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six locks: Improved optimistic spinning

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This adds a threshold for the maximum spin time, similar to the rwsem
code, and a flag to the lock itself indicating when we've spun too long
so other threads also refrain from spinning.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
This commit is contained in:
Kent Overstreet 2023-02-05 14:09:30 -05:00
parent 94c69fafa7
commit 91db806681
2 changed files with 39 additions and 16 deletions
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52
fs/bcachefs/six.c
View File

@ -346,30 +346,39 @@ static bool __six_relock_type(struct six_lock *lock, enum six_lock_type type,
#ifdef CONFIG_SIX_LOCK_SPIN_ON_OWNER
static inline int six_can_spin_on_owner(struct six_lock *lock)
static inline bool six_can_spin_on_owner(struct six_lock *lock)
{
struct task_struct *owner;
int retval = 1;
bool ret;
if (need_resched())
return 0;
return false;
rcu_read_lock();
owner = READ_ONCE(lock->owner);
if (owner)
retval = owner->on_cpu;
ret = !owner || owner_on_cpu(owner);
rcu_read_unlock();
/*
* if lock->owner is not set, the mutex owner may have just acquired
* it and not set the owner yet or the mutex has been released.
*/
return retval;
return ret;
}
static inline void six_set_nospin(struct six_lock *lock)
{
union six_lock_state old, new;
u64 v = READ_ONCE(lock->state.v);
do {
new.v = old.v = v;
new.nospin = true;
} while ((v = atomic64_cmpxchg(&lock->state.counter, old.v, new.v)) != old.v);
}
static inline bool six_spin_on_owner(struct six_lock *lock,
struct task_struct *owner)
struct task_struct *owner,
u64 end_time)
{
bool ret = true;
unsigned loop = 0;
rcu_read_lock();
while (lock->owner == owner) {
@ -381,7 +390,13 @@ static inline bool six_spin_on_owner(struct six_lock *lock,
*/
barrier();
if (!owner->on_cpu || need_resched()) {
if (!owner_on_cpu(owner) || need_resched()) {
ret = false;
break;
}
if (!(++loop & 0xf) && (time_after64(sched_clock(), end_time))) {
six_set_nospin(lock);
ret = false;
break;
}
@ -396,6 +411,7 @@ static inline bool six_spin_on_owner(struct six_lock *lock,
static inline bool six_optimistic_spin(struct six_lock *lock, enum six_lock_type type)
{
struct task_struct *task = current;
u64 end_time;
if (type == SIX_LOCK_write)
return false;
@ -407,6 +423,8 @@ static inline bool six_optimistic_spin(struct six_lock *lock, enum six_lock_type
if (!osq_lock(&lock->osq))
goto fail;
end_time = sched_clock() + 10 * NSEC_PER_USEC;
while (1) {
struct task_struct *owner;
@ -415,7 +433,7 @@ static inline bool six_optimistic_spin(struct six_lock *lock, enum six_lock_type
* release the lock or go to sleep.
*/
owner = READ_ONCE(lock->owner);
if (owner && !six_spin_on_owner(lock, owner))
if (owner && !six_spin_on_owner(lock, owner, end_time))
break;
if (do_six_trylock_type(lock, type, false)) {
@ -606,9 +624,13 @@ static void do_six_unlock_type(struct six_lock *lock, enum six_lock_type type)
smp_mb(); /* between unlocking and checking for waiters */
state.v = READ_ONCE(lock->state.v);
} else {
u64 v = l[type].unlock_val;
if (type != SIX_LOCK_read)
v -= lock->state.v & __SIX_VAL(nospin, 1);
EBUG_ON(!(lock->state.v & l[type].held_mask));
state.v = atomic64_add_return_release(l[type].unlock_val,
&lock->state.counter);
state.v = atomic64_add_return_release(v, &lock->state.counter);
}
six_lock_wakeup(lock, state, l[type].unlock_wakeup);

3
fs/bcachefs/six.h
View File

@ -83,9 +83,10 @@ union six_lock_state {
};
struct {
unsigned read_lock:27;
unsigned read_lock:26;
unsigned write_locking:1;
unsigned intent_lock:1;
unsigned nospin:1;
unsigned waiters:3;
/*
* seq works much like in seqlocks: it's incremented every time