Re: [RT LATENCY] 249 microsecond latency caused by slub'sunfreeze_partials() code.

[Joonsoo Kim]

Hello, Christoph.

On Mon, Apr 01, 2013 at 03:33:23PM +0000, Christoph Lameter wrote:
> Subject: slub: Fix object counts in acquire_slab V2
> 
> 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.

Yes. I sent a patch for this two month ago. :)

> 
> Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>
> 
> Index: linux/mm/slub.c
> ===================================================================
> --- linux.orig/mm/slub.c	2013-03-28 12:14:26.958358688 -0500
> +++ linux/mm/slub.c	2013-04-01 10:23:24.677584499 -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->inuse;
>  	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;
>  }

Good. I like your method which use lru.next in order to hand over
number of objects.

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

We need one more fix for correctness.
When available is assigned by put_cpu_partial, it doesn't count cpu slab's objects.
Please reference my old patch.

https://lkml.org/lkml/2013/1/21/64

Thanks.

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