linux-kernel - Re: [PATCH v3 08/16] zsmalloc: squeeze freelist into page->mapping

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Message-ID: <20160417155621.GE575@swordfish>
Date:	Mon, 18 Apr 2016 00:56:21 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	jlayton@...chiereds.net, bfields@...ldses.org,
	Vlastimil Babka <vbabka@...e.cz>,
	Joonsoo Kim <iamjoonsoo.kim@....com>, koct9i@...il.com,
	aquini@...hat.com, virtualization@...ts.linux-foundation.org,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Rik van Riel <riel@...hat.com>, rknize@...orola.com,
	Gioh Kim <gi-oh.kim@...fitbricks.com>,
	Sangseok Lee <sangseok.lee@....com>,
	Chan Gyun Jeong <chan.jeong@....com>,
	Al Viro <viro@...IV.linux.org.uk>,
	YiPing Xu <xuyiping@...ilicon.com>
Subject: Re: [PATCH v3 08/16] zsmalloc: squeeze freelist into page->mapping

Hello,

On (03/30/16 16:12), Minchan Kim wrote:
[..]
> +static void objidx_to_page_and_offset(struct size_class *class,
> +				struct page *first_page,
> +				unsigned long obj_idx,
> +				struct page **obj_page,
> +				unsigned long *offset_in_page)
>  {
> -	unsigned long obj;
> +	int i;
> +	unsigned long offset;
> +	struct page *cursor;
> +	int nr_page;
>  
> -	if (!page) {
> -		VM_BUG_ON(obj_idx);
> -		return NULL;
> -	}
> +	offset = obj_idx * class->size;

so we already know the `offset' before we call objidx_to_page_and_offset(),
thus we can drop `struct size_class *class' and `obj_idx', and pass
`long obj_offset'  (which is `obj_idx * class->size') instead, right?

we also _may be_ can return `cursor' from the function.

static struct page *objidx_to_page_and_offset(struct page *first_page,
					unsigned long obj_offset,
					unsigned long *offset_in_page);

this can save ~20 instructions, which is not so terrible for a hot path
like obj_malloc(). what do you think?

well, seems that `unsigned long *offset_in_page' can be calculated
outside of this function too, it's basically

	*offset_in_page = (obj_idx * class->size) & ~PAGE_MASK;

so we don't need to supply it to this function, nor modify it there.
which can save ~40 instructions on my system. does this sound silly?

	-ss

> +	cursor = first_page;
> +	nr_page = offset >> PAGE_SHIFT;
>  
> -	obj = page_to_pfn(page) << OBJ_INDEX_BITS;
> -	obj |= ((obj_idx) & OBJ_INDEX_MASK);
> -	obj <<= OBJ_TAG_BITS;
> +	*offset_in_page = offset & ~PAGE_MASK;
> +
> +	for (i = 0; i < nr_page; i++)
> +		cursor = get_next_page(cursor);
>  
> -	return (void *)obj;
> +	*obj_page = cursor;
>  }

	-ss