From: Matthew Wilcox <willy@infradead.org>
To: Hyeonggon Yoo <42.hyeyoo@gmail.com>
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, Jens Axboe <axboe@kernel.dk>,
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: Mon, 20 Sep 2021 23:01:16 +0100 [thread overview]
Message-ID: <YUkErK1vVZMht4s8@casper.infradead.org> (raw)
In-Reply-To: <20210920154816.31832-1-42.hyeyoo@gmail.com>
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)
next prev parent reply other threads:[~2021-09-21 2:05 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-20 15:48 [RFC v2 PATCH] mm, sl[au]b: Introduce lockless cache Hyeonggon Yoo
2021-09-20 22:01 ` Matthew Wilcox [this message]
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
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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