock_mcs_queue(struct
qspinlock *lock)
/* We must be the owner, just set the lock bit and acquire */
lock_set_locked(lock);
} else {
+ int iters = 0;
+ bool set_mustq = false;
+
again:
/* We're at the head of the waitqueue, wait for the lock. */
- while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL)
+ while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL) {
cpu_relax();
+ iters++;
+ if (!set_mustq && iters >= get_head_spins()) {
+ set_mustq = true;
+ lock_set_mustq(lock);
+ val |= _Q_MUST_Q_VAL;
+ }
+ }
+
/* If we're the last queued, must clean up the tail. */
if ((val & _Q_TAIL_CPU_MASK) == tail) {
if (trylock_clear_tail_cpu(lock, val))
@@ -329,9 +366,26 @@ static int steal_spins_get(void *data, u64 *val)
DEFINE_SIMPLE_ATTRIBUTE(fops_steal_spins, steal_spins_get, steal_spins_set,
"%llu\n");
+static int head_spins_set(void *data, u64 val)
+{
+ HEAD_SPINS = val;
+
+ return 0;
+}
+
+static int head_spins_get(void *data, u64 *val)
+{
+ *val = HEAD_SPINS;
+
+ return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(fops_head_spins, head_spins_get, head_spins_set,
"%llu\n");
+
static __init int spinlock_debugfs_init(void)
{
debugfs_create_file("qspl_steal_spins", 0600, arch_debugfs_dir, NULL,
&fops_steal_spins);
+ debugfs_create_file("qspl_head_spins", 0600, arch_debugfs_dir, NULL,
&fops_head_spins);
return 0;
}
--
2.35.1
