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Date:   Wed, 12 Feb 2020 14:52:47 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Wen Yang <wenyang@linux.alibaba.com>
Cc:     Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Xunlei Pang <xlpang@linux.alibaba.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/slub: Detach node lock from counting free objects
Message-Id: <20200212145247.bf89431272038de53dd9d975@linux-foundation.org>
In-Reply-To: <20200201031502.92218-1-wenyang@linux.alibaba.com>
References: <20200201031502.92218-1-wenyang@linux.alibaba.com>
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On Sat,  1 Feb 2020 11:15:02 +0800 Wen Yang <wenyang@linux.alibaba.com> wrote:

> The lock, protecting the node partial list, is taken when couting the free
> objects resident in that list. It introduces locking contention when the
> page(s) is moved between CPU and node partial lists in allocation path
> on another CPU. So reading "/proc/slabinfo" can possibily block the slab
> allocation on another CPU for a while, 200ms in extreme cases. If the
> slab object is to carry network packet, targeting the far-end disk array,
> it causes block IO jitter issue.
> 
> This fixes the block IO jitter issue by caching the total inuse objects in
> the node in advance. The value is retrieved without taking the node partial
> list lock on reading "/proc/slabinfo".
> 
> ...
>
> @@ -1768,7 +1774,9 @@ static void free_slab(struct kmem_cache *s, struct page *page)
>  
>  static void discard_slab(struct kmem_cache *s, struct page *page)
>  {
> -	dec_slabs_node(s, page_to_nid(page), page->objects);
> +	int inuse = page->objects;
> +
> +	dec_slabs_node(s, page_to_nid(page), page->objects, inuse);

Is this right?  dec_slabs_node(..., page->objects, page->objects)?

If no, we could simply pass the page* to inc_slabs_node/dec_slabs_node
and save a function argument.

If yes then why?

>  	free_slab(s, page);
>  }
>