linux-kernel - Re: [PATCH -mm v2 3/3] slub: make dead caches discard free slabs immediately

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Date:	Fri, 1 Apr 2016 11:04:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2 3/3] slub: make dead caches discard free slabs
 immediately

On Wed, Jan 28, 2015 at 07:22:51PM +0300, Vladimir Davydov wrote:
> +++ b/mm/slub.c
> @@ -2007,6 +2007,7 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
>  	int pages;
>  	int pobjects;
>  
> +	preempt_disable();
>  	do {
>  		pages = 0;
>  		pobjects = 0;
> @@ -2040,6 +2041,14 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
>  
>  	} while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page)
>  								!= oldpage);
> +	if (unlikely(!s->cpu_partial)) {
> +		unsigned long flags;
> +
> +		local_irq_save(flags);
> +		unfreeze_partials(s, this_cpu_ptr(s->cpu_slab));
> +		local_irq_restore(flags);
> +	}
> +	preempt_enable();
>  #endif
>  }
>  
> @@ -3369,7 +3378,7 @@ EXPORT_SYMBOL(kfree);
>   * being allocated from last increasing the chance that the last objects
>   * are freed in them.
>   */
> -int __kmem_cache_shrink(struct kmem_cache *s)
> +int __kmem_cache_shrink(struct kmem_cache *s, bool deactivate)
>  {
>  	int node;
>  	int i;
> @@ -3381,14 +3390,26 @@ int __kmem_cache_shrink(struct kmem_cache *s)
>  	unsigned long flags;
>  	int ret = 0;
>  
> +	if (deactivate) {
> +		/*
> +		 * Disable empty slabs caching. Used to avoid pinning offline
> +		 * memory cgroups by kmem pages that can be freed.
> +		 */
> +		s->cpu_partial = 0;
> +		s->min_partial = 0;
> +
> +		/*
> +		 * s->cpu_partial is checked locklessly (see put_cpu_partial),
> +		 * so we have to make sure the change is visible.
> +		 */
> +		kick_all_cpus_sync();
> +	}

Argh! what the heck! and without a single mention in the changelog.

Why are you spraying IPIs across the entire machine? Why isn't
synchronize_sched() good enough, that would allow you to get rid of the
local_irq_save/restore as well.