Re: [PATCH] mm: don't invoke __alloc_pages_direct_compact when order 0

[Minchan Kim <minchan@kernel.org>]

Hi Joonsoo,

On Sat, Jul 07, 2012 at 12:28:41AM +0900, Joonsoo Kim wrote:
> __alloc_pages_direct_compact has many arguments so invoking it is very costly.

It's already slow path so it's pointless for such optimization.

> And in almost invoking case, order is 0, so return immediately.

You can't make sure it.

> 
> Let's not invoke it when order 0

Let's not ruin git blame.

> 
> Signed-off-by: Joonsoo Kim <js1304@gmail.com>
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 6092f33..f4039aa 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2056,7 +2056,10 @@ out:
>  }
>  
>  #ifdef CONFIG_COMPACTION
> -/* Try memory compaction for high-order allocations before reclaim */
> +/*
> + * Try memory compaction for high-order allocations before reclaim
> + * Must be called with order > 0
> + */
>  static struct page *
>  __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>  	struct zonelist *zonelist, enum zone_type high_zoneidx,
> @@ -2067,8 +2070,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>  {
>  	struct page *page;
>  
> -	if (!order)
> -		return NULL;
> +	BUG_ON(!order);
>  
>  	if (compaction_deferred(preferred_zone, order)) {
>  		*deferred_compaction = true;
> @@ -2363,15 +2365,17 @@ rebalance:
>  	 * Try direct compaction. The first pass is asynchronous. Subsequent
>  	 * attempts after direct reclaim are synchronous
>  	 */
> -	page = __alloc_pages_direct_compact(gfp_mask, order,
> -					zonelist, high_zoneidx,
> -					nodemask,
> -					alloc_flags, preferred_zone,
> -					migratetype, sync_migration,
> -					&deferred_compaction,
> -					&did_some_progress);
> -	if (page)
> -		goto got_pg;
> +	if (unlikely(order)) {
> +		page = __alloc_pages_direct_compact(gfp_mask, order,
> +						zonelist, high_zoneidx,
> +						nodemask,
> +						alloc_flags, preferred_zone,
> +						migratetype, sync_migration,
> +						&deferred_compaction,
> +						&did_some_progress);
> +		if (page)
> +			goto got_pg;
> +	}
>  	sync_migration = true;
>  
>  	/*
> @@ -2446,15 +2450,17 @@ rebalance:
>  		 * direct reclaim and reclaim/compaction depends on compaction
>  		 * being called after reclaim so call directly if necessary
>  		 */
> -		page = __alloc_pages_direct_compact(gfp_mask, order,
> -					zonelist, high_zoneidx,
> -					nodemask,
> -					alloc_flags, preferred_zone,
> -					migratetype, sync_migration,
> -					&deferred_compaction,
> -					&did_some_progress);
> -		if (page)
> -			goto got_pg;
> +		if (unlikely(order)) {
> +			page = __alloc_pages_direct_compact(gfp_mask, order,
> +						zonelist, high_zoneidx,
> +						nodemask,
> +						alloc_flags, preferred_zone,
> +						migratetype, sync_migration,
> +						&deferred_compaction,
> +						&did_some_progress);
> +			if (page)
> +				goto got_pg;
> +		}
>  	}
>  
>  nopage:
> -- 
> 1.7.9.5
> 
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