Re: [PATCH v10 09/19] qspinlock: Prepare for unfair lock support
On Fri, May 09, 2014 at 09:19:32PM -0400, Waiman Long wrote: > On 05/08/2014 03:06 PM, Peter Zijlstra wrote: > >On Wed, May 07, 2014 at 11:01:37AM -0400, Waiman Long wrote: > >>If unfair lock is supported, the lock acquisition loop at the end of > >>the queue_spin_lock_slowpath() function may need to detect the fact > >>the lock can be stolen. Code are added for the stolen lock detection. > >> > >>A new qhead macro is also defined as a shorthand for mcs.locked. > >NAK, unfair should be a pure test-and-set lock. > > I have performance data showing that a simple test-and-set lock does not > scale well. That is the primary reason of ditching the test-and-set lock and > use a more complicated scheme which scales better. Nobody should give a fuck about scalability in this case anyway. Also, as I explained/asked earlier: lkml.kernel.org/r/20140314083001.gn27...@twins.programming.kicks-ass.net Lock holder preemption is _way_ worse with any kind of queueing. You've not explained how the simple 3 cpu example in that email gets better performance than a test-and-set lock. > Also, it will be hard to > make the unfair test-and-set lock code to coexist nicely with PV spinlock > code. That's just complete crap as the test-and-set lock is like 3 lines of code. pgpceVksftY6O.pgp Description: PGP signature
Re: [PATCH v10 09/19] qspinlock: Prepare for unfair lock support
On 05/08/2014 03:06 PM, Peter Zijlstra wrote:
On Wed, May 07, 2014 at 11:01:37AM -0400, Waiman Long wrote:
If unfair lock is supported, the lock acquisition loop at the end of
the queue_spin_lock_slowpath() function may need to detect the fact
the lock can be stolen. Code are added for the stolen lock detection.
A new qhead macro is also defined as a shorthand for mcs.locked.
NAK, unfair should be a pure test-and-set lock.
I have performance data showing that a simple test-and-set lock does not
scale well. That is the primary reason of ditching the test-and-set lock
and use a more complicated scheme which scales better. Also, it will be
hard to make the unfair test-and-set lock code to coexist nicely with PV
spinlock code.
/**
* get_qlock - Set the lock bit and own the lock
- * @lock: Pointer to queue spinlock structure
+ * @lock : Pointer to queue spinlock structure
+ * Return: 1 if lock acquired, 0 otherwise
*
* This routine should only be called when the caller is the only one
* entitled to acquire the lock.
*/
-static __always_inline void get_qlock(struct qspinlock *lock)
+static __always_inline int get_qlock(struct qspinlock *lock)
{
struct __qspinlock *l = (void *)lock;
barrier();
ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
barrier();
+ return 1;
}
and here you make a horribly named function more horrible;
try_set_locked() is that its now.
Will do.
-Longman
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Re: [PATCH v10 09/19] qspinlock: Prepare for unfair lock support
On Wed, May 07, 2014 at 11:01:37AM -0400, Waiman Long wrote:
> If unfair lock is supported, the lock acquisition loop at the end of
> the queue_spin_lock_slowpath() function may need to detect the fact
> the lock can be stolen. Code are added for the stolen lock detection.
>
> A new qhead macro is also defined as a shorthand for mcs.locked.
NAK, unfair should be a pure test-and-set lock.
> /**
> * get_qlock - Set the lock bit and own the lock
> - * @lock: Pointer to queue spinlock structure
> + * @lock : Pointer to queue spinlock structure
> + * Return: 1 if lock acquired, 0 otherwise
> *
> * This routine should only be called when the caller is the only one
> * entitled to acquire the lock.
> */
> -static __always_inline void get_qlock(struct qspinlock *lock)
> +static __always_inline int get_qlock(struct qspinlock *lock)
> {
> struct __qspinlock *l = (void *)lock;
>
> barrier();
> ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
> barrier();
> + return 1;
> }
and here you make a horribly named function more horrible;
try_set_locked() is that its now.
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[PATCH v10 09/19] qspinlock: Prepare for unfair lock support
If unfair lock is supported, the lock acquisition loop at the end of
the queue_spin_lock_slowpath() function may need to detect the fact
the lock can be stolen. Code are added for the stolen lock detection.
A new qhead macro is also defined as a shorthand for mcs.locked.
Signed-off-by: Waiman Long
---
kernel/locking/qspinlock.c | 26 +++---
1 files changed, 19 insertions(+), 7 deletions(-)
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index e98d7d4..9e7659e 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -64,6 +64,7 @@
struct qnode {
struct mcs_spinlock mcs;
};
+#define qhead mcs.locked /* The queue head flag */
/*
* Per-CPU queue node structures; we can never have more than 4 nested
@@ -216,18 +217,20 @@ xchg_tail(struct qspinlock *lock, u32 tail, u32 *pval)
/**
* get_qlock - Set the lock bit and own the lock
- * @lock: Pointer to queue spinlock structure
+ * @lock : Pointer to queue spinlock structure
+ * Return: 1 if lock acquired, 0 otherwise
*
* This routine should only be called when the caller is the only one
* entitled to acquire the lock.
*/
-static __always_inline void get_qlock(struct qspinlock *lock)
+static __always_inline int get_qlock(struct qspinlock *lock)
{
struct __qspinlock *l = (void *)lock;
barrier();
ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
barrier();
+ return 1;
}
/**
@@ -365,7 +368,7 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32
val)
tail = encode_tail(smp_processor_id(), idx);
node += idx;
- node->mcs.locked = 0;
+ node->qhead = 0;
node->mcs.next = NULL;
/*
@@ -391,7 +394,7 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32
val)
prev = decode_tail(old);
ACCESS_ONCE(prev->mcs.next) = (struct mcs_spinlock *)node;
- while (!smp_load_acquire(&node->mcs.locked))
+ while (!smp_load_acquire(&node->qhead))
arch_mutex_cpu_relax();
}
@@ -403,6 +406,7 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32
val)
*
* *,x,y -> *,0,0
*/
+retry_queue_wait:
while ((val = smp_load_acquire(&lock->val.counter))
& _Q_LOCKED_PENDING_MASK)
arch_mutex_cpu_relax();
@@ -419,12 +423,20 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32
val)
*/
for (;;) {
if (val != tail) {
- get_qlock(lock);
- break;
+ /*
+* The get_qlock function will only failed if the
+* lock was stolen.
+*/
+ if (get_qlock(lock))
+ break;
+ else
+ goto retry_queue_wait;
}
old = atomic_cmpxchg(&lock->val, val, _Q_LOCKED_VAL);
if (old == val)
goto release; /* No contention */
+ else if (old & _Q_LOCKED_MASK)
+ goto retry_queue_wait;
val = old;
}
@@ -435,7 +447,7 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32
val)
while (!(next = (struct qnode *)ACCESS_ONCE(node->mcs.next)))
arch_mutex_cpu_relax();
- arch_mcs_spin_unlock_contended(&next->mcs.locked);
+ arch_mcs_spin_unlock_contended(&next->qhead);
release:
/*
--
1.7.1
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