Re: [PATCH 0/6] ipc/sem.c: performance improvements, FIFO

From: Manfred Spraul
Date: Sat Jun 15 2013 - 01:27:25 EST

On 06/14/2013 09:05 PM, Mike Galbraith wrote:
32 of 64 cores DL980 without the -rt killing goto again loop removal I
showed you. Unstable, not wonderful throughput.
Unfortunately the -rt approach is defintively unstable:
@@ -285,9 +274,29 @@ static inline int sem_lock(struct sem_ar
* but have to wait for the global lock to be released.
if (unlikely(spin_is_locked(&sma->sem_perm.lock))) {
- spin_unlock(&sem->lock);
- spin_unlock_wait(&sma->sem_perm.lock);
- goto again;
+ spin_lock(&sma->sem_perm.lock);
+ if (sma->complex_count)
+ goto wait_array;
+ /*
+ * Acquiring our sem->lock under the global lock
+ * forces new complex operations to wait for us
+ * to exit our critical section.
+ */
+ spin_lock(&sem->lock);
+ spin_unlock(&sma->sem_perm.lock);

Assume there is one op (semctl(), whatever) that acquires the global lock - and a continuous stream of simple ops.
- spin_is_locked() returns true due to the semctl().
- then simple ops will switch to spin_lock(&sma->sem_perm.lock).
- since the spinlock is acquired, the next operation will get true from spin_is_locked().

It will stay that way around - as long as there is at least one op waiting for sma->sem_perm.lock.
With enough cpus, it will stay like this forever.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at