Re: [RFC v2 PATCH] mm, sl[au]b: Introduce lockless cache

[Hyeonggon Yoo]

On Mon, Sep 20, 2021 at 11:01:16PM +0100, Matthew Wilcox wrote:
> 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.

I think first we need to discuss if KMEM_LOCKLESS_CACHE_QUEUE_SIZE will be
okay with constant, or per kmem_cache size, or global boot parameter.

But even before that, we need to discuss how to manage magazine.
because that affect on what KMEM_LOCKLESS_CACHE_QUEUE_SIZE will be.

> > +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.

That looks good to me, as you said there's chance that 'size' and some of
queue's elements (hopefully queue[size - 1]) are in same cacheline.

Plus, in v2 I didn't consider cache line when determining position of
kmem_lockless_cache in kmem_cache, I think we need to reconsider this
too.

> > +/**
> > + * 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.

I want to ask you where idea of allocating 'half' the size of queue came from.
the paper you sent does not work with single queue(magazine). Instead,
it manages pool of magazines.

And after reading the paper, I see managing pool of magazine (where M is
an boot parameter) is valid approach to reduce hitting slowpath.

Thanks,
Hyeonggon Yoo

> > +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)