MINOR: pools: always pre-initialize allocated memory outside of the lock

When calling mmap(), in general the system gives us a page but does not
really allocate it until we first dereference it. And it turns out that
this time is much longer than the time to perform the mmap() syscall.
Unfortunately, when running with memory debugging enabled, we mmap/munmap()
each object resulting in lots of such calls and a high contention on the
allocator. And the first accesses to the page being done under the pool
lock is extremely damaging to other threads.

The simple fact of writing a 0 at the beginning of the page after
allocating it and placing the POOL_LINK pointer outside of the lock is
enough to boost the performance by 8x in debug mode and to save the
watchdog from triggering on lock contention. This is what this patch
does.

include/common/memory.h

@@ -421,6 +421,10 @@ static inline void *pool_alloc_area(size_t size)
 	ret = mmap(NULL, (size + 4095) & -4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
 	if (ret == MAP_FAILED)
 		return NULL;
+	/* let's dereference the page before returning so that the real
+	 * allocation in the system is performed without holding the lock.
+	 */
+	*(int *)ret = 0;
 	if (pad >= sizeof(void *))
 		*(void **)(ret + pad - sizeof(void *)) = ret + pad;
 	return ret + pad;

src/memory.c

@@ -337,6 +337,14 @@ void *__pool_refill_alloc(struct pool_head *pool, unsigned int avail)
 
 		HA_SPIN_UNLOCK(POOL_LOCK, &pool->lock);
 		ptr = pool_alloc_area(pool->size + POOL_EXTRA);
+#ifdef DEBUG_MEMORY_POOLS
+		/* keep track of where the element was allocated from. This
+		 * is done out of the lock so that the system really allocates
+		 * the data without harming other threads waiting on the lock.
+		 */
+		if (ptr)
+			*POOL_LINK(pool, ptr) = (void *)pool;
+#endif
 		HA_SPIN_LOCK(POOL_LOCK, &pool->lock);
 		if (!ptr) {
 			pool->failed++;
@@ -355,10 +363,6 @@ void *__pool_refill_alloc(struct pool_head *pool, unsigned int avail)
 		pool->free_list = ptr;
 	}
 	pool->used++;
-#ifdef DEBUG_MEMORY_POOLS
-	/* keep track of where the element was allocated from */
-	*POOL_LINK(pool, ptr) = (void *)pool;
-#endif
 	return ptr;
 }
 void *pool_refill_alloc(struct pool_head *pool, unsigned int avail)