@@ -12,6 +12,7 @@
struct qnode {
struct qnode *next;
struct qspinlock *lock;
+ int yield_cpu;
u8 locked; /* 1 if lock acquired */
};
@@ -28,6 +29,7 @@ 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_yield_prev __read_mostly = true;
+static bool pv_yield_propagate_owner __read_mostly = true;
static DEFINE_PER_CPU_ALIGNED(struct qnodes, qnodes);
@@ -251,14 +253,67 @@ static __always_inline void yield_head_to_locked_owner(struct qspinlock *lock, u
__yield_to_locked_owner(lock, val, paravirt, mustq);
}
+static __always_inline void propagate_yield_cpu(struct qnode *node, u32 val, int *set_yield_cpu, bool paravirt)
+{
+ struct qnode *next;
+ int owner;
+
+ if (!paravirt)
+ return;
+ if (!pv_yield_propagate_owner)
+ return;
+
+ owner = get_owner_cpu(val);
+ if (*set_yield_cpu == owner)
+ return;
+
+ next = READ_ONCE(node->next);
+ if (!next)
+ return;
+
+ if (vcpu_is_preempted(owner)) {
+ next->yield_cpu = owner;
+ *set_yield_cpu = owner;
+ } else if (*set_yield_cpu != -1) {
+ next->yield_cpu = owner;
+ *set_yield_cpu = owner;
+ }
+}
+
static __always_inline void yield_to_prev(struct qspinlock *lock, struct qnode *node, u32 val, bool paravirt)
{
int prev_cpu = decode_tail_cpu(val);
u32 yield_count;
+ int yield_cpu;
if (!paravirt)
goto relax;
+ if (!pv_yield_propagate_owner)
+ goto yield_prev;
+
+ yield_cpu = READ_ONCE(node->yield_cpu);
+ if (yield_cpu == -1) {
+ /* Propagate back the -1 CPU */
+ if (node->next && node->next->yield_cpu != -1)
+ node->next->yield_cpu = yield_cpu;
+ goto yield_prev;
+ }
+
+ yield_count = yield_count_of(yield_cpu);
+ if ((yield_count & 1) == 0)
+ goto yield_prev; /* owner vcpu is running */
+
+ smp_rmb();
+
+ if (yield_cpu == node->yield_cpu) {
+ if (node->next && node->next->yield_cpu != yield_cpu)
+ node->next->yield_cpu = yield_cpu;
+ yield_to_preempted(yield_cpu, yield_count);
+ return;
+ }
+
+yield_prev:
if (!pv_yield_prev)
goto relax;
@@ -331,6 +386,7 @@ static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, b
node = &qnodesp->nodes[idx];
node->next = NULL;
node->lock = lock;
+ node->yield_cpu = -1;
node->locked = 0;
tail = encode_tail_cpu(smp_processor_id());
@@ -351,16 +407,23 @@ static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, b
while (!node->locked)
yield_to_prev(lock, node, old, paravirt);
+ /* Clear out stale propagated yield_cpu */
+ if (paravirt && pv_yield_propagate_owner && node->yield_cpu != -1)
+ node->yield_cpu = -1;
+
smp_rmb(); /* acquire barrier for the mcs lock */
}
if (!maybe_stealers) {
+ int set_yield_cpu = -1;
+
/* We're at the head of the waitqueue, wait for the lock. */
for (;;) {
val = READ_ONCE(lock->val);
if (!(val & _Q_LOCKED_VAL))
break;
+ propagate_yield_cpu(node, val, &set_yield_cpu, paravirt);
yield_head_to_locked_owner(lock, val, paravirt);
}
@@ -374,6 +437,7 @@ static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, b
/* We must be the owner, just set the lock bit and acquire */
set_locked(lock);
} else {
+ int set_yield_cpu = -1;
int iters = 0;
bool mustq = false;
@@ -384,6 +448,7 @@ static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, b
if (!(val & _Q_LOCKED_VAL))
break;
+ propagate_yield_cpu(node, val, &set_yield_cpu, paravirt);
yield_head_to_locked_owner(lock, val, paravirt);
iters++;
@@ -553,6 +618,22 @@ static int pv_yield_prev_get(void *data, u64 *val)
DEFINE_SIMPLE_ATTRIBUTE(fops_pv_yield_prev, pv_yield_prev_get, pv_yield_prev_set, "%llu\n");
+static int pv_yield_propagate_owner_set(void *data, u64 val)
+{
+ pv_yield_propagate_owner = !!val;
+
+ return 0;
+}
+
+static int pv_yield_propagate_owner_get(void *data, u64 *val)
+{
+ *val = pv_yield_propagate_owner;
+
+ return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(fops_pv_yield_propagate_owner, pv_yield_propagate_owner_get, pv_yield_propagate_owner_set, "%llu\n");
+
static __init int spinlock_debugfs_init(void)
{
debugfs_create_file("qspl_steal_spins", 0600, arch_debugfs_dir, NULL, &fops_steal_spins);
@@ -561,6 +642,7 @@ static __init int spinlock_debugfs_init(void)
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_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);
}
return 0;
Having all CPUs poll the lock word for the owner CPU that should be yielded to defeats most of the purpose of using MCS queueing for scalability. Yet it may be desirable for queued waiters to to yield to a preempted owner. s390 addreses this problem by having queued waiters sample the lock word to find the owner much less frequently. In this approach, the waiters never sample it directly, but the queue head propagates the owner CPU back to the next waiter if it ever finds the owner has been preempted. Queued waiters then subsequently propagate the owner CPU back to the next waiter, and so on. Signed-off-by: Nicholas Piggin <npiggin@gmail.com> --- arch/powerpc/lib/qspinlock.c | 82 ++++++++++++++++++++++++++++++++++++ 1 file changed, 82 insertions(+)