Re: [PATCH v4 3/3 update] mm/free_pcppages_bulk: prefetch buddy while not holding lock

[Vlastimil Babka <vbabka@suse.cz>]

On 03/13/2018 08:04 AM, Aaron Lu wrote:
> On Tue, Mar 13, 2018 at 11:35:19AM +0800, Aaron Lu wrote:
>> On Mon, Mar 12, 2018 at 10:32:32AM -0700, Dave Hansen wrote:
>>> On 03/09/2018 12:24 AM, Aaron Lu wrote:
>>>> +			/*
>>>> +			 * We are going to put the page back to the global
>>>> +			 * pool, prefetch its buddy to speed up later access
>>>> +			 * under zone->lock. It is believed the overhead of
>>>> +			 * an additional test and calculating buddy_pfn here
>>>> +			 * can be offset by reduced memory latency later. To
>>>> +			 * avoid excessive prefetching due to large count, only
>>>> +			 * prefetch buddy for the last pcp->batch nr of pages.
>>>> +			 */
>>>> +			if (count > pcp->batch)
>>>> +				continue;
>>>> +			pfn = page_to_pfn(page);
>>>> +			buddy_pfn = __find_buddy_pfn(pfn, 0);
>>>> +			buddy = page + (buddy_pfn - pfn);
>>>> +			prefetch(buddy);
>>>
>>> FWIW, I think this needs to go into a helper function.  Is that possible?
>>
>> I'll give it a try.
>>
>>>
>>> There's too much logic happening here.  Also, 'count' going from
>>> batch_size->0 is totally non-obvious from the patch context.  It makes
>>> this hunk look totally wrong by itself.
> 
> I tried to avoid adding one more local variable but looks like it caused
> a lot of pain. What about the following? It doesn't use count any more
> but prefetch_nr to indicate how many prefetches have happened.
> 
> Also, I think it's not worth the risk of disordering pages in free_list
> by changing list_add_tail() to list_add() as Andrew reminded so I
> dropped that change too.

Looks fine, you can add

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index dafdcdec9c1f..00ea4483f679 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1099,6 +1099,15 @@ static bool bulkfree_pcp_prepare(struct page *page)
>  }
>  #endif /* CONFIG_DEBUG_VM */
>  
> +static inline void prefetch_buddy(struct page *page)
> +{
> +	unsigned long pfn = page_to_pfn(page);
> +	unsigned long buddy_pfn = __find_buddy_pfn(pfn, 0);
> +	struct page *buddy = page + (buddy_pfn - pfn);
> +
> +	prefetch(buddy);
> +}
> +
>  /*
>   * Frees a number of pages from the PCP lists
>   * Assumes all pages on list are in same zone, and of same order.
> @@ -1115,6 +1124,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  {
>  	int migratetype = 0;
>  	int batch_free = 0;
> +	int prefetch_nr = 0;
>  	bool isolated_pageblocks;
>  	struct page *page, *tmp;
>  	LIST_HEAD(head);
> @@ -1150,6 +1160,18 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  				continue;
>  
>  			list_add_tail(&page->lru, &head);
> +
> +			/*
> +			 * We are going to put the page back to the global
> +			 * pool, prefetch its buddy to speed up later access
> +			 * under zone->lock. It is believed the overhead of
> +			 * an additional test and calculating buddy_pfn here
> +			 * can be offset by reduced memory latency later. To
> +			 * avoid excessive prefetching due to large count, only
> +			 * prefetch buddy for the first pcp->batch nr of pages.
> +			 */
> +			if (prefetch_nr++ < pcp->batch)
> +				prefetch_buddy(page);
>  		} while (--count && --batch_free && !list_empty(list));
>  	}
>  
>