linux-kernel - Re: [RT LATENCY] 249 microsecond latency caused by slub's unfreeze

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Date:	Mon, 08 Apr 2013 08:25:26 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <clark@...hat.com>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [RT LATENCY] 249 microsecond latency caused by slub's
 unfreeze_partials() code.

Sorry for the late reply, but I just got back from vacation.

On Thu, 2013-03-28 at 17:29 +0000, Christoph Lameter wrote:
> Two patches against slub to enable deconfiguring cpu_partial support.
> 
> First one is a bug fix (Pekka please pick up this one or use Joonsoo's
> earlier one).
> 
> 
> Subject: slub: Fix object counts in acquire_slab
> 
> It seems that we were overallocating objects from the slab queues
> since get_partial_node() assumed that page->inuse was undisturbed by
> acquire_slab(). Save the # of objects in page->lru.next in acquire_slab()
> and pass it to get_partial_node() that way.
> 
> I have a vague memory that Joonsoo also ran into this issue awhile back.
> 
> Signed-off-by: Christoph Lameter <cl@...ux.com>
> 
> Index: linux/mm/slub.c
> ===================================================================
> --- linux.orig/mm/slub.c	2013-03-28 12:14:26.958358688 -0500
> +++ linux/mm/slub.c	2013-03-28 12:16:57.240785613 -0500
> @@ -1498,6 +1498,7 @@ static inline void *acquire_slab(struct
>  	void *freelist;
>  	unsigned long counters;
>  	struct page new;
> +	unsigned long objects;
> 
>  	/*
>  	 * Zap the freelist and set the frozen bit.
> @@ -1507,6 +1508,7 @@ static inline void *acquire_slab(struct
>  	freelist = page->freelist;
>  	counters = page->counters;
>  	new.counters = counters;
> +	objects = page->objects;
>  	if (mode) {
>  		new.inuse = page->objects;
>  		new.freelist = NULL;
> @@ -1524,6 +1526,7 @@ static inline void *acquire_slab(struct
>  		return NULL;
> 
>  	remove_partial(n, page);
> +	page->lru.next = (void *)objects;
>  	WARN_ON(!freelist);
>  	return freelist;
>  }
> @@ -1565,7 +1568,7 @@ static void *get_partial_node(struct kme
>  			c->page = page;
>  			stat(s, ALLOC_FROM_PARTIAL);
>  			object = t;
> -			available =  page->objects - page->inuse;
> +			available =  page->objects - (unsigned long)page->lru.next;

Ug, this is a really nasty hack. To avoid the curses you will get by the
one that takes over this code when you retire, I suggest just have
"objects" passed back as a reference parameter. I only see
acquire_slab() used here. Why not just pass in a pointer and have that
assigned objects instead of using a error prone use of lru.next?

-- Steve

>  		} else {
>  			available = put_cpu_partial(s, page, 0);
>  			stat(s, CPU_PARTIAL_NODE);


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