Re: [PATCH] mm, page_alloc: calculate first_deferred_pfn directly

[Wei Yang <richard.weiyang@gmail.com>]

On Thu, Dec 20, 2018 at 03:47:53PM -0800, Alexander Duyck wrote:
>On Fri, 2018-12-07 at 18:08 +0800, Wei Yang wrote:
>> After commit c9e97a1997fb ("mm: initialize pages on demand during
>> boot"), the behavior of DEFERRED_STRUCT_PAGE_INIT is changed to
>> initialize first section for highest zone on each node.
>> 
>> Instead of test each pfn during iteration, we could calculate the
>> first_deferred_pfn directly with necessary information.
>> 
>> By doing so, we also get some performance benefit during bootup:
>> 
>>     +----------+-----------+-----------+--------+
>>     |          |Base       |Patched    |Gain    |
>>     +----------+-----------+-----------+--------+
>>     | 1 Node   |0.011993   |0.011459   |-4.45%  |
>>     +----------+-----------+-----------+--------+
>>     | 4 Nodes  |0.006466   |0.006255   |-3.26%  |
>>     +----------+-----------+-----------+--------+
>> 
>> Test result is retrieved from dmesg time stamp by add printk around
>> free_area_init_nodes().
>> 
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>

Hi, Alexander

Thanks for your comment!

>I'm pretty sure the fundamental assumption made in this patch is wrong.
>
>It is assuming that the first deferred PFN will just be your start PFN
>+ PAGES_PER_SECTION aligned to the nearest PAGES_PER_SECTION, do I have
>that correct?

You are right.

>
>If I am not mistaken that can result in scenarios where you actually
>start out with 0 pages allocated if your first section is in a span
>belonging to another node, or is reserved memory for things like MMIO.

Yeah, sounds it is possible.

>
>Ideally we don't want to do that as we have to immediately jump into
>growing the zone with the code as it currently stands.

You are right.

>
>> ---
>>  mm/page_alloc.c | 57 +++++++++++++++++++++++++++------------------------------
>>  1 file changed, 27 insertions(+), 30 deletions(-)
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index baf473f80800..5f077bf07f3e 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -306,38 +306,33 @@ static inline bool __meminit early_page_uninitialised(unsigned long pfn)
>>  }
>>  
>>  /*
>> - * Returns true when the remaining initialisation should be deferred until
>> - * later in the boot cycle when it can be parallelised.
>> + * Calculate first_deferred_pfn in case:
>> + * - in MEMMAP_EARLY context
>> + * - this is the last zone
>> + *
>> + * If the first aligned section doesn't exceed the end_pfn, set it to
>> + * first_deferred_pfn and return it.
>>   */
>> -static bool __meminit
>> -defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
>> +unsigned long __meminit
>> +defer_pfn(int nid, unsigned long start_pfn, unsigned long end_pfn,
>> +	  enum memmap_context context)
>>  {
>> -	static unsigned long prev_end_pfn, nr_initialised;
>> +	struct pglist_data *pgdat = NODE_DATA(nid);
>> +	unsigned long pfn;
>>  
>> -	/*
>> -	 * prev_end_pfn static that contains the end of previous zone
>> -	 * No need to protect because called very early in boot before smp_init.
>> -	 */
>> -	if (prev_end_pfn != end_pfn) {
>> -		prev_end_pfn = end_pfn;
>> -		nr_initialised = 0;
>> -	}
>> +	if (context != MEMMAP_EARLY)
>> +		return end_pfn;
>>  
>> -	/* Always populate low zones for address-constrained allocations */
>> -	if (end_pfn < pgdat_end_pfn(NODE_DATA(nid)))
>> -		return false;
>> +	/* Always populate low zones */
>> +	if (end_pfn < pgdat_end_pfn(pgdat))
>> +		return end_pfn;
>>  
>> -	/*
>> -	 * We start only with one section of pages, more pages are added as
>> -	 * needed until the rest of deferred pages are initialized.
>> -	 */
>> -	nr_initialised++;
>> -	if ((nr_initialised > PAGES_PER_SECTION) &&
>> -	    (pfn & (PAGES_PER_SECTION - 1)) == 0) {
>> -		NODE_DATA(nid)->first_deferred_pfn = pfn;
>> -		return true;
>> +	pfn = roundup(start_pfn + PAGES_PER_SECTION - 1, PAGES_PER_SECTION);
>> +	if (end_pfn > pfn) {
>> +		pgdat->first_deferred_pfn = pfn;
>> +		end_pfn = pfn;
>>  	}
>> -	return false;
>> +	return end_pfn;
>
>Okay so I stand corrected. It looks like you are rounding up by
>(PAGES_PER_SECTION - 1) * 2 since if I am not mistaken roundup should
>do the same math you already did in side the function.
>
>>  }
>>  #else
>>  static inline bool early_page_uninitialised(unsigned long pfn)
>> @@ -345,9 +340,11 @@ static inline bool early_page_uninitialised(unsigned long pfn)
>>  	return false;
>>  }
>>  
>> -static inline bool defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
>> +unsigned long __meminit
>> +defer_pfn(int nid, unsigned long start_pfn, unsigned long end_pfn,
>> +	  enum memmap_context context)
>>  {
>> -	return false;
>> +	return end_pfn;
>>  }
>>  #endif
>>  
>> @@ -5514,6 +5511,8 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>>  	}
>>  #endif
>>  
>> +	end_pfn = defer_pfn(nid, start_pfn, end_pfn, context);
>> +
>
>A better approach for this might be to look at placing the loop within
>a loop similar to how I handled this for the deferred init. You only
>really need to be performing all of these checks once per section
>aligned point anyway.

I didn't really get your idea here. Do you have the commit id you handle
deferred init?

>
>Basically if you added another loop and limited the loop below so that
>you only fed it one section at a time then you could just pull the
>defer_init check out of this section and place it in the outer loop
>after you have already tried initializing at least one section worth of
>pages.
>
>You could probably also look at pulling in the logic that is currently
>sitting at the end of the current function that is initializing things
>until the end_pfn is aligned with PAGES_PER_SECTION.
>
>>  	for (pfn = start_pfn; pfn < end_pfn; pfn++) {
>>  		/*
>>  		 * There can be holes in boot-time mem_map[]s handed to this
>> @@ -5526,8 +5525,6 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
>>  				continue;
>>  			if (overlap_memmap_init(zone, &pfn))
>>  				continue;
>> -			if (defer_init(nid, pfn, end_pfn))
>> -				break;
>>  		}
>
>So the whole reason for the "defer_init" call being placed here is
>because there are checks to see if the prior PFN is valid, in our NUMA
>node, or is an overlapping region. If your first section or in this
>case 2 sections contain pages that fall into these categories you
>aren't going to initialize any pages.

Ok, I get your point. Let me do a summary, the approach in this patch
has one flaw: in case all pages in the first section fall into these two
categories, we will end up with no page initialized for this zone.

So my suggestion is:

  Find the first valid page and roundup it to PAGES_PER_SECTION. This
  would ensure we won't end up with zero initialized page.

Generally, my purpose in this patch is:

1. Don't affect the initialisation for non defer init zones.
   Current code will call defer_init() for each pfn, no matter this pfn
   should be defer_init or not. By taking this out, we try to minimize
   the effect on the initialisation process.
2. Iterate on less pfn for defer zone
   Current code will count on each pfn in defer zone. By roundup pfn
   directly, less calculation would be necessary. Defer init will handle
   the rest. Or if we really want at least PAGES_PER_SECTION pfn be
   initialized for defer zone, we can do the same math in defer_pfn().

Glad to talk with you and look forward your comments:-)

>
>>  
>>  		page = pfn_to_page(pfn);

-- 
Wei Yang
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