linux-kernel - Re: [RFC PATCH] mm: memblock: optimize memblock_find_in_range

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Date:	Mon, 07 Jan 2013 09:53:19 +0800
From:	Lin Feng <linfeng@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	akpm@...ux-foundation.org, mingo@...nel.org, yinghai@...nel.org,
	liwanp@...ux.vnet.ibm.com, benh@...nel.crashing.org,
	tangchen@...fujitsu.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: memblock: optimize memblock_find_in_range_node()
 to minimize the search work



On 01/04/2013 11:01 PM, Tejun Heo wrote:
> On Fri, Jan 04, 2013 at 05:24:53PM +0800, Lin Feng wrote:
>> The memblock array is in ascending order and we traverse the memblock array in
>> reverse order so we can add some simple check to reduce the search work.
>>
>> Tejun fix a underflow bug in 5d53cb27d8, but I think we could break there for
>> the same reason.
>>
>> Cc: Tejun Heo <tj@...nel.org>
>> Signed-off-by: Lin Feng <linfeng@...fujitsu.com>
>> ---
>>  mm/memblock.c | 9 ++++++++-
>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/memblock.c b/mm/memblock.c
>> index 6259055..a710557 100644
>> --- a/mm/memblock.c
>> +++ b/mm/memblock.c
>> @@ -111,11 +111,18 @@ phys_addr_t __init_memblock memblock_find_in_range_node(phys_addr_t start,
>>  	end = max(start, end);
>>  
>>  	for_each_free_mem_range_reverse(i, nid, &this_start, &this_end, NULL) {
>> +		/*
>> +		 * exclude the regions out of the candidate range, since it's
>> +		 * likely to find a suitable range, we ignore the worst case.
>> +		 */
>> +		if (this_start >= end)
>> +			continue;
>> +
>>  		this_start = clamp(this_start, start, end);
>>  		this_end = clamp(this_end, start, end);
>>  
>>  		if (this_end < size)
>> -			continue;
>> +			break;
> 
> I don't know.  This only saves looping when memblocks are below the
> requested size, right?  I don't think it would matter in any way and
> would prefer to keep the logic as simple as possible.
Hi Tejun,

You're right, when we hit the 'if (this_end < size)' branch, it's nearly 
the end of the whole search loops. I just got an impression that is 
there any candidate range after we hit the if clause when I first read
this code, so... ;-)

thanks,
linfeng
> 
> Thanks.
> 
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