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

From: Mike Galbraith
Date: Sat Jun 15 2013 - 03:30:21 EST


On Sat, 2013-06-15 at 07:48 +0200, Mike Galbraith wrote:
> On Sat, 2013-06-15 at 07:27 +0200, Manfred Spraul wrote:
>
> > 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.
>
> Yup, pondered that yesterday, scratching my head over how to do better.
> Hints highly welcome. Maybe if I figure out how to scratch dual lock
> thingy properly for -rt, non-rt will start acting sane too, as that spot
> seems to be itchy in both kernels.

Gee, just trying to flip back to a single semaphore lock mode if you had
to do the global wait thing fixed up -rt. 10 consecutive sem-waitzero 5
8 64 runs with the 3.8-rt9 kernel went like so, which is one hell of an
improvement.

Result matrix:
Thread 0: 20209311
Thread 1: 20255372
Thread 2: 20082611
...
Thread 61: 20162924
Thread 62: 20048995
Thread 63: 20142689

I must have screwed up something :)

static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
int nsops)
{
struct sem *sem;
int locknum;

if (nsops == 1 && !sma->complex_count) {
sem = sma->sem_base + sops->sem_num;

/*
* Another process is holding the global lock on the
* sem_array; we cannot enter our critical section,
* but have to wait for the global lock to be released.
*/
if (unlikely(spin_is_locked(&sma->sem_perm.lock))) {
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);
} else {
/* Lock just the semaphore we are interested in. */
spin_lock(&sem->lock);

/*
* If sma->complex_count was set prior to acquisition,
* we must fall back to the global array lock.
*/
if (unlikely(sma->complex_count)) {
spin_unlock(&sem->lock);
goto lock_array;
}
}

locknum = sops->sem_num;
} else {
int i;
/*
* Lock the semaphore array, and wait for all of the
* individual semaphore locks to go away. The code
* above ensures no new single-lock holders will enter
* their critical section while the array lock is held.
*/
lock_array:
spin_lock(&sma->sem_perm.lock);
wait_array:
for (i = 0; i < sma->sem_nsems; i++) {
sem = sma->sem_base + i;
#ifdef CONFIG_PREEMPT_RT_BASE
if (spin_is_locked(&sem->lock))
#endif
spin_unlock_wait(&sem->lock);
}
locknum = -1;

if (nsops == 1 && !sma->complex_count) {
sem = sma->sem_base + sops->sem_num;
spin_lock(&sem->lock);
spin_unlock(&sma->sem_perm.lock);
locknum = sops->sem_num;
}
}
return locknum;
}

Not very pretty, but it works markedly better.

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