On Mon, Sep 20, 2021 at 03:48:16PM +0000, Hyeonggon Yoo wrote:
> +#define KMEM_LOCKLESS_CACHE_QUEUE_SIZE 64
I would suggest that, to be nice to the percpu allocator, this be
one less than 2^n.
> +struct kmem_lockless_cache {
> + void *queue[KMEM_LOCKLESS_CACHE_QUEUE_SIZE];
> + unsigned int size;
> +};
I would also suggest that 'size' be first as it is going to be accessed
every time, and then there's a reasonable chance that queue[size - 1] will
be in the same cacheline. CPUs will tend to handle that better.
> +/**
> + * kmem_cache_alloc_cached - try to allocate from cache without lock
> + * @s: slab cache
> + * @flags: SLAB flags
> + *
> + * Try to allocate from cache without lock. If fails, fill the lockless cache
> + * using bulk alloc API
> + *
> + * Be sure that there's no race condition.
> + * Must create slab cache with SLAB_LOCKLESS_CACHE flag to use this function.
> + *
> + * Return: a pointer to free object on allocation success, NULL on failure.
> + */
> +void *kmem_cache_alloc_cached(struct kmem_cache *s, gfp_t gfpflags)
> +{
> + struct kmem_lockless_cache *cache = this_cpu_ptr(s->cache);
> +
> + BUG_ON(!(s->flags & SLAB_LOCKLESS_CACHE));
> +
> + if (cache->size) /* fastpath without lock */
> + return cache->queue[--cache->size];
> +
> + /* slowpath */
> + cache->size = kmem_cache_alloc_bulk(s, gfpflags,
> + KMEM_LOCKLESS_CACHE_QUEUE_SIZE, cache->queue);
Go back to the Bonwick paper and look at the magazine section again.
You have to allocate _half_ the size of the queue, otherwise you get
into pathological situations where you start to free and allocate
every time.
> +void kmem_cache_free_cached(struct kmem_cache *s, void *p)
> +{
> + struct kmem_lockless_cache *cache = this_cpu_ptr(s->cache);
> +
> + BUG_ON(!(s->flags & SLAB_LOCKLESS_CACHE));
> +
> + /* Is there better way to do this? */
> + if (cache->size == KMEM_LOCKLESS_CACHE_QUEUE_SIZE)
> + kmem_cache_free(s, cache->queue[--cache->size]);
Yes.
if (cache->size == KMEM_LOCKLESS_CACHE_QUEUE_SIZE) {
kmem_cache_free_bulk(s, KMEM_LOCKLESS_CACHE_QUEUE_SIZE / 2,
&cache->queue[KMEM_LOCKLESS_CACHE_QUEUE_SIZE / 2));
cache->size = KMEM_LOCKLESS_CACHE_QUEUE_SIZE / 2;
(check the maths on that; it might have some off-by-one)
