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From: Hyeonggon Yoo <42.hyeyoo@gmail.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: linux-mm@kvack.org, Christoph Lameter <cl@linux.com>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	linux-kernel@vger.kernel.org,
	Matthew Wilcox <willy@infradead.org>,
	John Garry <john.garry@huawei.com>,
	linux-block@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: [RFC v2 PATCH] mm, sl[au]b: Introduce lockless cache
Date: Wed, 22 Sep 2021 08:19:06 +0000	[thread overview]
Message-ID: <20210922081906.GA78305@kvm.asia-northeast3-a.c.our-ratio-313919.internal> (raw)
In-Reply-To: <ebea2af2-90d0-248f-8461-80f2e834dfea@kernel.dk>

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
> 

  reply	other threads:[~2021-09-22  8:19 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-20 15:48 Hyeonggon Yoo
2021-09-20 22:01 ` Matthew Wilcox
2021-09-21 10:56   ` Hyeonggon Yoo
2021-09-21 15:42   ` Hyeonggon Yoo
2021-09-21 16:17     ` Matthew Wilcox
2021-09-22  8:32       ` Hyeonggon Yoo
2021-09-22  9:11         ` Hyeonggon Yoo
2021-09-21 15:37 ` Jens Axboe
2021-09-22  8:19   ` Hyeonggon Yoo [this message]
2021-09-22 12:58     ` Jens Axboe
2021-09-23  3:34       ` Hyeonggon Yoo
2021-09-23 13:28       ` Jakub Kicinski
2021-09-23  3:55 ` Github link here Hyeonggon Yoo

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