Provide an option that holds off queueing indefinitely while the lock
owner is preempted. This could reduce queueing latencies for very
overcommitted vcpu situations.
This is disabled by default.
---
arch/powerpc/lib/qspinlock.c | 91 +++++++++++++++++++++++++++++++-----
1 file changed, 79 insertions(+), 12 deletions(-)
diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
index 24f68bd71e2b..5cfd69931e31 100644
--- a/arch/powerpc/lib/qspinlock.c
+++ b/arch/powerpc/lib/qspinlock.c
@@ -35,6 +35,7 @@ static int HEAD_SPINS __read_mostly = (1<<8);
static bool pv_yield_owner __read_mostly = true;
static bool pv_yield_allow_steal __read_mostly = false;
+static bool pv_spin_on_preempted_owner __read_mostly = false;
static bool pv_yield_prev __read_mostly = true;
static bool pv_yield_propagate_owner __read_mostly = true;
static bool pv_prod_head __read_mostly = false;
@@ -220,13 +221,15 @@ static struct qnode *get_tail_qnode(struct qspinlock
*lock, u32 val)
BUG();
}
-static __always_inline void __yield_to_locked_owner(struct qspinlock *lock,
u32 val, bool paravirt, bool clear_mustq)
+static __always_inline void __yield_to_locked_owner(struct qspinlock *lock,
u32 val, bool paravirt, bool clear_mustq, bool *preempted)
{
int owner;
u32 yield_count;
BUG_ON(!(val & _Q_LOCKED_VAL));
+ *preempted = false;
+
if (!paravirt)
goto relax;
@@ -241,6 +244,8 @@ static __always_inline void __yield_to_locked_owner(struct
qspinlock *lock, u32
spin_end();
+ *preempted = true;
+
/*
* Read the lock word after sampling the yield count. On the other side
* there may a wmb because the yield count update is done by the
@@ -265,14 +270,14 @@ static __always_inline void
__yield_to_locked_owner(struct qspinlock *lock, u32
spin_cpu_relax();
}
-static __always_inline void yield_to_locked_owner(struct qspinlock *lock, u32
val, bool paravirt)
+static __always_inline void yield_to_locked_owner(struct qspinlock *lock, u32
val, bool paravirt, bool *preempted)
{
- __yield_to_locked_owner(lock, val, paravirt, false);
+ __yield_to_locked_owner(lock, val, paravirt, false, preempted);
}
-static __always_inline void yield_head_to_locked_owner(struct qspinlock *lock,
u32 val, bool paravirt, bool clear_mustq)
+static __always_inline void yield_head_to_locked_owner(struct qspinlock *lock,
u32 val, bool paravirt, bool clear_mustq, bool *preempted)
{
- __yield_to_locked_owner(lock, val, paravirt, clear_mustq);
+ __yield_to_locked_owner(lock, val, paravirt, clear_mustq, preempted);
}
static __always_inline void propagate_yield_cpu(struct qnode *node, u32 val,
int *set_yield_cpu, bool paravirt)
@@ -364,12 +369,33 @@ static __always_inline void yield_to_prev(struct
qspinlock *lock, struct qnode *
static __always_inline bool try_to_steal_lock(struct qspinlock *lock, bool
paravirt)
{
- int iters;
+ int iters = 0;
+
+ if (!STEAL_SPINS) {
+ if (paravirt && pv_spin_on_preempted_owner) {
+ spin_begin();
+ for (;;) {
+ u32 val = READ_ONCE(lock->val);
+ bool preempted;
+
+ if (val & _Q_MUST_Q_VAL)
+ break;
+ if (!(val & _Q_LOCKED_VAL))
+ break;
+ if (!vcpu_is_preempted(get_owner_cpu(val)))
+ break;
+ yield_to_locked_owner(lock, val, paravirt,
&preempted);
+ }
+ spin_end();
+ }
+ return false;
+ }
/* Attempt to steal the lock */
spin_begin();
for (;;) {
u32 val = READ_ONCE(lock->val);
+ bool preempted;
if (val & _Q_MUST_Q_VAL)
break;
@@ -382,9 +408,22 @@ static __always_inline bool try_to_steal_lock(struct
qspinlock *lock, bool parav
continue;
}
- yield_to_locked_owner(lock, val, paravirt);
-
- iters++;
+ yield_to_locked_owner(lock, val, paravirt, &preempted);
+
+ if (paravirt && preempted) {
+ if (!pv_spin_on_preempted_owner)
+ iters++;
+ /*
+ * pv_spin_on_preempted_owner don't increase iters
+ * while the owner is preempted -- we won't interfere
+ * with it by definition. This could introduce some
+ * latency issue if we continually observe preempted
+ * owners, but hopefully that's a rare corner case of
+ * a badly oversubscribed system.
+ */
+ } else {
+ iters++;
+ }
if (iters >= get_steal_spins(paravirt, false))
break;
@@ -463,8 +502,10 @@ static __always_inline void
queued_spin_lock_mcs_queue(struct qspinlock *lock, b
/* We're at the head of the waitqueue, wait for the lock. */
spin_begin();
while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL) {
+ bool preempted;
+
propagate_yield_cpu(node, val, &set_yield_cpu,
paravirt);
- yield_head_to_locked_owner(lock, val, paravirt, false);
+ yield_head_to_locked_owner(lock, val, paravirt, false,
&preempted);
}
spin_end();
@@ -486,11 +527,20 @@ static __always_inline void
queued_spin_lock_mcs_queue(struct qspinlock *lock, b
/* We're at the head of the waitqueue, wait for the lock. */
spin_begin();
while ((val = READ_ONCE(lock->val)) & _Q_LOCKED_VAL) {
+ bool preempted;
+
propagate_yield_cpu(node, val, &set_yield_cpu,
paravirt);
yield_head_to_locked_owner(lock, val, paravirt,
- pv_yield_allow_steal && set_mustq);
+ pv_yield_allow_steal && set_mustq,
+ &preempted);
+
+ if (paravirt && preempted) {
+ if (!pv_spin_on_preempted_owner)
+ iters++;
+ } else {
+ iters++;
+ }
- iters++;
if (!set_mustq && iters >= get_head_spins(paravirt)) {
set_mustq = true;
lock_set_mustq(lock);
@@ -663,6 +713,22 @@ static int pv_yield_allow_steal_get(void *data, u64 *val)
DEFINE_SIMPLE_ATTRIBUTE(fops_pv_yield_allow_steal, pv_yield_allow_steal_get,
pv_yield_allow_steal_set, "%llu\n");
+static int pv_spin_on_preempted_owner_set(void *data, u64 val)
+{
+ pv_spin_on_preempted_owner = !!val;
+
+ return 0;
+}
+
+static int pv_spin_on_preempted_owner_get(void *data, u64 *val)
+{
+ *val = pv_spin_on_preempted_owner;
+
+ return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(fops_pv_spin_on_preempted_owner,
pv_spin_on_preempted_owner_get, pv_spin_on_preempted_owner_set, "%llu\n");
+
static int pv_yield_prev_set(void *data, u64 val)
{
pv_yield_prev = !!val;
@@ -719,6 +785,7 @@ static __init int spinlock_debugfs_init(void)
if (is_shared_processor()) {
debugfs_create_file("qspl_pv_yield_owner", 0600,
arch_debugfs_dir, NULL, &fops_pv_yield_owner);
debugfs_create_file("qspl_pv_yield_allow_steal", 0600,
arch_debugfs_dir, NULL, &fops_pv_yield_allow_steal);
+ debugfs_create_file("qspl_pv_spin_on_preempted_owner", 0600,
arch_debugfs_dir, NULL, &fops_pv_spin_on_preempted_owner);
debugfs_create_file("qspl_pv_yield_prev", 0600,
arch_debugfs_dir, NULL, &fops_pv_yield_prev);
debugfs_create_file("qspl_pv_yield_propagate_owner", 0600,
arch_debugfs_dir, NULL, &fops_pv_yield_propagate_owner);
debugfs_create_file("qspl_pv_prod_head", 0600,
arch_debugfs_dir, NULL, &fops_pv_prod_head);
--
2.35.1
