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

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Tue, Sep 21, 2021 at 09:37:40AM -0600, Jens Axboe wrote:
> > @@ -424,6 +431,57 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
> >  }
> >  EXPORT_SYMBOL(kmem_cache_create);
> >  
> > +/**
> > + * 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);
> > +	if (cache->size)
> > +		return cache->queue[--cache->size];
> > +	else
> > +		return NULL;
> > +}
> > +EXPORT_SYMBOL(kmem_cache_alloc_cached);

Hello Jens, I'm so happy that you gave comment.

> What I implemented for IOPOLL doesn't need to care about interrupts,
> hence preemption disable is enough. But we do need that, at least.

To be honest, that was my mistake. I was mistakenly using percpu API.
it's a shame :> Thank you for pointing that.

Fixed it in v3 (work in progress now)

> There are basically two types of use cases for this:
> 
> 1) Freeing can happen from interrupts
> 2) Freeing cannot happen from interrupts
>

I considered only case 2) when writing code. Well, To support 1),
I think there are two ways:

 a) internally call kmem_cache_free when in_interrupt() is true
 b) caller must disable interrupt when freeing

I think a) is okay, how do you think?

note that b) can be problematic with kmem_cache_free_bulk
as it says interrupts must be enabled.

> How does this work for preempt? You seem to assume that the function is
> invoked with preempt disabled, but then it could only be used with
> GFP_ATOMIC.

I wrote it just same prototype with kmem_cache_alloc, and the gfpflags
parameter is unnecessary as you said. Okay, let's remove it in v3.

> And if you don't care about users that free from irq/softirq, then that
> should be mentioned. Locking context should be mentioned, too. The above
> may be just fine IFF both alloc and free are protected by a lock higher
> up. If not, both need preemption disabled and GFP_ATOMIC. I'd suggest
> making the get/put cpu part of the API internally.

Actually I didn't put much effort in documentation. (Especially
on what context is expected before calling them)

comments will be updated in v3, with your comment in mind.

> > +/**
> > + * kmem_cache_free_cached - return object to cache
> > + * @s: slab cache
> > + * @p: pointer to free
> > + */
> > +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));
> 
> Don't use BUG_ON, just do:
> 
> 	if (WARN_ON_ONCE(!(s->flags & SLAB_LOCKLESS_CACHE))) {
> 		kmem_cache_free(s, p);
> 		return;
> 	}
>

Ok. I agree WARN is better than BUG.

Thanks,
Hyeonggon Yoo

> -- 
> Jens Axboe
>